diff --git a/toolchain/codegen/testdata/assembly/basic.carbon b/toolchain/codegen/testdata/assembly/basic.carbon
index 3b8c3e727c52..a0fc2061d5c2 100644
--- a/toolchain/codegen/testdata/assembly/basic.carbon
+++ b/toolchain/codegen/testdata/assembly/basic.carbon
@@ -5,6 +5,6 @@
 // ARGS: compile --no-prelude-import --target=x86_64-unknown-linux-gnu --output=- %s
 // NOAUTOUPDATE
 // SET-CHECK-SUBSET
-// CHECK:STDOUT: Main:
+// CHECK:STDOUT: _CMain.Main:
 
 fn Main() {}
diff --git a/toolchain/lower/BUILD b/toolchain/lower/BUILD
index 9e8e1b69b8ae..273956b9fd4d 100644
--- a/toolchain/lower/BUILD
+++ b/toolchain/lower/BUILD
@@ -32,6 +32,8 @@ cc_library(
         "constant.h",
         "file_context.cpp",
         "function_context.cpp",
+        "mangler.cpp",
+        "mangler.h",
     ] +
     # Glob handler files to avoid missing any.
     glob([
diff --git a/toolchain/lower/file_context.cpp b/toolchain/lower/file_context.cpp
index 82ef3ea7e40c..dec3328b7ae3 100644
--- a/toolchain/lower/file_context.cpp
+++ b/toolchain/lower/file_context.cpp
@@ -11,6 +11,7 @@
 #include "toolchain/base/kind_switch.h"
 #include "toolchain/lower/constant.h"
 #include "toolchain/lower/function_context.h"
+#include "toolchain/lower/mangler.h"
 #include "toolchain/sem_ir/entry_point.h"
 #include "toolchain/sem_ir/file.h"
 #include "toolchain/sem_ir/function.h"
@@ -233,22 +234,15 @@ auto FileContext::BuildFunctionDecl(SemIR::FunctionId function_id)
 
   // Compute the return type to use for the LLVM function. If the initializing
   // representation doesn't produce a value, set the return type to void.
+  // TODO: For the `Run` entry point, remap return type to i32 if it doesn't
+  // return a value.
   llvm::Type* function_return_type =
       return_info.init_repr.kind == SemIR::InitRepr::ByCopy
           ? return_type
           : llvm::Type::getVoidTy(llvm_context());
 
-  std::string mangled_name;
-  if (SemIR::IsEntryPoint(sem_ir(), function_id)) {
-    // TODO: Add an implicit `return 0` if `Run` doesn't return `i32`.
-    mangled_name = "main";
-  } else {
-    // TODO: Decide on a name mangling scheme.
-    auto name = sem_ir().names().GetAsStringIfIdentifier(function.name_id);
-    CARBON_CHECK(name) << "Unexpected special name for function: "
-                       << function.name_id;
-    mangled_name = *name;
-  }
+  Mangler m(*this);
+  std::string mangled_name = m.Mangle(function_id);
 
   llvm::FunctionType* function_type = llvm::FunctionType::get(
       function_return_type, param_types, /*isVarArg=*/false);
diff --git a/toolchain/lower/mangler.cpp b/toolchain/lower/mangler.cpp
new file mode 100644
index 000000000000..29cdc5737a33
--- /dev/null
+++ b/toolchain/lower/mangler.cpp
@@ -0,0 +1,115 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/lower/mangler.h"
+
+#include "toolchain/base/kind_switch.h"
+#include "toolchain/sem_ir/entry_point.h"
+
+namespace Carbon::Lower {
+
+auto Mangler::MangleInverseQualifiedNameScope(llvm::raw_ostream& os,
+                                              SemIR::NameScopeId name_scope_id)
+    -> void {
+  // Maintain a stack of names for delayed rendering of interface impls.
+  struct NameEntry {
+    SemIR::NameScopeId name_scope_id;
+
+    // The prefix emitted before this name component. If '\0', no prefix will be
+    // emitted.
+    // - Namespace components are separated by '.'.
+    // - The two components of an interface are separated by ':'.
+    char prefix;
+  };
+  llvm::SmallVector<NameEntry> names_to_render;
+  names_to_render.push_back({.name_scope_id = name_scope_id, .prefix = '.'});
+  while (!names_to_render.empty()) {
+    auto [name_scope_id, prefix] = names_to_render.pop_back_val();
+    if (prefix) {
+      os << prefix;
+    }
+    if (name_scope_id == SemIR::NameScopeId::Package) {
+      if (auto package_id = sem_ir().package_id(); package_id.is_valid()) {
+        os << sem_ir().identifiers().Get(package_id);
+      } else {
+        os << "Main";
+      }
+      continue;
+    }
+    const auto& name_scope = sem_ir().name_scopes().Get(name_scope_id);
+    CARBON_KIND_SWITCH(sem_ir().insts().Get(name_scope.inst_id)) {
+      case CARBON_KIND(SemIR::ImplDecl impl_decl): {
+        const auto& impl = sem_ir().impls().Get(impl_decl.impl_id);
+
+        auto interface_type =
+            types().GetAs<SemIR::InterfaceType>(impl.constraint_id);
+        const auto& interface =
+            sem_ir().interfaces().Get(interface_type.interface_id);
+        names_to_render.push_back(
+            {.name_scope_id = interface.scope_id, .prefix = ':'});
+
+        CARBON_KIND_SWITCH(types().GetAsInst(impl.self_id)) {
+          case CARBON_KIND(SemIR::ClassType class_type): {
+            auto next_name_scope_id =
+                sem_ir().classes().Get(class_type.class_id).scope_id;
+            names_to_render.push_back(
+                {.name_scope_id = next_name_scope_id, .prefix = '\0'});
+            break;
+          }
+          case CARBON_KIND(SemIR::BuiltinInst builtin_inst): {
+            os << builtin_inst.builtin_inst_kind.label();
+            break;
+          }
+          default:
+            CARBON_FATAL() << "Attempting to mangle unsupported SemIR.";
+            break;
+        }
+        // Skip the tail of the loop that adds the parent name scope to the
+        // stack - the scope in which the impl was defined is not part of the
+        // mangling, the constraint and interface alone uniquelify identify an
+        // impl.
+        continue;
+      }
+      case CARBON_KIND(SemIR::ClassDecl class_decl): {
+        os << names().GetAsStringIfIdentifier(
+            sem_ir().classes().Get(class_decl.class_id).name_id);
+        break;
+      }
+      case CARBON_KIND(SemIR::InterfaceDecl interface_decl): {
+        os << names().GetAsStringIfIdentifier(
+            sem_ir().interfaces().Get(interface_decl.interface_id).name_id);
+        break;
+      }
+      case SemIR::Namespace::Kind: {
+        os << names().GetAsStringIfIdentifier(name_scope.name_id);
+        break;
+      }
+      default:
+        CARBON_FATAL() << "Attempting to mangle unsupported SemIR.";
+        break;
+    }
+    names_to_render.push_back(
+        {.name_scope_id = name_scope.parent_scope_id, .prefix = '.'});
+  }
+}
+
+auto Mangler::Mangle(SemIR::FunctionId function_id) -> std::string {
+  // FIXME: Add support for generic entities.
+
+  const auto& function = sem_ir().functions().Get(function_id);
+  if (SemIR::IsEntryPoint(sem_ir(), function_id)) {
+    return "main";
+  }
+  std::string result;
+  llvm::raw_string_ostream os(result);
+  os << "_C";
+
+  os << names().GetAsStringIfIdentifier(function.name_id);
+
+  MangleInverseQualifiedNameScope(os, function.parent_scope_id);
+
+  return os.str();
+}
+
+}  // namespace Carbon::Lower
diff --git a/toolchain/lower/mangler.h b/toolchain/lower/mangler.h
new file mode 100644
index 000000000000..0ab3bcdd5b57
--- /dev/null
+++ b/toolchain/lower/mangler.h
@@ -0,0 +1,48 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#ifndef CARBON_TOOLCHAIN_LOWER_MANGLER_H_
+#define CARBON_TOOLCHAIN_LOWER_MANGLER_H_
+
+#include <string>
+
+#include "toolchain/lower/file_context.h"
+#include "toolchain/sem_ir/ids.h"
+
+namespace Carbon::Lower {
+
+// A class for producing mangled (deterministically unique, at least partially
+// human readable) names for externally referenceable entities such as
+// functions.
+class Mangler {
+ public:
+  // Initialize a new Mangler instance for mangling entities within the
+  // specified `FileContext`.
+  explicit Mangler(FileContext& file_context) : file_context_(file_context) {}
+
+  // Produce a deterministically unique mangled name for the function specified
+  // by `function_id`.
+  auto Mangle(SemIR::FunctionId function_id) -> std::string;
+
+ private:
+  // Mangle this qualified name with inner scope first, working outwards. This
+  // may reduce the incidence of common prefixes in the name mangling. (i.e.:
+  // every standard library name won't have a common prefix that has to be
+  // skipped and compared before getting to the interesting part)
+  auto MangleInverseQualifiedNameScope(llvm::raw_ostream& os,
+                                       SemIR::NameScopeId name_scope_id)
+      -> void;
+
+  auto sem_ir() const -> const SemIR::File& { return file_context_.sem_ir(); }
+
+  auto names() const -> SemIR::NameStoreWrapper { return sem_ir().names(); }
+
+  auto types() const -> SemIR::TypeStore { return sem_ir().types(); }
+
+  FileContext& file_context_;
+};
+
+}  // namespace Carbon::Lower
+
+#endif  // CARBON_TOOLCHAIN_LOWER_MANGLER_H_
diff --git a/toolchain/lower/testdata/alias/local.carbon b/toolchain/lower/testdata/alias/local.carbon
index df04a9ea44e0..357d7d157db0 100644
--- a/toolchain/lower/testdata/alias/local.carbon
+++ b/toolchain/lower/testdata/alias/local.carbon
@@ -17,7 +17,7 @@ fn F() -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'local.carbon'
 // CHECK:STDOUT: source_filename = "local.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F() !dbg !4 {
+// CHECK:STDOUT: define i32 @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %a.var = alloca i32, align 4, !dbg !7
 // CHECK:STDOUT:   store i32 0, ptr %a.var, align 4, !dbg !8
@@ -32,7 +32,7 @@ fn F() -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "local.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/array/array_in_place.carbon b/toolchain/lower/testdata/array/array_in_place.carbon
index 91432af7ba67..f446adac9069 100644
--- a/toolchain/lower/testdata/array/array_in_place.carbon
+++ b/toolchain/lower/testdata/array/array_in_place.carbon
@@ -17,15 +17,15 @@ fn G() {
 // CHECK:STDOUT: ; ModuleID = 'array_in_place.carbon'
 // CHECK:STDOUT: source_filename = "array_in_place.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @F(ptr sret({ i32, i32, i32 }))
+// CHECK:STDOUT: declare void @_CF.Main(ptr sret({ i32, i32, i32 }))
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @G() !dbg !4 {
+// CHECK:STDOUT: define void @_CG.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %v.var = alloca [2 x { i32, i32, i32 }], align 8, !dbg !7
 // CHECK:STDOUT:   %.loc14_42.2.array.index = getelementptr inbounds [2 x { i32, i32, i32 }], ptr %v.var, i32 0, i32 0, !dbg !8
-// CHECK:STDOUT:   call void @F(ptr %.loc14_42.2.array.index), !dbg !9
+// CHECK:STDOUT:   call void @_CF.Main(ptr %.loc14_42.2.array.index), !dbg !9
 // CHECK:STDOUT:   %.loc14_42.5.array.index = getelementptr inbounds [2 x { i32, i32, i32 }], ptr %v.var, i32 0, i32 1, !dbg !8
-// CHECK:STDOUT:   call void @F(ptr %.loc14_42.5.array.index), !dbg !10
+// CHECK:STDOUT:   call void @_CF.Main(ptr %.loc14_42.5.array.index), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -36,7 +36,7 @@ fn G() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "array_in_place.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/array/assign_return_value.carbon b/toolchain/lower/testdata/array/assign_return_value.carbon
index b24670f0b6f0..c744d8b8c330 100644
--- a/toolchain/lower/testdata/array/assign_return_value.carbon
+++ b/toolchain/lower/testdata/array/assign_return_value.carbon
@@ -19,7 +19,7 @@ fn Run() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @tuple.loc11_39 = internal constant { i32, i32 } { i32 12, i32 24 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F(ptr sret({ i32, i32 }) %return) !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main(ptr sret({ i32, i32 }) %return) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc11_38.2.tuple.elem = getelementptr inbounds nuw { i32, i32 }, ptr %return, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc11_38.4.tuple.elem = getelementptr inbounds nuw { i32, i32 }, ptr %return, i32 0, i32 1, !dbg !7
@@ -31,7 +31,7 @@ fn Run() {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %t.var = alloca [2 x i32], align 4, !dbg !10
 // CHECK:STDOUT:   %.loc14_22.1.temp = alloca { i32, i32 }, align 8, !dbg !11
-// CHECK:STDOUT:   call void @F(ptr %.loc14_22.1.temp), !dbg !11
+// CHECK:STDOUT:   call void @_CF.Main(ptr %.loc14_22.1.temp), !dbg !11
 // CHECK:STDOUT:   %.loc14_22.3.tuple.elem = getelementptr inbounds nuw { i32, i32 }, ptr %.loc14_22.1.temp, i32 0, i32 0, !dbg !11
 // CHECK:STDOUT:   %.loc14_22.4 = load i32, ptr %.loc14_22.3.tuple.elem, align 4, !dbg !11
 // CHECK:STDOUT:   %.loc14_22.6.array.index = getelementptr inbounds [2 x i32], ptr %t.var, i32 0, i32 0, !dbg !11
@@ -55,7 +55,7 @@ fn Run() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "assign_return_value.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 31, scope: !4)
diff --git a/toolchain/lower/testdata/array/function_param.carbon b/toolchain/lower/testdata/array/function_param.carbon
index 76aac8284789..6ddec2e2fe99 100644
--- a/toolchain/lower/testdata/array/function_param.carbon
+++ b/toolchain/lower/testdata/array/function_param.carbon
@@ -21,21 +21,21 @@ fn G() -> i32 {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @array.loc16_11.1 = internal constant [3 x i32] [i32 1, i32 2, i32 3]
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F(ptr %arr, i32 %i) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CF.Main(ptr %arr, i32 %i) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc12_15.2.array.index = getelementptr inbounds [3 x i32], ptr %arr, i32 0, i32 %i, !dbg !7
 // CHECK:STDOUT:   %.loc12_15.3 = load i32, ptr %.loc12_15.2.array.index, align 4, !dbg !7
 // CHECK:STDOUT:   ret i32 %.loc12_15.3, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @G() !dbg !9 {
+// CHECK:STDOUT: define i32 @_CG.Main() !dbg !9 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc16_20.2.temp = alloca [3 x i32], align 4, !dbg !10
 // CHECK:STDOUT:   %.loc16_20.4.array.index = getelementptr inbounds [3 x i32], ptr %.loc16_20.2.temp, i32 0, i32 0, !dbg !10
 // CHECK:STDOUT:   %.loc16_20.7.array.index = getelementptr inbounds [3 x i32], ptr %.loc16_20.2.temp, i32 0, i32 1, !dbg !10
 // CHECK:STDOUT:   %.loc16_20.10.array.index = getelementptr inbounds [3 x i32], ptr %.loc16_20.2.temp, i32 0, i32 2, !dbg !10
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %.loc16_20.2.temp, ptr align 4 @array.loc16_11.1, i64 12, i1 false), !dbg !11
-// CHECK:STDOUT:   %F.call = call i32 @F(ptr %.loc16_20.2.temp, i32 1), !dbg !11
+// CHECK:STDOUT:   %F.call = call i32 @_CF.Main(ptr %.loc16_20.2.temp, i32 1), !dbg !11
 // CHECK:STDOUT:   ret i32 %F.call, !dbg !12
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -51,12 +51,12 @@ fn G() -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "function_param.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 10, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 12, column: 3, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 16, column: 12, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 16, column: 10, scope: !9)
 // CHECK:STDOUT: !12 = !DILocation(line: 16, column: 3, scope: !9)
diff --git a/toolchain/lower/testdata/basics/false_true.carbon b/toolchain/lower/testdata/basics/false_true.carbon
index 892dcbb604a8..e87edff19b18 100644
--- a/toolchain/lower/testdata/basics/false_true.carbon
+++ b/toolchain/lower/testdata/basics/false_true.carbon
@@ -19,12 +19,12 @@ fn T() -> bool {
 // CHECK:STDOUT: ; ModuleID = 'false_true.carbon'
 // CHECK:STDOUT: source_filename = "false_true.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @F() !dbg !4 {
+// CHECK:STDOUT: define i1 @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i1 false, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @T() !dbg !8 {
+// CHECK:STDOUT: define i1 @_CT.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i1 true, !dbg !9
 // CHECK:STDOUT: }
@@ -36,9 +36,9 @@ fn T() -> bool {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "false_true.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 3, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "T", linkageName: "T", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "T", linkageName: "_CT.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 16, column: 3, scope: !8)
diff --git a/toolchain/lower/testdata/basics/int_types.carbon b/toolchain/lower/testdata/basics/int_types.carbon
index 5c2cb942b8fc..fe588c84723f 100644
--- a/toolchain/lower/testdata/basics/int_types.carbon
+++ b/toolchain/lower/testdata/basics/int_types.carbon
@@ -16,22 +16,22 @@ fn F_u65536(a: u65536) -> u65536 { return a; }
 // CHECK:STDOUT: ; ModuleID = 'int_types.carbon'
 // CHECK:STDOUT: source_filename = "int_types.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i8 @F_i8(i8 %a) !dbg !4 {
+// CHECK:STDOUT: define i8 @_CF_i8.Main(i8 %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i8 %a, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i16 @F_u16(i16 %a) !dbg !8 {
+// CHECK:STDOUT: define i16 @_CF_u16.Main(i16 %a) !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i16 %a, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @F_i64(i64 %a) !dbg !10 {
+// CHECK:STDOUT: define i64 @_CF_i64.Main(i64 %a) !dbg !10 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i64 %a, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i65536 @F_u65536(i65536 %a) !dbg !12 {
+// CHECK:STDOUT: define i65536 @_CF_u65536.Main(i65536 %a) !dbg !12 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i65536 %a, !dbg !13
 // CHECK:STDOUT: }
@@ -43,13 +43,13 @@ fn F_u65536(a: u65536) -> u65536 { return a; }
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "int_types.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F_i8", linkageName: "F_i8", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F_i8", linkageName: "_CF_i8.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 24, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "F_u16", linkageName: "F_u16", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "F_u16", linkageName: "_CF_u16.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 12, column: 27, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "F_i64", linkageName: "F_i64", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "F_i64", linkageName: "_CF_i64.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 13, column: 27, scope: !10)
-// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "F_u65536", linkageName: "F_u65536", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "F_u65536", linkageName: "_CF_u65536.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !13 = !DILocation(line: 14, column: 36, scope: !12)
diff --git a/toolchain/lower/testdata/basics/numeric_literals.carbon b/toolchain/lower/testdata/basics/numeric_literals.carbon
index a06a1e38aad3..ac9faab62d5e 100644
--- a/toolchain/lower/testdata/basics/numeric_literals.carbon
+++ b/toolchain/lower/testdata/basics/numeric_literals.carbon
@@ -33,7 +33,7 @@ fn F() {
 // CHECK:STDOUT: @array.1.loc19_4 = internal constant [4 x i32] [i32 8, i32 9, i32 8, i32 8]
 // CHECK:STDOUT: @array.2.loc27_4 = internal constant [6 x double] [double 9.000000e-01, double 8.000000e+00, double 8.000000e+01, double 1.000000e+07, double 1.000000e+08, double 1.000000e-08]
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %ints.var = alloca [4 x i32], align 4, !dbg !7
 // CHECK:STDOUT:   %.loc19_3.3.array.index = getelementptr inbounds [4 x i32], ptr %ints.var, i32 0, i32 0, !dbg !8
@@ -67,7 +67,7 @@ fn F() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "numeric_literals.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/basics/type_values.carbon b/toolchain/lower/testdata/basics/type_values.carbon
index e1e826b65aeb..46d753ddb488 100644
--- a/toolchain/lower/testdata/basics/type_values.carbon
+++ b/toolchain/lower/testdata/basics/type_values.carbon
@@ -28,17 +28,17 @@ fn F64() -> type {
 // CHECK:STDOUT:
 // CHECK:STDOUT: %type = type {}
 // CHECK:STDOUT:
-// CHECK:STDOUT: define %type @I32() !dbg !4 {
+// CHECK:STDOUT: define %type @_CI32.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret %type zeroinitializer, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define %type @I48() !dbg !8 {
+// CHECK:STDOUT: define %type @_CI48.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret %type zeroinitializer, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define %type @F64() !dbg !10 {
+// CHECK:STDOUT: define %type @_CF64.Main() !dbg !10 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret %type zeroinitializer, !dbg !11
 // CHECK:STDOUT: }
@@ -50,11 +50,11 @@ fn F64() -> type {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "type_values.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "I32", linkageName: "I32", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "I32", linkageName: "_CI32.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 15, column: 3, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "I48", linkageName: "I48", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "I48", linkageName: "_CI48.Main", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 19, column: 3, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "F64", linkageName: "F64", scope: null, file: !3, line: 22, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "F64", linkageName: "_CF64.Main", scope: null, file: !3, line: 22, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 23, column: 3, scope: !10)
diff --git a/toolchain/lower/testdata/basics/zero.carbon b/toolchain/lower/testdata/basics/zero.carbon
index f30827b032f4..b3bf6e2519af 100644
--- a/toolchain/lower/testdata/basics/zero.carbon
+++ b/toolchain/lower/testdata/basics/zero.carbon
@@ -15,7 +15,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'zero.carbon'
 // CHECK:STDOUT: source_filename = "zero.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Main() !dbg !4 {
+// CHECK:STDOUT: define i32 @_CMain.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i32 0, !dbg !7
 // CHECK:STDOUT: }
@@ -27,7 +27,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "zero.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 3, scope: !4)
diff --git a/toolchain/lower/testdata/builtins/float.carbon b/toolchain/lower/testdata/builtins/float.carbon
index 357ec1dcd1fd..ff3bff56ba69 100644
--- a/toolchain/lower/testdata/builtins/float.carbon
+++ b/toolchain/lower/testdata/builtins/float.carbon
@@ -44,67 +44,67 @@ fn TestGreaterEq(a: f64, b: f64) -> bool { return GreaterEq(a, b); }
 // CHECK:STDOUT: ; ModuleID = 'float.carbon'
 // CHECK:STDOUT: source_filename = "float.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define double @TestNegate(double %a) !dbg !4 {
+// CHECK:STDOUT: define double @_CTestNegate.Main(double %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.negate = fneg double %a, !dbg !7
 // CHECK:STDOUT:   ret double %float.negate, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define double @TestAdd(double %a, double %b) !dbg !9 {
+// CHECK:STDOUT: define double @_CTestAdd.Main(double %a, double %b) !dbg !9 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.add = fadd double %a, %b, !dbg !10
 // CHECK:STDOUT:   ret double %float.add, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define double @TestSub(double %a, double %b) !dbg !12 {
+// CHECK:STDOUT: define double @_CTestSub.Main(double %a, double %b) !dbg !12 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.sub = fsub double %a, %b, !dbg !13
 // CHECK:STDOUT:   ret double %float.sub, !dbg !14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define double @TestMul(double %a, double %b) !dbg !15 {
+// CHECK:STDOUT: define double @_CTestMul.Main(double %a, double %b) !dbg !15 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.mul = fmul double %a, %b, !dbg !16
 // CHECK:STDOUT:   ret double %float.mul, !dbg !17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define double @TestDiv(double %a, double %b) !dbg !18 {
+// CHECK:STDOUT: define double @_CTestDiv.Main(double %a, double %b) !dbg !18 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.div = fdiv double %a, %b, !dbg !19
 // CHECK:STDOUT:   ret double %float.div, !dbg !20
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestEq(double %a, double %b) !dbg !21 {
+// CHECK:STDOUT: define i1 @_CTestEq.Main(double %a, double %b) !dbg !21 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.eq = fcmp oeq double %a, %b, !dbg !22
 // CHECK:STDOUT:   ret i1 %float.eq, !dbg !23
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestNeq(double %a, double %b) !dbg !24 {
+// CHECK:STDOUT: define i1 @_CTestNeq.Main(double %a, double %b) !dbg !24 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.neq = fcmp one double %a, %b, !dbg !25
 // CHECK:STDOUT:   ret i1 %float.neq, !dbg !26
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestLess(double %a, double %b) !dbg !27 {
+// CHECK:STDOUT: define i1 @_CTestLess.Main(double %a, double %b) !dbg !27 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.less = fcmp olt double %a, %b, !dbg !28
 // CHECK:STDOUT:   ret i1 %float.less, !dbg !29
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestLessEq(double %a, double %b) !dbg !30 {
+// CHECK:STDOUT: define i1 @_CTestLessEq.Main(double %a, double %b) !dbg !30 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.less_eq = fcmp ole double %a, %b, !dbg !31
 // CHECK:STDOUT:   ret i1 %float.less_eq, !dbg !32
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestGreater(double %a, double %b) !dbg !33 {
+// CHECK:STDOUT: define i1 @_CTestGreater.Main(double %a, double %b) !dbg !33 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.greater = fcmp ogt double %a, %b, !dbg !34
 // CHECK:STDOUT:   ret i1 %float.greater, !dbg !35
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestGreaterEq(double %a, double %b) !dbg !36 {
+// CHECK:STDOUT: define i1 @_CTestGreaterEq.Main(double %a, double %b) !dbg !36 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %float.greater_eq = fcmp oge double %a, %b, !dbg !37
 // CHECK:STDOUT:   ret i1 %float.greater_eq, !dbg !38
@@ -117,38 +117,38 @@ fn TestGreaterEq(a: f64, b: f64) -> bool { return GreaterEq(a, b); }
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "float.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "TestNegate", linkageName: "TestNegate", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "TestNegate", linkageName: "_CTestNegate.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 39, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 12, column: 32, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "TestAdd", linkageName: "TestAdd", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "TestAdd", linkageName: "_CTestAdd.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 15, column: 44, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 37, scope: !9)
-// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "TestSub", linkageName: "TestSub", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "TestSub", linkageName: "_CTestSub.Main", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !13 = !DILocation(line: 18, column: 44, scope: !12)
 // CHECK:STDOUT: !14 = !DILocation(line: 18, column: 37, scope: !12)
-// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "TestMul", linkageName: "TestMul", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "TestMul", linkageName: "_CTestMul.Main", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !16 = !DILocation(line: 21, column: 44, scope: !15)
 // CHECK:STDOUT: !17 = !DILocation(line: 21, column: 37, scope: !15)
-// CHECK:STDOUT: !18 = distinct !DISubprogram(name: "TestDiv", linkageName: "TestDiv", scope: null, file: !3, line: 24, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !18 = distinct !DISubprogram(name: "TestDiv", linkageName: "_CTestDiv.Main", scope: null, file: !3, line: 24, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !19 = !DILocation(line: 24, column: 44, scope: !18)
 // CHECK:STDOUT: !20 = !DILocation(line: 24, column: 37, scope: !18)
-// CHECK:STDOUT: !21 = distinct !DISubprogram(name: "TestEq", linkageName: "TestEq", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !21 = distinct !DISubprogram(name: "TestEq", linkageName: "_CTestEq.Main", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !22 = !DILocation(line: 27, column: 44, scope: !21)
 // CHECK:STDOUT: !23 = !DILocation(line: 27, column: 37, scope: !21)
-// CHECK:STDOUT: !24 = distinct !DISubprogram(name: "TestNeq", linkageName: "TestNeq", scope: null, file: !3, line: 30, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !24 = distinct !DISubprogram(name: "TestNeq", linkageName: "_CTestNeq.Main", scope: null, file: !3, line: 30, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !25 = !DILocation(line: 30, column: 45, scope: !24)
 // CHECK:STDOUT: !26 = !DILocation(line: 30, column: 38, scope: !24)
-// CHECK:STDOUT: !27 = distinct !DISubprogram(name: "TestLess", linkageName: "TestLess", scope: null, file: !3, line: 33, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !27 = distinct !DISubprogram(name: "TestLess", linkageName: "_CTestLess.Main", scope: null, file: !3, line: 33, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !28 = !DILocation(line: 33, column: 46, scope: !27)
 // CHECK:STDOUT: !29 = !DILocation(line: 33, column: 39, scope: !27)
-// CHECK:STDOUT: !30 = distinct !DISubprogram(name: "TestLessEq", linkageName: "TestLessEq", scope: null, file: !3, line: 36, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !30 = distinct !DISubprogram(name: "TestLessEq", linkageName: "_CTestLessEq.Main", scope: null, file: !3, line: 36, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !31 = !DILocation(line: 36, column: 48, scope: !30)
 // CHECK:STDOUT: !32 = !DILocation(line: 36, column: 41, scope: !30)
-// CHECK:STDOUT: !33 = distinct !DISubprogram(name: "TestGreater", linkageName: "TestGreater", scope: null, file: !3, line: 39, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !33 = distinct !DISubprogram(name: "TestGreater", linkageName: "_CTestGreater.Main", scope: null, file: !3, line: 39, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !34 = !DILocation(line: 39, column: 49, scope: !33)
 // CHECK:STDOUT: !35 = !DILocation(line: 39, column: 42, scope: !33)
-// CHECK:STDOUT: !36 = distinct !DISubprogram(name: "TestGreaterEq", linkageName: "TestGreaterEq", scope: null, file: !3, line: 42, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !36 = distinct !DISubprogram(name: "TestGreaterEq", linkageName: "_CTestGreaterEq.Main", scope: null, file: !3, line: 42, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !37 = !DILocation(line: 42, column: 51, scope: !36)
 // CHECK:STDOUT: !38 = !DILocation(line: 42, column: 44, scope: !36)
diff --git a/toolchain/lower/testdata/builtins/int.carbon b/toolchain/lower/testdata/builtins/int.carbon
index 39063db372f5..51ee33fd7c20 100644
--- a/toolchain/lower/testdata/builtins/int.carbon
+++ b/toolchain/lower/testdata/builtins/int.carbon
@@ -65,109 +65,109 @@ fn TestGreaterEq(a: i32, b: i32) -> bool { return GreaterEq(a, b); }
 // CHECK:STDOUT: ; ModuleID = 'int.carbon'
 // CHECK:STDOUT: source_filename = "int.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestNegate(i32 %a) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CTestNegate.Main(i32 %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.snegate = sub i32 0, %a, !dbg !7
 // CHECK:STDOUT:   ret i32 %int.snegate, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestAdd(i32 %a, i32 %b) !dbg !9 {
+// CHECK:STDOUT: define i32 @_CTestAdd.Main(i32 %a, i32 %b) !dbg !9 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.sadd = add i32 %a, %b, !dbg !10
 // CHECK:STDOUT:   ret i32 %int.sadd, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestSub(i32 %a, i32 %b) !dbg !12 {
+// CHECK:STDOUT: define i32 @_CTestSub.Main(i32 %a, i32 %b) !dbg !12 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.ssub = sub i32 %a, %b, !dbg !13
 // CHECK:STDOUT:   ret i32 %int.ssub, !dbg !14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestMul(i32 %a, i32 %b) !dbg !15 {
+// CHECK:STDOUT: define i32 @_CTestMul.Main(i32 %a, i32 %b) !dbg !15 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.smul = mul i32 %a, %b, !dbg !16
 // CHECK:STDOUT:   ret i32 %int.smul, !dbg !17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestDiv(i32 %a, i32 %b) !dbg !18 {
+// CHECK:STDOUT: define i32 @_CTestDiv.Main(i32 %a, i32 %b) !dbg !18 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.sdiv = sdiv i32 %a, %b, !dbg !19
 // CHECK:STDOUT:   ret i32 %int.sdiv, !dbg !20
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestMod(i32 %a, i32 %b) !dbg !21 {
+// CHECK:STDOUT: define i32 @_CTestMod.Main(i32 %a, i32 %b) !dbg !21 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.smod = srem i32 %a, %b, !dbg !22
 // CHECK:STDOUT:   ret i32 %int.smod, !dbg !23
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestComplement(i32 %a) !dbg !24 {
+// CHECK:STDOUT: define i32 @_CTestComplement.Main(i32 %a) !dbg !24 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.complement = xor i32 -1, %a, !dbg !25
 // CHECK:STDOUT:   ret i32 %int.complement, !dbg !26
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestAnd(i32 %a, i32 %b) !dbg !27 {
+// CHECK:STDOUT: define i32 @_CTestAnd.Main(i32 %a, i32 %b) !dbg !27 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.and = and i32 %a, %b, !dbg !28
 // CHECK:STDOUT:   ret i32 %int.and, !dbg !29
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestOr(i32 %a, i32 %b) !dbg !30 {
+// CHECK:STDOUT: define i32 @_CTestOr.Main(i32 %a, i32 %b) !dbg !30 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.or = or i32 %a, %b, !dbg !31
 // CHECK:STDOUT:   ret i32 %int.or, !dbg !32
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestXor(i32 %a, i32 %b) !dbg !33 {
+// CHECK:STDOUT: define i32 @_CTestXor.Main(i32 %a, i32 %b) !dbg !33 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.xor = xor i32 %a, %b, !dbg !34
 // CHECK:STDOUT:   ret i32 %int.xor, !dbg !35
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestLeftShift(i32 %a, i32 %b) !dbg !36 {
+// CHECK:STDOUT: define i32 @_CTestLeftShift.Main(i32 %a, i32 %b) !dbg !36 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.left_shift = shl i32 %a, %b, !dbg !37
 // CHECK:STDOUT:   ret i32 %int.left_shift, !dbg !38
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestRightShift(i32 %a, i32 %b) !dbg !39 {
+// CHECK:STDOUT: define i32 @_CTestRightShift.Main(i32 %a, i32 %b) !dbg !39 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.right_shift = ashr i32 %a, %b, !dbg !40
 // CHECK:STDOUT:   ret i32 %int.right_shift, !dbg !41
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestEq(i32 %a, i32 %b) !dbg !42 {
+// CHECK:STDOUT: define i1 @_CTestEq.Main(i32 %a, i32 %b) !dbg !42 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.eq = icmp eq i32 %a, %b, !dbg !43
 // CHECK:STDOUT:   ret i1 %int.eq, !dbg !44
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestNeq(i32 %a, i32 %b) !dbg !45 {
+// CHECK:STDOUT: define i1 @_CTestNeq.Main(i32 %a, i32 %b) !dbg !45 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.neq = icmp ne i32 %a, %b, !dbg !46
 // CHECK:STDOUT:   ret i1 %int.neq, !dbg !47
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestLess(i32 %a, i32 %b) !dbg !48 {
+// CHECK:STDOUT: define i1 @_CTestLess.Main(i32 %a, i32 %b) !dbg !48 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.less = icmp slt i32 %a, %b, !dbg !49
 // CHECK:STDOUT:   ret i1 %int.less, !dbg !50
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestLessEq(i32 %a, i32 %b) !dbg !51 {
+// CHECK:STDOUT: define i1 @_CTestLessEq.Main(i32 %a, i32 %b) !dbg !51 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.less_eq = icmp sle i32 %a, %b, !dbg !52
 // CHECK:STDOUT:   ret i1 %int.less_eq, !dbg !53
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestGreater(i32 %a, i32 %b) !dbg !54 {
+// CHECK:STDOUT: define i1 @_CTestGreater.Main(i32 %a, i32 %b) !dbg !54 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.greater = icmp sgt i32 %a, %b, !dbg !55
 // CHECK:STDOUT:   ret i1 %int.greater, !dbg !56
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestGreaterEq(i32 %a, i32 %b) !dbg !57 {
+// CHECK:STDOUT: define i1 @_CTestGreaterEq.Main(i32 %a, i32 %b) !dbg !57 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.greater_eq = icmp sge i32 %a, %b, !dbg !58
 // CHECK:STDOUT:   ret i1 %int.greater_eq, !dbg !59
@@ -180,59 +180,59 @@ fn TestGreaterEq(a: i32, b: i32) -> bool { return GreaterEq(a, b); }
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "int.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "TestNegate", linkageName: "TestNegate", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "TestNegate", linkageName: "_CTestNegate.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 39, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 12, column: 32, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "TestAdd", linkageName: "TestAdd", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "TestAdd", linkageName: "_CTestAdd.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 15, column: 44, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 37, scope: !9)
-// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "TestSub", linkageName: "TestSub", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "TestSub", linkageName: "_CTestSub.Main", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !13 = !DILocation(line: 18, column: 44, scope: !12)
 // CHECK:STDOUT: !14 = !DILocation(line: 18, column: 37, scope: !12)
-// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "TestMul", linkageName: "TestMul", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "TestMul", linkageName: "_CTestMul.Main", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !16 = !DILocation(line: 21, column: 44, scope: !15)
 // CHECK:STDOUT: !17 = !DILocation(line: 21, column: 37, scope: !15)
-// CHECK:STDOUT: !18 = distinct !DISubprogram(name: "TestDiv", linkageName: "TestDiv", scope: null, file: !3, line: 24, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !18 = distinct !DISubprogram(name: "TestDiv", linkageName: "_CTestDiv.Main", scope: null, file: !3, line: 24, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !19 = !DILocation(line: 24, column: 44, scope: !18)
 // CHECK:STDOUT: !20 = !DILocation(line: 24, column: 37, scope: !18)
-// CHECK:STDOUT: !21 = distinct !DISubprogram(name: "TestMod", linkageName: "TestMod", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !21 = distinct !DISubprogram(name: "TestMod", linkageName: "_CTestMod.Main", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !22 = !DILocation(line: 27, column: 44, scope: !21)
 // CHECK:STDOUT: !23 = !DILocation(line: 27, column: 37, scope: !21)
-// CHECK:STDOUT: !24 = distinct !DISubprogram(name: "TestComplement", linkageName: "TestComplement", scope: null, file: !3, line: 30, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !24 = distinct !DISubprogram(name: "TestComplement", linkageName: "_CTestComplement.Main", scope: null, file: !3, line: 30, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !25 = !DILocation(line: 30, column: 43, scope: !24)
 // CHECK:STDOUT: !26 = !DILocation(line: 30, column: 36, scope: !24)
-// CHECK:STDOUT: !27 = distinct !DISubprogram(name: "TestAnd", linkageName: "TestAnd", scope: null, file: !3, line: 33, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !27 = distinct !DISubprogram(name: "TestAnd", linkageName: "_CTestAnd.Main", scope: null, file: !3, line: 33, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !28 = !DILocation(line: 33, column: 44, scope: !27)
 // CHECK:STDOUT: !29 = !DILocation(line: 33, column: 37, scope: !27)
-// CHECK:STDOUT: !30 = distinct !DISubprogram(name: "TestOr", linkageName: "TestOr", scope: null, file: !3, line: 36, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !30 = distinct !DISubprogram(name: "TestOr", linkageName: "_CTestOr.Main", scope: null, file: !3, line: 36, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !31 = !DILocation(line: 36, column: 43, scope: !30)
 // CHECK:STDOUT: !32 = !DILocation(line: 36, column: 36, scope: !30)
-// CHECK:STDOUT: !33 = distinct !DISubprogram(name: "TestXor", linkageName: "TestXor", scope: null, file: !3, line: 39, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !33 = distinct !DISubprogram(name: "TestXor", linkageName: "_CTestXor.Main", scope: null, file: !3, line: 39, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !34 = !DILocation(line: 39, column: 44, scope: !33)
 // CHECK:STDOUT: !35 = !DILocation(line: 39, column: 37, scope: !33)
-// CHECK:STDOUT: !36 = distinct !DISubprogram(name: "TestLeftShift", linkageName: "TestLeftShift", scope: null, file: !3, line: 42, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !36 = distinct !DISubprogram(name: "TestLeftShift", linkageName: "_CTestLeftShift.Main", scope: null, file: !3, line: 42, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !37 = !DILocation(line: 42, column: 50, scope: !36)
 // CHECK:STDOUT: !38 = !DILocation(line: 42, column: 43, scope: !36)
-// CHECK:STDOUT: !39 = distinct !DISubprogram(name: "TestRightShift", linkageName: "TestRightShift", scope: null, file: !3, line: 45, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !39 = distinct !DISubprogram(name: "TestRightShift", linkageName: "_CTestRightShift.Main", scope: null, file: !3, line: 45, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !40 = !DILocation(line: 45, column: 51, scope: !39)
 // CHECK:STDOUT: !41 = !DILocation(line: 45, column: 44, scope: !39)
-// CHECK:STDOUT: !42 = distinct !DISubprogram(name: "TestEq", linkageName: "TestEq", scope: null, file: !3, line: 48, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !42 = distinct !DISubprogram(name: "TestEq", linkageName: "_CTestEq.Main", scope: null, file: !3, line: 48, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !43 = !DILocation(line: 48, column: 44, scope: !42)
 // CHECK:STDOUT: !44 = !DILocation(line: 48, column: 37, scope: !42)
-// CHECK:STDOUT: !45 = distinct !DISubprogram(name: "TestNeq", linkageName: "TestNeq", scope: null, file: !3, line: 51, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !45 = distinct !DISubprogram(name: "TestNeq", linkageName: "_CTestNeq.Main", scope: null, file: !3, line: 51, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !46 = !DILocation(line: 51, column: 45, scope: !45)
 // CHECK:STDOUT: !47 = !DILocation(line: 51, column: 38, scope: !45)
-// CHECK:STDOUT: !48 = distinct !DISubprogram(name: "TestLess", linkageName: "TestLess", scope: null, file: !3, line: 54, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !48 = distinct !DISubprogram(name: "TestLess", linkageName: "_CTestLess.Main", scope: null, file: !3, line: 54, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !49 = !DILocation(line: 54, column: 46, scope: !48)
 // CHECK:STDOUT: !50 = !DILocation(line: 54, column: 39, scope: !48)
-// CHECK:STDOUT: !51 = distinct !DISubprogram(name: "TestLessEq", linkageName: "TestLessEq", scope: null, file: !3, line: 57, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !51 = distinct !DISubprogram(name: "TestLessEq", linkageName: "_CTestLessEq.Main", scope: null, file: !3, line: 57, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !52 = !DILocation(line: 57, column: 48, scope: !51)
 // CHECK:STDOUT: !53 = !DILocation(line: 57, column: 41, scope: !51)
-// CHECK:STDOUT: !54 = distinct !DISubprogram(name: "TestGreater", linkageName: "TestGreater", scope: null, file: !3, line: 60, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !54 = distinct !DISubprogram(name: "TestGreater", linkageName: "_CTestGreater.Main", scope: null, file: !3, line: 60, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !55 = !DILocation(line: 60, column: 49, scope: !54)
 // CHECK:STDOUT: !56 = !DILocation(line: 60, column: 42, scope: !54)
-// CHECK:STDOUT: !57 = distinct !DISubprogram(name: "TestGreaterEq", linkageName: "TestGreaterEq", scope: null, file: !3, line: 63, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !57 = distinct !DISubprogram(name: "TestGreaterEq", linkageName: "_CTestGreaterEq.Main", scope: null, file: !3, line: 63, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !58 = !DILocation(line: 63, column: 51, scope: !57)
 // CHECK:STDOUT: !59 = !DILocation(line: 63, column: 44, scope: !57)
diff --git a/toolchain/lower/testdata/builtins/method_vs_nonmethod.carbon b/toolchain/lower/testdata/builtins/method_vs_nonmethod.carbon
index 5cefc7af7d5d..c759e6557a38 100644
--- a/toolchain/lower/testdata/builtins/method_vs_nonmethod.carbon
+++ b/toolchain/lower/testdata/builtins/method_vs_nonmethod.carbon
@@ -17,13 +17,13 @@ fn TestAddMethod(a: i32, b: i32) -> i32 { return a.(AddMethod)(b); }
 // CHECK:STDOUT: ; ModuleID = 'method_vs_nonmethod.carbon'
 // CHECK:STDOUT: source_filename = "method_vs_nonmethod.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestAddNonmethod(i32 %a, i32 %b) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CTestAddNonmethod.Main(i32 %a, i32 %b) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.sadd = add i32 %a, %b, !dbg !7
 // CHECK:STDOUT:   ret i32 %int.sadd, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TestAddMethod(i32 %a, i32 %b) !dbg !9 {
+// CHECK:STDOUT: define i32 @_CTestAddMethod.Main(i32 %a, i32 %b) !dbg !9 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.sadd = add i32 %a, %b, !dbg !10
 // CHECK:STDOUT:   ret i32 %int.sadd, !dbg !11
@@ -36,11 +36,11 @@ fn TestAddMethod(a: i32, b: i32) -> i32 { return a.(AddMethod)(b); }
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "method_vs_nonmethod.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "TestAddNonmethod", linkageName: "TestAddNonmethod", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "TestAddNonmethod", linkageName: "_CTestAddNonmethod.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 53, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 14, column: 46, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "TestAddMethod", linkageName: "TestAddMethod", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "TestAddMethod", linkageName: "_CTestAddMethod.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 15, column: 50, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 43, scope: !9)
diff --git a/toolchain/lower/testdata/builtins/overloaded_operator.carbon b/toolchain/lower/testdata/builtins/overloaded_operator.carbon
index ff5b82a41599..3b9064f1ea49 100644
--- a/toolchain/lower/testdata/builtins/overloaded_operator.carbon
+++ b/toolchain/lower/testdata/builtins/overloaded_operator.carbon
@@ -21,7 +21,7 @@ fn AddThreeIntegers(a: i32, b: i32, c: i32) -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'overloaded_operator.carbon'
 // CHECK:STDOUT: source_filename = "overloaded_operator.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @AddThreeIntegers(i32 %a, i32 %b, i32 %c) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CAddThreeIntegers.Main(i32 %a, i32 %b, i32 %c) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.sadd.loc18_12 = add i32 %a, %b, !dbg !7
 // CHECK:STDOUT:   %.loc18_12.3.temp = alloca i32, align 4, !dbg !7
@@ -38,7 +38,7 @@ fn AddThreeIntegers(a: i32, b: i32, c: i32) -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "overloaded_operator.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "AddThreeIntegers", linkageName: "AddThreeIntegers", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "AddThreeIntegers", linkageName: "_CAddThreeIntegers.Main", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 18, column: 10, scope: !4)
diff --git a/toolchain/lower/testdata/builtins/print.carbon b/toolchain/lower/testdata/builtins/print.carbon
index 577cef9d00a3..b3666bc72949 100644
--- a/toolchain/lower/testdata/builtins/print.carbon
+++ b/toolchain/lower/testdata/builtins/print.carbon
@@ -17,7 +17,7 @@ fn Main() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @printf.int.format = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !4 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %print.int.printf = call i32 (ptr, ...) @printf(ptr @printf.int.format, i32 1), !dbg !7
 // CHECK:STDOUT:   ret void, !dbg !8
@@ -32,7 +32,7 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "print.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 3, scope: !4)
diff --git a/toolchain/lower/testdata/builtins/types.carbon b/toolchain/lower/testdata/builtins/types.carbon
index dc84466143c3..25b716c6c55a 100644
--- a/toolchain/lower/testdata/builtins/types.carbon
+++ b/toolchain/lower/testdata/builtins/types.carbon
@@ -21,7 +21,7 @@ fn F() {
 // CHECK:STDOUT: ; ModuleID = 'types.carbon'
 // CHECK:STDOUT: source_filename = "types.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %i.var = alloca i32, align 4, !dbg !7
 // CHECK:STDOUT:   store i32 0, ptr %i.var, align 4, !dbg !8
@@ -39,7 +39,7 @@ fn F() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "types.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 16, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/builtins/uint.carbon b/toolchain/lower/testdata/builtins/uint.carbon
index 275f433bbafc..25c13a04d162 100644
--- a/toolchain/lower/testdata/builtins/uint.carbon
+++ b/toolchain/lower/testdata/builtins/uint.carbon
@@ -65,109 +65,109 @@ fn TestGreaterEq(a: u64, b: u64) -> bool { return GreaterEq(a, b); }
 // CHECK:STDOUT: ; ModuleID = 'uint.carbon'
 // CHECK:STDOUT: source_filename = "uint.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestNegate(i64 %a) !dbg !4 {
+// CHECK:STDOUT: define i64 @_CTestNegate.Main(i64 %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.unegate = sub i64 0, %a, !dbg !7
 // CHECK:STDOUT:   ret i64 %int.unegate, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestAdd(i64 %a, i64 %b) !dbg !9 {
+// CHECK:STDOUT: define i64 @_CTestAdd.Main(i64 %a, i64 %b) !dbg !9 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.uadd = add i64 %a, %b, !dbg !10
 // CHECK:STDOUT:   ret i64 %int.uadd, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestSub(i64 %a, i64 %b) !dbg !12 {
+// CHECK:STDOUT: define i64 @_CTestSub.Main(i64 %a, i64 %b) !dbg !12 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.usub = sub i64 %a, %b, !dbg !13
 // CHECK:STDOUT:   ret i64 %int.usub, !dbg !14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestMul(i64 %a, i64 %b) !dbg !15 {
+// CHECK:STDOUT: define i64 @_CTestMul.Main(i64 %a, i64 %b) !dbg !15 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.umul = mul i64 %a, %b, !dbg !16
 // CHECK:STDOUT:   ret i64 %int.umul, !dbg !17
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestDiv(i64 %a, i64 %b) !dbg !18 {
+// CHECK:STDOUT: define i64 @_CTestDiv.Main(i64 %a, i64 %b) !dbg !18 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.udiv = udiv i64 %a, %b, !dbg !19
 // CHECK:STDOUT:   ret i64 %int.udiv, !dbg !20
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestMod(i64 %a, i64 %b) !dbg !21 {
+// CHECK:STDOUT: define i64 @_CTestMod.Main(i64 %a, i64 %b) !dbg !21 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.umod = urem i64 %a, %b, !dbg !22
 // CHECK:STDOUT:   ret i64 %int.umod, !dbg !23
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestComplement(i64 %a) !dbg !24 {
+// CHECK:STDOUT: define i64 @_CTestComplement.Main(i64 %a) !dbg !24 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.complement = xor i64 -1, %a, !dbg !25
 // CHECK:STDOUT:   ret i64 %int.complement, !dbg !26
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestAnd(i64 %a, i64 %b) !dbg !27 {
+// CHECK:STDOUT: define i64 @_CTestAnd.Main(i64 %a, i64 %b) !dbg !27 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.and = and i64 %a, %b, !dbg !28
 // CHECK:STDOUT:   ret i64 %int.and, !dbg !29
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestOr(i64 %a, i64 %b) !dbg !30 {
+// CHECK:STDOUT: define i64 @_CTestOr.Main(i64 %a, i64 %b) !dbg !30 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.or = or i64 %a, %b, !dbg !31
 // CHECK:STDOUT:   ret i64 %int.or, !dbg !32
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestXor(i64 %a, i64 %b) !dbg !33 {
+// CHECK:STDOUT: define i64 @_CTestXor.Main(i64 %a, i64 %b) !dbg !33 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.xor = xor i64 %a, %b, !dbg !34
 // CHECK:STDOUT:   ret i64 %int.xor, !dbg !35
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestLeftShift(i64 %a, i64 %b) !dbg !36 {
+// CHECK:STDOUT: define i64 @_CTestLeftShift.Main(i64 %a, i64 %b) !dbg !36 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.left_shift = shl i64 %a, %b, !dbg !37
 // CHECK:STDOUT:   ret i64 %int.left_shift, !dbg !38
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i64 @TestRightShift(i64 %a, i64 %b) !dbg !39 {
+// CHECK:STDOUT: define i64 @_CTestRightShift.Main(i64 %a, i64 %b) !dbg !39 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.right_shift = lshr i64 %a, %b, !dbg !40
 // CHECK:STDOUT:   ret i64 %int.right_shift, !dbg !41
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestEq(i64 %a, i64 %b) !dbg !42 {
+// CHECK:STDOUT: define i1 @_CTestEq.Main(i64 %a, i64 %b) !dbg !42 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.eq = icmp eq i64 %a, %b, !dbg !43
 // CHECK:STDOUT:   ret i1 %int.eq, !dbg !44
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestNeq(i64 %a, i64 %b) !dbg !45 {
+// CHECK:STDOUT: define i1 @_CTestNeq.Main(i64 %a, i64 %b) !dbg !45 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.neq = icmp ne i64 %a, %b, !dbg !46
 // CHECK:STDOUT:   ret i1 %int.neq, !dbg !47
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestLess(i64 %a, i64 %b) !dbg !48 {
+// CHECK:STDOUT: define i1 @_CTestLess.Main(i64 %a, i64 %b) !dbg !48 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.less = icmp ult i64 %a, %b, !dbg !49
 // CHECK:STDOUT:   ret i1 %int.less, !dbg !50
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestLessEq(i64 %a, i64 %b) !dbg !51 {
+// CHECK:STDOUT: define i1 @_CTestLessEq.Main(i64 %a, i64 %b) !dbg !51 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.less_eq = icmp ule i64 %a, %b, !dbg !52
 // CHECK:STDOUT:   ret i1 %int.less_eq, !dbg !53
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestGreater(i64 %a, i64 %b) !dbg !54 {
+// CHECK:STDOUT: define i1 @_CTestGreater.Main(i64 %a, i64 %b) !dbg !54 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.greater = icmp ugt i64 %a, %b, !dbg !55
 // CHECK:STDOUT:   ret i1 %int.greater, !dbg !56
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @TestGreaterEq(i64 %a, i64 %b) !dbg !57 {
+// CHECK:STDOUT: define i1 @_CTestGreaterEq.Main(i64 %a, i64 %b) !dbg !57 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %int.greater_eq = icmp uge i64 %a, %b, !dbg !58
 // CHECK:STDOUT:   ret i1 %int.greater_eq, !dbg !59
@@ -180,59 +180,59 @@ fn TestGreaterEq(a: u64, b: u64) -> bool { return GreaterEq(a, b); }
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "uint.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "TestNegate", linkageName: "TestNegate", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "TestNegate", linkageName: "_CTestNegate.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 39, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 12, column: 32, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "TestAdd", linkageName: "TestAdd", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "TestAdd", linkageName: "_CTestAdd.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 15, column: 44, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 37, scope: !9)
-// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "TestSub", linkageName: "TestSub", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "TestSub", linkageName: "_CTestSub.Main", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !13 = !DILocation(line: 18, column: 44, scope: !12)
 // CHECK:STDOUT: !14 = !DILocation(line: 18, column: 37, scope: !12)
-// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "TestMul", linkageName: "TestMul", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "TestMul", linkageName: "_CTestMul.Main", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !16 = !DILocation(line: 21, column: 44, scope: !15)
 // CHECK:STDOUT: !17 = !DILocation(line: 21, column: 37, scope: !15)
-// CHECK:STDOUT: !18 = distinct !DISubprogram(name: "TestDiv", linkageName: "TestDiv", scope: null, file: !3, line: 24, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !18 = distinct !DISubprogram(name: "TestDiv", linkageName: "_CTestDiv.Main", scope: null, file: !3, line: 24, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !19 = !DILocation(line: 24, column: 44, scope: !18)
 // CHECK:STDOUT: !20 = !DILocation(line: 24, column: 37, scope: !18)
-// CHECK:STDOUT: !21 = distinct !DISubprogram(name: "TestMod", linkageName: "TestMod", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !21 = distinct !DISubprogram(name: "TestMod", linkageName: "_CTestMod.Main", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !22 = !DILocation(line: 27, column: 44, scope: !21)
 // CHECK:STDOUT: !23 = !DILocation(line: 27, column: 37, scope: !21)
-// CHECK:STDOUT: !24 = distinct !DISubprogram(name: "TestComplement", linkageName: "TestComplement", scope: null, file: !3, line: 30, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !24 = distinct !DISubprogram(name: "TestComplement", linkageName: "_CTestComplement.Main", scope: null, file: !3, line: 30, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !25 = !DILocation(line: 30, column: 43, scope: !24)
 // CHECK:STDOUT: !26 = !DILocation(line: 30, column: 36, scope: !24)
-// CHECK:STDOUT: !27 = distinct !DISubprogram(name: "TestAnd", linkageName: "TestAnd", scope: null, file: !3, line: 33, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !27 = distinct !DISubprogram(name: "TestAnd", linkageName: "_CTestAnd.Main", scope: null, file: !3, line: 33, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !28 = !DILocation(line: 33, column: 44, scope: !27)
 // CHECK:STDOUT: !29 = !DILocation(line: 33, column: 37, scope: !27)
-// CHECK:STDOUT: !30 = distinct !DISubprogram(name: "TestOr", linkageName: "TestOr", scope: null, file: !3, line: 36, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !30 = distinct !DISubprogram(name: "TestOr", linkageName: "_CTestOr.Main", scope: null, file: !3, line: 36, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !31 = !DILocation(line: 36, column: 43, scope: !30)
 // CHECK:STDOUT: !32 = !DILocation(line: 36, column: 36, scope: !30)
-// CHECK:STDOUT: !33 = distinct !DISubprogram(name: "TestXor", linkageName: "TestXor", scope: null, file: !3, line: 39, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !33 = distinct !DISubprogram(name: "TestXor", linkageName: "_CTestXor.Main", scope: null, file: !3, line: 39, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !34 = !DILocation(line: 39, column: 44, scope: !33)
 // CHECK:STDOUT: !35 = !DILocation(line: 39, column: 37, scope: !33)
-// CHECK:STDOUT: !36 = distinct !DISubprogram(name: "TestLeftShift", linkageName: "TestLeftShift", scope: null, file: !3, line: 42, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !36 = distinct !DISubprogram(name: "TestLeftShift", linkageName: "_CTestLeftShift.Main", scope: null, file: !3, line: 42, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !37 = !DILocation(line: 42, column: 50, scope: !36)
 // CHECK:STDOUT: !38 = !DILocation(line: 42, column: 43, scope: !36)
-// CHECK:STDOUT: !39 = distinct !DISubprogram(name: "TestRightShift", linkageName: "TestRightShift", scope: null, file: !3, line: 45, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !39 = distinct !DISubprogram(name: "TestRightShift", linkageName: "_CTestRightShift.Main", scope: null, file: !3, line: 45, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !40 = !DILocation(line: 45, column: 51, scope: !39)
 // CHECK:STDOUT: !41 = !DILocation(line: 45, column: 44, scope: !39)
-// CHECK:STDOUT: !42 = distinct !DISubprogram(name: "TestEq", linkageName: "TestEq", scope: null, file: !3, line: 48, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !42 = distinct !DISubprogram(name: "TestEq", linkageName: "_CTestEq.Main", scope: null, file: !3, line: 48, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !43 = !DILocation(line: 48, column: 44, scope: !42)
 // CHECK:STDOUT: !44 = !DILocation(line: 48, column: 37, scope: !42)
-// CHECK:STDOUT: !45 = distinct !DISubprogram(name: "TestNeq", linkageName: "TestNeq", scope: null, file: !3, line: 51, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !45 = distinct !DISubprogram(name: "TestNeq", linkageName: "_CTestNeq.Main", scope: null, file: !3, line: 51, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !46 = !DILocation(line: 51, column: 45, scope: !45)
 // CHECK:STDOUT: !47 = !DILocation(line: 51, column: 38, scope: !45)
-// CHECK:STDOUT: !48 = distinct !DISubprogram(name: "TestLess", linkageName: "TestLess", scope: null, file: !3, line: 54, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !48 = distinct !DISubprogram(name: "TestLess", linkageName: "_CTestLess.Main", scope: null, file: !3, line: 54, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !49 = !DILocation(line: 54, column: 46, scope: !48)
 // CHECK:STDOUT: !50 = !DILocation(line: 54, column: 39, scope: !48)
-// CHECK:STDOUT: !51 = distinct !DISubprogram(name: "TestLessEq", linkageName: "TestLessEq", scope: null, file: !3, line: 57, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !51 = distinct !DISubprogram(name: "TestLessEq", linkageName: "_CTestLessEq.Main", scope: null, file: !3, line: 57, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !52 = !DILocation(line: 57, column: 48, scope: !51)
 // CHECK:STDOUT: !53 = !DILocation(line: 57, column: 41, scope: !51)
-// CHECK:STDOUT: !54 = distinct !DISubprogram(name: "TestGreater", linkageName: "TestGreater", scope: null, file: !3, line: 60, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !54 = distinct !DISubprogram(name: "TestGreater", linkageName: "_CTestGreater.Main", scope: null, file: !3, line: 60, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !55 = !DILocation(line: 60, column: 49, scope: !54)
 // CHECK:STDOUT: !56 = !DILocation(line: 60, column: 42, scope: !54)
-// CHECK:STDOUT: !57 = distinct !DISubprogram(name: "TestGreaterEq", linkageName: "TestGreaterEq", scope: null, file: !3, line: 63, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !57 = distinct !DISubprogram(name: "TestGreaterEq", linkageName: "_CTestGreaterEq.Main", scope: null, file: !3, line: 63, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !58 = !DILocation(line: 63, column: 51, scope: !57)
 // CHECK:STDOUT: !59 = !DILocation(line: 63, column: 44, scope: !57)
diff --git a/toolchain/lower/testdata/class/adapt.carbon b/toolchain/lower/testdata/class/adapt.carbon
index 26e74311b3be..b51a4a961d39 100644
--- a/toolchain/lower/testdata/class/adapt.carbon
+++ b/toolchain/lower/testdata/class/adapt.carbon
@@ -56,7 +56,7 @@ fn DoStuff(a: Int) -> Int {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @struct.loc9_28 = internal constant { i32, i32 } { i32 1, i32 2 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Make(ptr sret({ i32, i32 }) %return) !dbg !4 {
+// CHECK:STDOUT: define void @_CMake.PairOfInts.Main(ptr sret({ i32, i32 }) %return) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc9_27.2.a = getelementptr inbounds nuw { i32, i32 }, ptr %return, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc9_27.4.b = getelementptr inbounds nuw { i32, i32 }, ptr %return, i32 0, i32 1, !dbg !7
@@ -64,24 +64,24 @@ fn DoStuff(a: Int) -> Int {
 // CHECK:STDOUT:   ret void, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Make.1(ptr sret({ i32, i32 }) %return) !dbg !9 {
+// CHECK:STDOUT: define void @_CMake.PairAdapter.Main(ptr sret({ i32, i32 }) %return) !dbg !9 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @Make(ptr %return), !dbg !10
+// CHECK:STDOUT:   call void @_CMake.PairOfInts.Main(ptr %return), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @GetB(ptr %self) !dbg !12 {
+// CHECK:STDOUT: define i32 @_CGetB.PairAdapter.Main(ptr %self) !dbg !12 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc22_14.1.b = getelementptr inbounds nuw { i32, i32 }, ptr %self, i32 0, i32 1, !dbg !13
 // CHECK:STDOUT:   %.loc22_14.2 = load i32, ptr %.loc22_14.1.b, align 4, !dbg !13
 // CHECK:STDOUT:   ret i32 %.loc22_14.2, !dbg !14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Use() !dbg !15 {
+// CHECK:STDOUT: define i32 @_CUse.Main() !dbg !15 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %pa.var = alloca { i32, i32 }, align 8, !dbg !16
-// CHECK:STDOUT:   call void @Make.1(ptr %pa.var), !dbg !17
-// CHECK:STDOUT:   %GetB.call = call i32 @GetB(ptr %pa.var), !dbg !18
+// CHECK:STDOUT:   call void @_CMake.PairAdapter.Main(ptr %pa.var), !dbg !17
+// CHECK:STDOUT:   %GetB.call = call i32 @_CGetB.PairAdapter.Main(ptr %pa.var), !dbg !18
 // CHECK:STDOUT:   ret i32 %GetB.call, !dbg !19
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -97,18 +97,18 @@ fn DoStuff(a: Int) -> Int {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "adapt_class.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Make", linkageName: "Make", scope: null, file: !3, line: 8, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Make", linkageName: "_CMake.PairOfInts.Main", scope: null, file: !3, line: 8, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 9, column: 12, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 9, column: 5, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "Make", linkageName: "Make.1", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "Make", linkageName: "_CMake.PairAdapter.Main", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 17, column: 12, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 17, column: 5, scope: !9)
-// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "GetB", linkageName: "GetB", scope: null, file: !3, line: 20, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "GetB", linkageName: "_CGetB.PairAdapter.Main", scope: null, file: !3, line: 20, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !13 = !DILocation(line: 22, column: 12, scope: !12)
 // CHECK:STDOUT: !14 = !DILocation(line: 22, column: 5, scope: !12)
-// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "Use", linkageName: "Use", scope: null, file: !3, line: 26, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "Use", linkageName: "_CUse.Main", scope: null, file: !3, line: 26, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !16 = !DILocation(line: 27, column: 7, scope: !15)
 // CHECK:STDOUT: !17 = !DILocation(line: 27, column: 25, scope: !15)
 // CHECK:STDOUT: !18 = !DILocation(line: 28, column: 10, scope: !15)
@@ -116,7 +116,7 @@ fn DoStuff(a: Int) -> Int {
 // CHECK:STDOUT: ; ModuleID = 'adapt_int.carbon'
 // CHECK:STDOUT: source_filename = "adapt_int.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @DoStuff(i32 %a) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CDoStuff.Main(i32 %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i32 %a, !dbg !7
 // CHECK:STDOUT: }
@@ -128,7 +128,7 @@ fn DoStuff(a: Int) -> Int {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "adapt_int.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "DoStuff", linkageName: "DoStuff", scope: null, file: !3, line: 8, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "DoStuff", linkageName: "_CDoStuff.Main", scope: null, file: !3, line: 8, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 9, column: 3, scope: !4)
diff --git a/toolchain/lower/testdata/class/base.carbon b/toolchain/lower/testdata/class/base.carbon
index a5f7d4d38d91..b992df020138 100644
--- a/toolchain/lower/testdata/class/base.carbon
+++ b/toolchain/lower/testdata/class/base.carbon
@@ -35,7 +35,7 @@ fn Convert(p: Derived*) -> Base* {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @struct.2.loc22_36 = internal constant { { i32 }, i32 } { { i32 } { i32 4 }, i32 7 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Make(ptr sret({ { i32 }, i32 }) %return) !dbg !4 {
+// CHECK:STDOUT: define void @_CMake.Main(ptr sret({ { i32 }, i32 }) %return) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc22_35.2.base = getelementptr inbounds nuw { { i32 }, i32 }, ptr %return, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc22_26.2.b = getelementptr inbounds nuw { i32 }, ptr %.loc22_35.2.base, i32 0, i32 0, !dbg !8
@@ -44,7 +44,7 @@ fn Convert(p: Derived*) -> Base* {
 // CHECK:STDOUT:   ret void, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Access(ptr sret({ i32, i32 }) %return, ptr %d) !dbg !10 {
+// CHECK:STDOUT: define void @_CAccess.Main(ptr sret({ i32, i32 }) %return, ptr %d) !dbg !10 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc26_12.1.d = getelementptr inbounds nuw { { i32 }, i32 }, ptr %d, i32 0, i32 1, !dbg !11
 // CHECK:STDOUT:   %.loc26_12.2 = load i32, ptr %.loc26_12.1.d, align 4, !dbg !11
@@ -58,7 +58,7 @@ fn Convert(p: Derived*) -> Base* {
 // CHECK:STDOUT:   ret void, !dbg !14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define ptr @Convert(ptr %p) !dbg !15 {
+// CHECK:STDOUT: define ptr @_CConvert.Main(ptr %p) !dbg !15 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc30_11.2.base = getelementptr inbounds nuw { { i32 }, i32 }, ptr %p, i32 0, i32 0, !dbg !16
 // CHECK:STDOUT:   ret ptr %.loc30_11.2.base, !dbg !16
@@ -76,16 +76,16 @@ fn Convert(p: Derived*) -> Base* {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "base.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Make", linkageName: "Make", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Make", linkageName: "_CMake.Main", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 22, column: 10, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 22, column: 19, scope: !4)
 // CHECK:STDOUT: !9 = !DILocation(line: 22, column: 3, scope: !4)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Access", linkageName: "Access", scope: null, file: !3, line: 25, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Access", linkageName: "_CAccess.Main", scope: null, file: !3, line: 25, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 26, column: 11, scope: !10)
 // CHECK:STDOUT: !12 = !DILocation(line: 26, column: 16, scope: !10)
 // CHECK:STDOUT: !13 = !DILocation(line: 26, column: 10, scope: !10)
 // CHECK:STDOUT: !14 = !DILocation(line: 26, column: 3, scope: !10)
-// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "Convert", linkageName: "Convert", scope: null, file: !3, line: 29, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !15 = distinct !DISubprogram(name: "Convert", linkageName: "_CConvert.Main", scope: null, file: !3, line: 29, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !16 = !DILocation(line: 30, column: 3, scope: !15)
diff --git a/toolchain/lower/testdata/class/basic.carbon b/toolchain/lower/testdata/class/basic.carbon
index f3e3b0f6b0a5..97a1858eecb7 100644
--- a/toolchain/lower/testdata/class/basic.carbon
+++ b/toolchain/lower/testdata/class/basic.carbon
@@ -23,13 +23,13 @@ fn Run() {
 // CHECK:STDOUT: ; ModuleID = 'basic.carbon'
 // CHECK:STDOUT: source_filename = "basic.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @F(ptr sret({ i32, ptr }), ptr)
+// CHECK:STDOUT: declare void @_CF.Main(ptr sret({ i32, ptr }), ptr)
 // CHECK:STDOUT:
 // CHECK:STDOUT: define void @main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %c.var = alloca { i32, ptr }, align 8, !dbg !7
 // CHECK:STDOUT:   %d.var = alloca { i32, ptr }, align 8, !dbg !8
-// CHECK:STDOUT:   call void @F(ptr %d.var, ptr %c.var), !dbg !9
+// CHECK:STDOUT:   call void @_CF.Main(ptr %d.var, ptr %c.var), !dbg !9
 // CHECK:STDOUT:   ret void, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
diff --git a/toolchain/lower/testdata/class/field.carbon b/toolchain/lower/testdata/class/field.carbon
index 670cdfc593ad..720f401147fa 100644
--- a/toolchain/lower/testdata/class/field.carbon
+++ b/toolchain/lower/testdata/class/field.carbon
@@ -27,7 +27,7 @@ fn Run() -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'field.carbon'
 // CHECK:STDOUT: source_filename = "field.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F(ptr %c) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CF.Main(ptr %c) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc17_13.1.b = getelementptr inbounds nuw { i32, ptr }, ptr %c, i32 0, i32 1, !dbg !7
 // CHECK:STDOUT:   %.loc17_13.2 = load ptr, ptr %.loc17_13.1.b, align 8, !dbg !7
@@ -43,7 +43,7 @@ fn Run() -> i32 {
 // CHECK:STDOUT:   store i32 1, ptr %.loc22_4.a, align 4, !dbg !12
 // CHECK:STDOUT:   %.loc23_4.b = getelementptr inbounds nuw { i32, ptr }, ptr %c.var, i32 0, i32 1, !dbg !13
 // CHECK:STDOUT:   store ptr %c.var, ptr %.loc23_4.b, align 8, !dbg !13
-// CHECK:STDOUT:   %F.call = call i32 @F(ptr %c.var), !dbg !14
+// CHECK:STDOUT:   %F.call = call i32 @_CF.Main(ptr %c.var), !dbg !14
 // CHECK:STDOUT:   ret i32 %F.call, !dbg !15
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -54,7 +54,7 @@ fn Run() -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "field.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 17, column: 12, scope: !4)
diff --git a/toolchain/lower/testdata/class/method.carbon b/toolchain/lower/testdata/class/method.carbon
index 2769ab0f2c6f..c70b6c06938b 100644
--- a/toolchain/lower/testdata/class/method.carbon
+++ b/toolchain/lower/testdata/class/method.carbon
@@ -23,14 +23,14 @@ fn F(p: C*) {
 // CHECK:STDOUT: ; ModuleID = 'method.carbon'
 // CHECK:STDOUT: source_filename = "method.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare i32 @Get(ptr)
+// CHECK:STDOUT: declare i32 @_CGet.C.Main(ptr)
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @Set(ptr, i32)
+// CHECK:STDOUT: declare void @_CSet.C.Main(ptr, i32)
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F(ptr %p) !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main(ptr %p) !dbg !4 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %Get.call = call i32 @Get(ptr %p), !dbg !7
-// CHECK:STDOUT:   call void @Set(ptr %p, i32 %Get.call), !dbg !8
+// CHECK:STDOUT:   %Get.call = call i32 @_CGet.C.Main(ptr %p), !dbg !7
+// CHECK:STDOUT:   call void @_CSet.C.Main(ptr %p, i32 %Get.call), !dbg !8
 // CHECK:STDOUT:   ret void, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -41,7 +41,7 @@ fn F(p: C*) {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "method.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 19, column: 16, scope: !4)
diff --git a/toolchain/lower/testdata/class/self.carbon b/toolchain/lower/testdata/class/self.carbon
index 55636f479d5e..40f2dc115df1 100644
--- a/toolchain/lower/testdata/class/self.carbon
+++ b/toolchain/lower/testdata/class/self.carbon
@@ -26,14 +26,14 @@ fn C.Set[addr self: C*]() {
 // CHECK:STDOUT: ; ModuleID = 'self.carbon'
 // CHECK:STDOUT: source_filename = "self.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Get(ptr %self) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CGet.C.Main(ptr %self) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc19_14.1.a = getelementptr inbounds nuw { i32 }, ptr %self, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc19_14.2 = load i32, ptr %.loc19_14.1.a, align 4, !dbg !7
 // CHECK:STDOUT:   ret i32 %.loc19_14.2, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Set(ptr %self) !dbg !9 {
+// CHECK:STDOUT: define void @_CSet.C.Main(ptr %self) !dbg !9 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc23_10.a = getelementptr inbounds nuw { i32 }, ptr %self, i32 0, i32 0, !dbg !10
 // CHECK:STDOUT:   store i32 1, ptr %.loc23_10.a, align 4, !dbg !10
@@ -47,11 +47,11 @@ fn C.Set[addr self: C*]() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "self.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Get", linkageName: "Get", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Get", linkageName: "_CGet.C.Main", scope: null, file: !3, line: 18, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 19, column: 10, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 19, column: 3, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "Set", linkageName: "Set", scope: null, file: !3, line: 22, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "Set", linkageName: "_CSet.C.Main", scope: null, file: !3, line: 22, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 23, column: 3, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 22, column: 1, scope: !9)
diff --git a/toolchain/lower/testdata/class/value_access.carbon b/toolchain/lower/testdata/class/value_access.carbon
index 8604499c6304..55c6a4364a2a 100644
--- a/toolchain/lower/testdata/class/value_access.carbon
+++ b/toolchain/lower/testdata/class/value_access.carbon
@@ -22,7 +22,7 @@ fn F(c: C) -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'value_access.carbon'
 // CHECK:STDOUT: source_filename = "value_access.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F(ptr %c) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CF.Main(ptr %c) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc19_11.1.a = getelementptr inbounds nuw { { i32, i32, i32 } }, ptr %c, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc19_11.2.tuple.elem = getelementptr inbounds nuw { i32, i32, i32 }, ptr %.loc19_11.1.a, i32 0, i32 0, !dbg !7
@@ -50,7 +50,7 @@ fn F(c: C) -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "value_access.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 19, column: 10, scope: !4)
diff --git a/toolchain/lower/testdata/debug/nodebug.carbon b/toolchain/lower/testdata/debug/nodebug.carbon
index d7c25618baf5..5450be65aecd 100644
--- a/toolchain/lower/testdata/debug/nodebug.carbon
+++ b/toolchain/lower/testdata/debug/nodebug.carbon
@@ -16,7 +16,7 @@ fn F() {
 // CHECK:STDOUT: ; ModuleID = 'nodebug.carbon'
 // CHECK:STDOUT: source_filename = "nodebug.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() {
+// CHECK:STDOUT: define void @_CF.Main() {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
diff --git a/toolchain/lower/testdata/function/call/empty_struct.carbon b/toolchain/lower/testdata/function/call/empty_struct.carbon
index 56d9da938111..c320614ff2da 100644
--- a/toolchain/lower/testdata/function/call/empty_struct.carbon
+++ b/toolchain/lower/testdata/function/call/empty_struct.carbon
@@ -19,15 +19,15 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'empty_struct.carbon'
 // CHECK:STDOUT: source_filename = "empty_struct.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Echo() !dbg !4 {
+// CHECK:STDOUT: define void @_CEcho.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %b.var = alloca {}, align 8, !dbg !9
-// CHECK:STDOUT:   call void @Echo(), !dbg !10
+// CHECK:STDOUT:   call void @_CEcho.Main(), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -38,11 +38,11 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "empty_struct.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Echo", linkageName: "Echo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Echo", linkageName: "_CEcho.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 3, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 16, column: 7, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 16, column: 15, scope: !8)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/empty_tuple.carbon b/toolchain/lower/testdata/function/call/empty_tuple.carbon
index f79c32e899d7..c05c94f1d634 100644
--- a/toolchain/lower/testdata/function/call/empty_tuple.carbon
+++ b/toolchain/lower/testdata/function/call/empty_tuple.carbon
@@ -19,15 +19,15 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'empty_tuple.carbon'
 // CHECK:STDOUT: source_filename = "empty_tuple.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Echo() !dbg !4 {
+// CHECK:STDOUT: define void @_CEcho.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %b.var = alloca {}, align 8, !dbg !9
-// CHECK:STDOUT:   call void @Echo(), !dbg !10
+// CHECK:STDOUT:   call void @_CEcho.Main(), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -38,11 +38,11 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "empty_tuple.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Echo", linkageName: "Echo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Echo", linkageName: "_CEcho.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 3, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 16, column: 7, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 16, column: 15, scope: !8)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/i32.carbon b/toolchain/lower/testdata/function/call/i32.carbon
index 3c15547d13de..3e4a9e4914a2 100644
--- a/toolchain/lower/testdata/function/call/i32.carbon
+++ b/toolchain/lower/testdata/function/call/i32.carbon
@@ -19,15 +19,15 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'i32.carbon'
 // CHECK:STDOUT: source_filename = "i32.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Echo(i32 %a) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CEcho.Main(i32 %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i32 %a, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %b.var = alloca i32, align 4, !dbg !9
-// CHECK:STDOUT:   %Echo.call = call i32 @Echo(i32 1), !dbg !10
+// CHECK:STDOUT:   %Echo.call = call i32 @_CEcho.Main(i32 1), !dbg !10
 // CHECK:STDOUT:   store i32 %Echo.call, ptr %b.var, align 4, !dbg !11
 // CHECK:STDOUT:   ret void, !dbg !12
 // CHECK:STDOUT: }
@@ -39,11 +39,11 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "i32.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Echo", linkageName: "Echo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Echo", linkageName: "_CEcho.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 3, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 16, column: 7, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 16, column: 16, scope: !8)
 // CHECK:STDOUT: !11 = !DILocation(line: 16, column: 3, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/implicit_empty_tuple_as_arg.carbon b/toolchain/lower/testdata/function/call/implicit_empty_tuple_as_arg.carbon
index f1ca435bf05f..c68cea9e8963 100644
--- a/toolchain/lower/testdata/function/call/implicit_empty_tuple_as_arg.carbon
+++ b/toolchain/lower/testdata/function/call/implicit_empty_tuple_as_arg.carbon
@@ -20,22 +20,22 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'implicit_empty_tuple_as_arg.carbon'
 // CHECK:STDOUT: source_filename = "implicit_empty_tuple_as_arg.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Foo() !dbg !4 {
+// CHECK:STDOUT: define void @_CFoo.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Bar() !dbg !8 {
+// CHECK:STDOUT: define void @_CBar.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !10 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !10 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %x.var = alloca {}, align 8, !dbg !11
-// CHECK:STDOUT:   call void @Foo(), !dbg !12
+// CHECK:STDOUT:   call void @_CFoo.Main(), !dbg !12
 // CHECK:STDOUT:   %.loc17_22.1.temp = alloca {}, align 8, !dbg !12
-// CHECK:STDOUT:   call void @Bar(), !dbg !13
+// CHECK:STDOUT:   call void @_CBar.Main(), !dbg !13
 // CHECK:STDOUT:   ret void, !dbg !14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -46,13 +46,13 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "implicit_empty_tuple_as_arg.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "Foo", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "_CFoo.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Bar", linkageName: "Bar", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Bar", linkageName: "_CBar.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 23, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 17, column: 7, scope: !10)
 // CHECK:STDOUT: !12 = !DILocation(line: 17, column: 19, scope: !10)
 // CHECK:STDOUT: !13 = !DILocation(line: 17, column: 15, scope: !10)
diff --git a/toolchain/lower/testdata/function/call/params_one.carbon b/toolchain/lower/testdata/function/call/params_one.carbon
index 5ab33ecca831..da5f946c7319 100644
--- a/toolchain/lower/testdata/function/call/params_one.carbon
+++ b/toolchain/lower/testdata/function/call/params_one.carbon
@@ -17,14 +17,14 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'params_one.carbon'
 // CHECK:STDOUT: source_filename = "params_one.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Foo(i32 %a) !dbg !4 {
+// CHECK:STDOUT: define void @_CFoo.Main(i32 %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @Foo(i32 1), !dbg !9
+// CHECK:STDOUT:   call void @_CFoo.Main(i32 1), !dbg !9
 // CHECK:STDOUT:   ret void, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -35,10 +35,10 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "params_one.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "Foo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "_CFoo.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 3, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 13, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/params_one_comma.carbon b/toolchain/lower/testdata/function/call/params_one_comma.carbon
index 7855e9fa03ff..267c93bf89c0 100644
--- a/toolchain/lower/testdata/function/call/params_one_comma.carbon
+++ b/toolchain/lower/testdata/function/call/params_one_comma.carbon
@@ -18,15 +18,15 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'params_one_comma.carbon'
 // CHECK:STDOUT: source_filename = "params_one_comma.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Foo(i32 %a) !dbg !4 {
+// CHECK:STDOUT: define void @_CFoo.Main(i32 %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @Foo(i32 1), !dbg !9
-// CHECK:STDOUT:   call void @Foo(i32 1), !dbg !10
+// CHECK:STDOUT:   call void @_CFoo.Main(i32 1), !dbg !9
+// CHECK:STDOUT:   call void @_CFoo.Main(i32 1), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -37,11 +37,11 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "params_one_comma.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "Foo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "_CFoo.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 3, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 15, column: 3, scope: !8)
 // CHECK:STDOUT: !11 = !DILocation(line: 13, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/params_two.carbon b/toolchain/lower/testdata/function/call/params_two.carbon
index fa1f2275a7b0..06ffd7467bef 100644
--- a/toolchain/lower/testdata/function/call/params_two.carbon
+++ b/toolchain/lower/testdata/function/call/params_two.carbon
@@ -17,14 +17,14 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'params_two.carbon'
 // CHECK:STDOUT: source_filename = "params_two.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Foo(i32 %a, i32 %b) !dbg !4 {
+// CHECK:STDOUT: define void @_CFoo.Main(i32 %a, i32 %b) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @Foo(i32 1, i32 2), !dbg !9
+// CHECK:STDOUT:   call void @_CFoo.Main(i32 1, i32 2), !dbg !9
 // CHECK:STDOUT:   ret void, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -35,10 +35,10 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "params_two.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "Foo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "_CFoo.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 3, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 13, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/params_two_comma.carbon b/toolchain/lower/testdata/function/call/params_two_comma.carbon
index 871eee2ca5a6..b3137f22b323 100644
--- a/toolchain/lower/testdata/function/call/params_two_comma.carbon
+++ b/toolchain/lower/testdata/function/call/params_two_comma.carbon
@@ -18,15 +18,15 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'params_two_comma.carbon'
 // CHECK:STDOUT: source_filename = "params_two_comma.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Foo(i32 %a, i32 %b) !dbg !4 {
+// CHECK:STDOUT: define void @_CFoo.Main(i32 %a, i32 %b) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @Foo(i32 1, i32 2), !dbg !9
-// CHECK:STDOUT:   call void @Foo(i32 1, i32 2), !dbg !10
+// CHECK:STDOUT:   call void @_CFoo.Main(i32 1, i32 2), !dbg !9
+// CHECK:STDOUT:   call void @_CFoo.Main(i32 1, i32 2), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -37,11 +37,11 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "params_two_comma.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "Foo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "_CFoo.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 3, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 15, column: 3, scope: !8)
 // CHECK:STDOUT: !11 = !DILocation(line: 13, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/params_zero.carbon b/toolchain/lower/testdata/function/call/params_zero.carbon
index 42114b82b829..26a6884ef6d4 100644
--- a/toolchain/lower/testdata/function/call/params_zero.carbon
+++ b/toolchain/lower/testdata/function/call/params_zero.carbon
@@ -17,14 +17,14 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'params_zero.carbon'
 // CHECK:STDOUT: source_filename = "params_zero.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Foo() !dbg !4 {
+// CHECK:STDOUT: define void @_CFoo.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @Foo(), !dbg !9
+// CHECK:STDOUT:   call void @_CFoo.Main(), !dbg !9
 // CHECK:STDOUT:   ret void, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -35,10 +35,10 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "params_zero.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "Foo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "_CFoo.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 3, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 13, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/return_implicit.carbon b/toolchain/lower/testdata/function/call/return_implicit.carbon
index 7f9669e4731c..a1e3c1a51126 100644
--- a/toolchain/lower/testdata/function/call/return_implicit.carbon
+++ b/toolchain/lower/testdata/function/call/return_implicit.carbon
@@ -19,15 +19,15 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'return_implicit.carbon'
 // CHECK:STDOUT: source_filename = "return_implicit.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @MakeImplicitEmptyTuple() !dbg !4 {
+// CHECK:STDOUT: define void @_CMakeImplicitEmptyTuple.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %b.var = alloca {}, align 8, !dbg !9
-// CHECK:STDOUT:   call void @MakeImplicitEmptyTuple(), !dbg !10
+// CHECK:STDOUT:   call void @_CMakeImplicitEmptyTuple.Main(), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -38,11 +38,11 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "return_implicit.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "MakeImplicitEmptyTuple", linkageName: "MakeImplicitEmptyTuple", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "MakeImplicitEmptyTuple", linkageName: "_CMakeImplicitEmptyTuple.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 16, column: 7, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 16, column: 15, scope: !8)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/struct_param.carbon b/toolchain/lower/testdata/function/call/struct_param.carbon
index 2cf59d120afb..c7e0ef7f6745 100644
--- a/toolchain/lower/testdata/function/call/struct_param.carbon
+++ b/toolchain/lower/testdata/function/call/struct_param.carbon
@@ -19,14 +19,14 @@ fn Main() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @struct.3.loc14_4.3 = internal constant { i32, i32 } { i32 2, i32 3 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F({ i32 } %b, ptr %c) !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main({ i32 } %b, ptr %c) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @F({ i32 } { i32 1 }, ptr @struct.3.loc14_4.3), !dbg !9
+// CHECK:STDOUT:   call void @_CF.Main({ i32 } { i32 1 }, ptr @struct.3.loc14_4.3), !dbg !9
 // CHECK:STDOUT:   ret void, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -37,10 +37,10 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "struct_param.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 3, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 13, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/tuple_param.carbon b/toolchain/lower/testdata/function/call/tuple_param.carbon
index 1b33d29ece49..5ad45eaf9e73 100644
--- a/toolchain/lower/testdata/function/call/tuple_param.carbon
+++ b/toolchain/lower/testdata/function/call/tuple_param.carbon
@@ -19,14 +19,14 @@ fn Main() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @tuple.3.loc14_4.3 = internal constant { i32, i32 } { i32 2, i32 3 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F({ i32 } %b, ptr %c) !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main({ i32 } %b, ptr %c) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @F({ i32 } { i32 1 }, ptr @tuple.3.loc14_4.3), !dbg !9
+// CHECK:STDOUT:   call void @_CF.Main({ i32 } { i32 1 }, ptr @tuple.3.loc14_4.3), !dbg !9
 // CHECK:STDOUT:   ret void, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -37,10 +37,10 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "tuple_param.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 3, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 13, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/function/call/tuple_param_with_return_slot.carbon b/toolchain/lower/testdata/function/call/tuple_param_with_return_slot.carbon
index 02d943490641..4a164beea3b4 100644
--- a/toolchain/lower/testdata/function/call/tuple_param_with_return_slot.carbon
+++ b/toolchain/lower/testdata/function/call/tuple_param_with_return_slot.carbon
@@ -21,7 +21,7 @@ fn Main() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @tuple.3.loc16_4.4 = internal constant { i32, i32 } { i32 2, i32 3 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F(ptr sret({ i32, i32, i32 }) %return, { i32 } %b, ptr %c) !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main(ptr sret({ i32, i32, i32 }) %return, { i32 } %b, ptr %c) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc12_12.tuple.index = extractvalue { i32 } %b, 0, !dbg !7
 // CHECK:STDOUT:   %.loc12_17.tuple.index = getelementptr inbounds nuw { i32, i32 }, ptr %c, i32 0, i32 0, !dbg !8
@@ -37,10 +37,10 @@ fn Main() {
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !12 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !12 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc16_4.1.temp = alloca { i32, i32, i32 }, align 8, !dbg !13
-// CHECK:STDOUT:   call void @F(ptr %.loc16_4.1.temp, { i32 } { i32 1 }, ptr @tuple.3.loc16_4.4), !dbg !13
+// CHECK:STDOUT:   call void @_CF.Main(ptr %.loc16_4.1.temp, { i32 } { i32 1 }, ptr @tuple.3.loc16_4.4), !dbg !13
 // CHECK:STDOUT:   ret void, !dbg !14
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -54,7 +54,7 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "tuple_param_with_return_slot.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 11, scope: !4)
@@ -62,6 +62,6 @@ fn Main() {
 // CHECK:STDOUT: !9 = !DILocation(line: 12, column: 21, scope: !4)
 // CHECK:STDOUT: !10 = !DILocation(line: 12, column: 10, scope: !4)
 // CHECK:STDOUT: !11 = !DILocation(line: 12, column: 3, scope: !4)
-// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !13 = !DILocation(line: 16, column: 3, scope: !12)
 // CHECK:STDOUT: !14 = !DILocation(line: 15, column: 1, scope: !12)
diff --git a/toolchain/lower/testdata/function/call/var_param.carbon b/toolchain/lower/testdata/function/call/var_param.carbon
index 1e2b97ba4668..0ebccd348889 100644
--- a/toolchain/lower/testdata/function/call/var_param.carbon
+++ b/toolchain/lower/testdata/function/call/var_param.carbon
@@ -18,17 +18,17 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'var_param.carbon'
 // CHECK:STDOUT: source_filename = "var_param.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @DoNothing(i32 %a) !dbg !4 {
+// CHECK:STDOUT: define void @_CDoNothing.Main(i32 %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %a.var = alloca i32, align 4, !dbg !9
 // CHECK:STDOUT:   store i32 0, ptr %a.var, align 4, !dbg !10
 // CHECK:STDOUT:   %.loc15 = load i32, ptr %a.var, align 4, !dbg !11
-// CHECK:STDOUT:   call void @DoNothing(i32 %.loc15), !dbg !12
+// CHECK:STDOUT:   call void @_CDoNothing.Main(i32 %.loc15), !dbg !12
 // CHECK:STDOUT:   ret void, !dbg !13
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -39,11 +39,11 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "var_param.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "DoNothing", linkageName: "DoNothing", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "DoNothing", linkageName: "_CDoNothing.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 7, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 14, column: 3, scope: !8)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 13, scope: !8)
diff --git a/toolchain/lower/testdata/function/declaration/simple.carbon b/toolchain/lower/testdata/function/declaration/simple.carbon
index 21a4aa4745f9..6b5f82ba6edd 100644
--- a/toolchain/lower/testdata/function/declaration/simple.carbon
+++ b/toolchain/lower/testdata/function/declaration/simple.carbon
@@ -15,11 +15,11 @@ fn G(n: i32) { F(n); }
 // CHECK:STDOUT: ; ModuleID = 'simple.carbon'
 // CHECK:STDOUT: source_filename = "simple.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @F(i32)
+// CHECK:STDOUT: declare void @_CF.Main(i32)
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @G(i32 %n) !dbg !4 {
+// CHECK:STDOUT: define void @_CG.Main(i32 %n) !dbg !4 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @F(i32 %n), !dbg !7
+// CHECK:STDOUT:   call void @_CF.Main(i32 %n), !dbg !7
 // CHECK:STDOUT:   ret void, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -30,7 +30,7 @@ fn G(n: i32) { F(n); }
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "simple.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 13, column: 16, scope: !4)
diff --git a/toolchain/lower/testdata/function/definition/empty_struct.carbon b/toolchain/lower/testdata/function/definition/empty_struct.carbon
index 19fab4912ff2..62b6f3e5e3e8 100644
--- a/toolchain/lower/testdata/function/definition/empty_struct.carbon
+++ b/toolchain/lower/testdata/function/definition/empty_struct.carbon
@@ -14,7 +14,7 @@ fn Echo(a: {}) {
 // CHECK:STDOUT: ; ModuleID = 'empty_struct.carbon'
 // CHECK:STDOUT: source_filename = "empty_struct.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Echo() !dbg !4 {
+// CHECK:STDOUT: define void @_CEcho.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
@@ -26,7 +26,7 @@ fn Echo(a: {}) {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "empty_struct.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Echo", linkageName: "Echo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Echo", linkageName: "_CEcho.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
diff --git a/toolchain/lower/testdata/function/definition/params_one.carbon b/toolchain/lower/testdata/function/definition/params_one.carbon
index 0ffe22f00391..81b438967618 100644
--- a/toolchain/lower/testdata/function/definition/params_one.carbon
+++ b/toolchain/lower/testdata/function/definition/params_one.carbon
@@ -13,7 +13,7 @@ fn Foo(a: i32) {}
 // CHECK:STDOUT: ; ModuleID = 'params_one.carbon'
 // CHECK:STDOUT: source_filename = "params_one.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Foo(i32 %a) !dbg !4 {
+// CHECK:STDOUT: define void @_CFoo.Main(i32 %a) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
@@ -25,7 +25,7 @@ fn Foo(a: i32) {}
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "params_one.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "Foo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "_CFoo.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
diff --git a/toolchain/lower/testdata/function/definition/params_two.carbon b/toolchain/lower/testdata/function/definition/params_two.carbon
index 23322afc2473..2ade1989c27b 100644
--- a/toolchain/lower/testdata/function/definition/params_two.carbon
+++ b/toolchain/lower/testdata/function/definition/params_two.carbon
@@ -13,7 +13,7 @@ fn Foo(a: i32, b: i32) {}
 // CHECK:STDOUT: ; ModuleID = 'params_two.carbon'
 // CHECK:STDOUT: source_filename = "params_two.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Foo(i32 %a, i32 %b) !dbg !4 {
+// CHECK:STDOUT: define void @_CFoo.Main(i32 %a, i32 %b) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
@@ -25,7 +25,7 @@ fn Foo(a: i32, b: i32) {}
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "params_two.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "Foo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "_CFoo.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
diff --git a/toolchain/lower/testdata/function/definition/params_zero.carbon b/toolchain/lower/testdata/function/definition/params_zero.carbon
index 0bc600ff6269..3faed3f0c4f6 100644
--- a/toolchain/lower/testdata/function/definition/params_zero.carbon
+++ b/toolchain/lower/testdata/function/definition/params_zero.carbon
@@ -13,7 +13,7 @@ fn Foo() {}
 // CHECK:STDOUT: ; ModuleID = 'params_zero.carbon'
 // CHECK:STDOUT: source_filename = "params_zero.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Foo() !dbg !4 {
+// CHECK:STDOUT: define void @_CFoo.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
@@ -25,7 +25,7 @@ fn Foo() {}
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "params_zero.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "Foo", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Foo", linkageName: "_CFoo.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
diff --git a/toolchain/lower/testdata/function/definition/raw_name.carbon b/toolchain/lower/testdata/function/definition/raw_name.carbon
index 7737ab7493ce..b25e5b7bd1be 100644
--- a/toolchain/lower/testdata/function/definition/raw_name.carbon
+++ b/toolchain/lower/testdata/function/definition/raw_name.carbon
@@ -14,7 +14,7 @@ fn r#self() {}
 // CHECK:STDOUT: ; ModuleID = 'raw_name.carbon'
 // CHECK:STDOUT: source_filename = "raw_name.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @self() !dbg !4 {
+// CHECK:STDOUT: define void @_Cself.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
@@ -26,7 +26,7 @@ fn r#self() {}
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "raw_name.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "self", linkageName: "self", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "self", linkageName: "_Cself.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
diff --git a/toolchain/lower/testdata/global/class_obj.carbon b/toolchain/lower/testdata/global/class_obj.carbon
index 3388d87b7cf6..00d3ed5d6284 100644
--- a/toolchain/lower/testdata/global/class_obj.carbon
+++ b/toolchain/lower/testdata/global/class_obj.carbon
@@ -16,9 +16,9 @@ var a: A = {};
 // CHECK:STDOUT:
 // CHECK:STDOUT: @a = internal global {}
 // CHECK:STDOUT: @struct.loc12_14 = internal constant {} zeroinitializer
-// CHECK:STDOUT: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 0, ptr @__global_init, ptr null }]
+// CHECK:STDOUT: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 0, ptr @_C__global_init.Main, ptr null }]
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @__global_init() !dbg !4 {
+// CHECK:STDOUT: define void @_C__global_init.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 1 @a, ptr align 1 @struct.loc12_14, i64 0, i1 false), !dbg !7
 // CHECK:STDOUT:   ret void, !dbg !8
@@ -36,7 +36,7 @@ var a: A = {};
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "class_obj.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "__global_init", linkageName: "__global_init", scope: null, file: !3, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "__global_init", linkageName: "_C__global_init.Main", scope: null, file: !3, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 1, scope: !4)
diff --git a/toolchain/lower/testdata/global/class_with_fun.carbon b/toolchain/lower/testdata/global/class_with_fun.carbon
index 28ab6e829ff0..c0570446d2f3 100644
--- a/toolchain/lower/testdata/global/class_with_fun.carbon
+++ b/toolchain/lower/testdata/global/class_with_fun.carbon
@@ -21,15 +21,15 @@ var a: A = {};
 // CHECK:STDOUT: @a = internal global {}
 // CHECK:STDOUT: @struct.loc13_12 = internal constant {} zeroinitializer
 // CHECK:STDOUT: @struct.loc16_14 = internal constant {} zeroinitializer
-// CHECK:STDOUT: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 0, ptr @__global_init, ptr null }]
+// CHECK:STDOUT: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 0, ptr @_C__global_init.Main, ptr null }]
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @ret_a(ptr sret({}) %return) !dbg !4 {
+// CHECK:STDOUT: define void @_Cret_a.Main(ptr sret({}) %return) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 1 %return, ptr align 1 @struct.loc13_12, i64 0, i1 false), !dbg !7
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @__global_init() !dbg !8 {
+// CHECK:STDOUT: define void @_C__global_init.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 1 @a, ptr align 1 @struct.loc16_14, i64 0, i1 false), !dbg !9
 // CHECK:STDOUT:   ret void, !dbg !10
@@ -50,10 +50,10 @@ var a: A = {};
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "class_with_fun.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "ret_a", linkageName: "ret_a", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "ret_a", linkageName: "_Cret_a.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 13, column: 3, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "__global_init", linkageName: "__global_init", scope: null, file: !3, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "__global_init", linkageName: "_C__global_init.Main", scope: null, file: !3, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 16, column: 1, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 0, scope: !8)
diff --git a/toolchain/lower/testdata/global/simple_init.carbon b/toolchain/lower/testdata/global/simple_init.carbon
index 231e11e20893..6591ab48b3df 100644
--- a/toolchain/lower/testdata/global/simple_init.carbon
+++ b/toolchain/lower/testdata/global/simple_init.carbon
@@ -13,9 +13,9 @@ var a: i32 = 0;
 // CHECK:STDOUT: source_filename = "simple_init.carbon"
 // CHECK:STDOUT:
 // CHECK:STDOUT: @a = internal global i32
-// CHECK:STDOUT: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 0, ptr @__global_init, ptr null }]
+// CHECK:STDOUT: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 0, ptr @_C__global_init.Main, ptr null }]
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @__global_init() !dbg !4 {
+// CHECK:STDOUT: define void @_C__global_init.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   store i32 0, ptr @a, align 4, !dbg !7
 // CHECK:STDOUT:   ret void, !dbg !8
@@ -31,7 +31,7 @@ var a: i32 = 0;
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "simple_init.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "__global_init", linkageName: "__global_init", scope: null, file: !3, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "__global_init", linkageName: "_C__global_init.Main", scope: null, file: !3, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 10, column: 1, scope: !4)
diff --git a/toolchain/lower/testdata/global/simple_with_fun.carbon b/toolchain/lower/testdata/global/simple_with_fun.carbon
index f1ff64fb42a2..761b9abb0261 100644
--- a/toolchain/lower/testdata/global/simple_with_fun.carbon
+++ b/toolchain/lower/testdata/global/simple_with_fun.carbon
@@ -18,16 +18,16 @@ var a: i32 = test_a();
 // CHECK:STDOUT: source_filename = "simple_with_fun.carbon"
 // CHECK:STDOUT:
 // CHECK:STDOUT: @a = internal global i32
-// CHECK:STDOUT: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 0, ptr @__global_init, ptr null }]
+// CHECK:STDOUT: @llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [{ i32, ptr, ptr } { i32 0, ptr @_C__global_init.Main, ptr null }]
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @test_a() !dbg !4 {
+// CHECK:STDOUT: define i32 @_Ctest_a.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i32 0, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @__global_init() !dbg !8 {
+// CHECK:STDOUT: define void @_C__global_init.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %test_a.call = call i32 @test_a(), !dbg !9
+// CHECK:STDOUT:   %test_a.call = call i32 @_Ctest_a.Main(), !dbg !9
 // CHECK:STDOUT:   store i32 %test_a.call, ptr @a, align 4, !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
@@ -42,11 +42,11 @@ var a: i32 = test_a();
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "simple_with_fun.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "test_a", linkageName: "test_a", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "test_a", linkageName: "_Ctest_a.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 3, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "__global_init", linkageName: "__global_init", scope: null, file: !3, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "__global_init", linkageName: "_C__global_init.Main", scope: null, file: !3, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 15, column: 14, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 15, column: 1, scope: !8)
 // CHECK:STDOUT: !11 = !DILocation(line: 0, scope: !8)
diff --git a/toolchain/lower/testdata/if/else.carbon b/toolchain/lower/testdata/if/else.carbon
index 567f88fbebd4..0bf402bef0ad 100644
--- a/toolchain/lower/testdata/if/else.carbon
+++ b/toolchain/lower/testdata/if/else.carbon
@@ -24,35 +24,35 @@ fn If(b: bool) {
 // CHECK:STDOUT: ; ModuleID = 'else.carbon'
 // CHECK:STDOUT: source_filename = "else.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @G() !dbg !8 {
+// CHECK:STDOUT: define void @_CG.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @H() !dbg !10 {
+// CHECK:STDOUT: define void @_CH.Main() !dbg !10 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @If(i1 %b) !dbg !12 {
+// CHECK:STDOUT: define void @_CIf.Main(i1 %b) !dbg !12 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   br i1 %b, label %if.then, label %if.else, !dbg !13
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.then:                                          ; preds = %entry
-// CHECK:STDOUT:   call void @F(), !dbg !14
+// CHECK:STDOUT:   call void @_CF.Main(), !dbg !14
 // CHECK:STDOUT:   br label %if.done, !dbg !15
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.else:                                          ; preds = %entry
-// CHECK:STDOUT:   call void @G(), !dbg !16
+// CHECK:STDOUT:   call void @_CG.Main(), !dbg !16
 // CHECK:STDOUT:   br label %if.done, !dbg !15
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.done:                                          ; preds = %if.else, %if.then
-// CHECK:STDOUT:   call void @H(), !dbg !17
+// CHECK:STDOUT:   call void @_CH.Main(), !dbg !17
 // CHECK:STDOUT:   ret void, !dbg !18
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -63,15 +63,15 @@ fn If(b: bool) {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "else.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 12, column: 1, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "H", linkageName: "H", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "H", linkageName: "_CH.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 13, column: 1, scope: !10)
-// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "If", linkageName: "If", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !12 = distinct !DISubprogram(name: "If", linkageName: "_CIf.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !13 = !DILocation(line: 16, column: 6, scope: !12)
 // CHECK:STDOUT: !14 = !DILocation(line: 17, column: 5, scope: !12)
 // CHECK:STDOUT: !15 = !DILocation(line: 16, column: 3, scope: !12)
diff --git a/toolchain/lower/testdata/if/no_else.carbon b/toolchain/lower/testdata/if/no_else.carbon
index a95c0e1e6e4c..21b9cbc7b6b1 100644
--- a/toolchain/lower/testdata/if/no_else.carbon
+++ b/toolchain/lower/testdata/if/no_else.carbon
@@ -21,26 +21,26 @@ fn If(b: bool) {
 // CHECK:STDOUT: ; ModuleID = 'no_else.carbon'
 // CHECK:STDOUT: source_filename = "no_else.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @G() !dbg !8 {
+// CHECK:STDOUT: define void @_CG.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @If(i1 %b) !dbg !10 {
+// CHECK:STDOUT: define void @_CIf.Main(i1 %b) !dbg !10 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   br i1 %b, label %if.then, label %if.else, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.then:                                          ; preds = %entry
-// CHECK:STDOUT:   call void @F(), !dbg !12
+// CHECK:STDOUT:   call void @_CF.Main(), !dbg !12
 // CHECK:STDOUT:   br label %if.else, !dbg !13
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.else:                                          ; preds = %if.then, %entry
-// CHECK:STDOUT:   call void @G(), !dbg !14
+// CHECK:STDOUT:   call void @_CG.Main(), !dbg !14
 // CHECK:STDOUT:   ret void, !dbg !15
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -51,13 +51,13 @@ fn If(b: bool) {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "no_else.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 12, column: 1, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "If", linkageName: "If", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "If", linkageName: "_CIf.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 6, scope: !10)
 // CHECK:STDOUT: !12 = !DILocation(line: 16, column: 5, scope: !10)
 // CHECK:STDOUT: !13 = !DILocation(line: 15, column: 3, scope: !10)
diff --git a/toolchain/lower/testdata/if_expr/basic.carbon b/toolchain/lower/testdata/if_expr/basic.carbon
index e2f814687cc5..919be2d4f742 100644
--- a/toolchain/lower/testdata/if_expr/basic.carbon
+++ b/toolchain/lower/testdata/if_expr/basic.carbon
@@ -18,26 +18,26 @@ fn Select(b: bool) -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'basic.carbon'
 // CHECK:STDOUT: source_filename = "basic.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F() !dbg !4 {
+// CHECK:STDOUT: define i32 @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i32 1, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @G() !dbg !8 {
+// CHECK:STDOUT: define i32 @_CG.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i32 2, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Select(i1 %b) !dbg !10 {
+// CHECK:STDOUT: define i32 @_CSelect.Main(i1 %b) !dbg !10 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   br i1 %b, label %if.expr.then, label %if.expr.else, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.expr.then:                                     ; preds = %entry
-// CHECK:STDOUT:   %F.call = call i32 @F(), !dbg !12
+// CHECK:STDOUT:   %F.call = call i32 @_CF.Main(), !dbg !12
 // CHECK:STDOUT:   br label %if.expr.result, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.expr.else:                                     ; preds = %entry
-// CHECK:STDOUT:   %G.call = call i32 @G(), !dbg !13
+// CHECK:STDOUT:   %G.call = call i32 @_CG.Main(), !dbg !13
 // CHECK:STDOUT:   br label %if.expr.result, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.expr.result:                                   ; preds = %if.expr.else, %if.expr.then
@@ -52,13 +52,13 @@ fn Select(b: bool) -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "basic.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 17, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 12, column: 17, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Select", linkageName: "Select", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Select", linkageName: "_CSelect.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 10, scope: !10)
 // CHECK:STDOUT: !12 = !DILocation(line: 15, column: 20, scope: !10)
 // CHECK:STDOUT: !13 = !DILocation(line: 15, column: 29, scope: !10)
diff --git a/toolchain/lower/testdata/if_expr/empty_block.carbon b/toolchain/lower/testdata/if_expr/empty_block.carbon
index fda57da20c9d..fb3e33cfb415 100644
--- a/toolchain/lower/testdata/if_expr/empty_block.carbon
+++ b/toolchain/lower/testdata/if_expr/empty_block.carbon
@@ -15,7 +15,7 @@ fn Select(b: bool, c: bool, d: bool) -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'empty_block.carbon'
 // CHECK:STDOUT: source_filename = "empty_block.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Select(i1 %b, i1 %c, i1 %d) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CSelect.Main(i1 %b, i1 %c, i1 %d) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   br i1 %b, label %if.expr.then.loc12_10, label %if.expr.else.loc12_10, !dbg !7
 // CHECK:STDOUT:
@@ -57,7 +57,7 @@ fn Select(b: bool, c: bool, d: bool) -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "empty_block.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Select", linkageName: "Select", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Select", linkageName: "_CSelect.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 10, scope: !4)
diff --git a/toolchain/lower/testdata/impl/assoc_fn_alias.carbon b/toolchain/lower/testdata/impl/assoc_fn_alias.carbon
index 46eb2091ba3e..0da3a2fe68de 100644
--- a/toolchain/lower/testdata/impl/assoc_fn_alias.carbon
+++ b/toolchain/lower/testdata/impl/assoc_fn_alias.carbon
@@ -30,16 +30,16 @@ fn Call(a: A) -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'assoc_fn_alias.carbon'
 // CHECK:STDOUT: source_filename = "assoc_fn_alias.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F(ptr %self) !dbg !4 {
+// CHECK:STDOUT: define i32 @"_CF.A.Main:I.Main"(ptr %self) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc22_16.1.n = getelementptr inbounds nuw { i32 }, ptr %self, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc22_16.2 = load i32, ptr %.loc22_16.1.n, align 4, !dbg !7
 // CHECK:STDOUT:   ret i32 %.loc22_16.2, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Call(ptr %a) !dbg !9 {
+// CHECK:STDOUT: define i32 @_CCall.Main(ptr %a) !dbg !9 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %F.call = call i32 @F(ptr %a), !dbg !10
+// CHECK:STDOUT:   %F.call = call i32 @"_CF.A.Main:I.Main"(ptr %a), !dbg !10
 // CHECK:STDOUT:   ret i32 %F.call, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -50,11 +50,11 @@ fn Call(a: A) -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "assoc_fn_alias.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.A.Main:I.Main", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 22, column: 12, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 22, column: 5, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "Call", linkageName: "Call", scope: null, file: !3, line: 26, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "Call", linkageName: "_CCall.Main", scope: null, file: !3, line: 26, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 27, column: 10, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 27, column: 3, scope: !9)
diff --git a/toolchain/lower/testdata/impl/extend_impl.carbon b/toolchain/lower/testdata/impl/extend_impl.carbon
index 9e2b3799470f..9583cac774df 100644
--- a/toolchain/lower/testdata/impl/extend_impl.carbon
+++ b/toolchain/lower/testdata/impl/extend_impl.carbon
@@ -31,20 +31,20 @@ fn InstanceAccess(a: A) -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'extend_impl.carbon'
 // CHECK:STDOUT: source_filename = "extend_impl.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F() !dbg !4 {
+// CHECK:STDOUT: define i32 @"_CF.A.Main:I.Main"() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i32 0, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @TypeAccess() !dbg !8 {
+// CHECK:STDOUT: define i32 @_CTypeAccess.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %F.call = call i32 @F(), !dbg !9
+// CHECK:STDOUT:   %F.call = call i32 @"_CF.A.Main:I.Main"(), !dbg !9
 // CHECK:STDOUT:   ret i32 %F.call, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @InstanceAccess(ptr %a) !dbg !11 {
+// CHECK:STDOUT: define i32 @_CInstanceAccess.Main(ptr %a) !dbg !11 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %F.call = call i32 @F(), !dbg !12
+// CHECK:STDOUT:   %F.call = call i32 @"_CF.A.Main:I.Main"(), !dbg !12
 // CHECK:STDOUT:   ret i32 %F.call, !dbg !13
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -55,13 +55,13 @@ fn InstanceAccess(a: A) -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "extend_impl.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.A.Main:I.Main", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 18, column: 7, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "TypeAccess", linkageName: "TypeAccess", scope: null, file: !3, line: 23, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "TypeAccess", linkageName: "_CTypeAccess.Main", scope: null, file: !3, line: 23, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 24, column: 10, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 24, column: 3, scope: !8)
-// CHECK:STDOUT: !11 = distinct !DISubprogram(name: "InstanceAccess", linkageName: "InstanceAccess", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !11 = distinct !DISubprogram(name: "InstanceAccess", linkageName: "_CInstanceAccess.Main", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !12 = !DILocation(line: 28, column: 10, scope: !11)
 // CHECK:STDOUT: !13 = !DILocation(line: 28, column: 3, scope: !11)
diff --git a/toolchain/lower/testdata/impl/impl.carbon b/toolchain/lower/testdata/impl/impl.carbon
index 060c07a290c2..ff653b26b811 100644
--- a/toolchain/lower/testdata/impl/impl.carbon
+++ b/toolchain/lower/testdata/impl/impl.carbon
@@ -29,16 +29,16 @@ fn Call(a: A) -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'impl.carbon'
 // CHECK:STDOUT: source_filename = "impl.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F(ptr %self) !dbg !4 {
+// CHECK:STDOUT: define i32 @"_CF.A.Main:I.Main"(ptr %self) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc21_16.1.n = getelementptr inbounds nuw { i32 }, ptr %self, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc21_16.2 = load i32, ptr %.loc21_16.1.n, align 4, !dbg !7
 // CHECK:STDOUT:   ret i32 %.loc21_16.2, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Call(ptr %a) !dbg !9 {
+// CHECK:STDOUT: define i32 @_CCall.Main(ptr %a) !dbg !9 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %F.call = call i32 @F(ptr %a), !dbg !10
+// CHECK:STDOUT:   %F.call = call i32 @"_CF.A.Main:I.Main"(ptr %a), !dbg !10
 // CHECK:STDOUT:   ret i32 %F.call, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -49,11 +49,11 @@ fn Call(a: A) -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "impl.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 20, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.A.Main:I.Main", scope: null, file: !3, line: 20, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 21, column: 12, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 21, column: 5, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "Call", linkageName: "Call", scope: null, file: !3, line: 25, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "Call", linkageName: "_CCall.Main", scope: null, file: !3, line: 25, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 26, column: 10, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 26, column: 3, scope: !9)
diff --git a/toolchain/lower/testdata/impl/instance_method.carbon b/toolchain/lower/testdata/impl/instance_method.carbon
index c6c20f0fe98d..62947f7d0115 100644
--- a/toolchain/lower/testdata/impl/instance_method.carbon
+++ b/toolchain/lower/testdata/impl/instance_method.carbon
@@ -29,14 +29,14 @@ fn Call(a: A*) -> A* {
 // CHECK:STDOUT: ; ModuleID = 'instance_method.carbon'
 // CHECK:STDOUT: source_filename = "instance_method.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define ptr @Get(ptr %self) !dbg !4 {
+// CHECK:STDOUT: define ptr @"_CGet.A.Main:GetSelf.Main"(ptr %self) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret ptr %self, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define ptr @Call(ptr %a) !dbg !8 {
+// CHECK:STDOUT: define ptr @_CCall.Main(ptr %a) !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %Get.call = call ptr @Get(ptr %a), !dbg !9
+// CHECK:STDOUT:   %Get.call = call ptr @"_CGet.A.Main:GetSelf.Main"(ptr %a), !dbg !9
 // CHECK:STDOUT:   ret ptr %Get.call, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -47,10 +47,10 @@ fn Call(a: A*) -> A* {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "instance_method.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Get", linkageName: "Get", scope: null, file: !3, line: 19, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Get", linkageName: "_CGet.A.Main:GetSelf.Main", scope: null, file: !3, line: 19, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 20, column: 7, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Call", linkageName: "Call", scope: null, file: !3, line: 25, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Call", linkageName: "_CCall.Main", scope: null, file: !3, line: 25, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 26, column: 10, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 26, column: 3, scope: !8)
diff --git a/toolchain/lower/testdata/index/array_element_access.carbon b/toolchain/lower/testdata/index/array_element_access.carbon
index 20ee6928f201..ca4ed8469e09 100644
--- a/toolchain/lower/testdata/index/array_element_access.carbon
+++ b/toolchain/lower/testdata/index/array_element_access.carbon
@@ -24,7 +24,7 @@ fn Run() {
 // CHECK:STDOUT: @tuple.loc10_37 = internal constant { i32, i32 } { i32 1, i32 2 }
 // CHECK:STDOUT: @array.loc12_35 = internal constant [2 x i32] [i32 1, i32 2]
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @A(ptr sret({ i32, i32 }) %return) !dbg !4 {
+// CHECK:STDOUT: define void @_CA.Main(ptr sret({ i32, i32 }) %return) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc10_36.2.tuple.elem = getelementptr inbounds nuw { i32, i32 }, ptr %return, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc10_36.4.tuple.elem = getelementptr inbounds nuw { i32, i32 }, ptr %return, i32 0, i32 1, !dbg !7
@@ -32,7 +32,7 @@ fn Run() {
 // CHECK:STDOUT:   ret void, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @B(ptr sret([2 x i32]) %return) !dbg !9 {
+// CHECK:STDOUT: define void @_CB.Main(ptr sret([2 x i32]) %return) !dbg !9 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc12_34.3.array.index = getelementptr inbounds [2 x i32], ptr %return, i32 0, i32 0, !dbg !10
 // CHECK:STDOUT:   %.loc12_34.6.array.index = getelementptr inbounds [2 x i32], ptr %return, i32 0, i32 1, !dbg !10
@@ -44,7 +44,7 @@ fn Run() {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %a.var = alloca [2 x i32], align 4, !dbg !13
 // CHECK:STDOUT:   %.loc15_22.1.temp = alloca { i32, i32 }, align 8, !dbg !14
-// CHECK:STDOUT:   call void @A(ptr %.loc15_22.1.temp), !dbg !14
+// CHECK:STDOUT:   call void @_CA.Main(ptr %.loc15_22.1.temp), !dbg !14
 // CHECK:STDOUT:   %.loc15_22.3.tuple.elem = getelementptr inbounds nuw { i32, i32 }, ptr %.loc15_22.1.temp, i32 0, i32 0, !dbg !14
 // CHECK:STDOUT:   %.loc15_22.4 = load i32, ptr %.loc15_22.3.tuple.elem, align 4, !dbg !14
 // CHECK:STDOUT:   %.loc15_22.6.array.index = getelementptr inbounds [2 x i32], ptr %a.var, i32 0, i32 0, !dbg !14
@@ -62,7 +62,7 @@ fn Run() {
 // CHECK:STDOUT:   store i32 %.loc17_19.2, ptr %c.var, align 4, !dbg !20
 // CHECK:STDOUT:   %d.var = alloca i32, align 4, !dbg !21
 // CHECK:STDOUT:   %.loc18_17.1.temp = alloca [2 x i32], align 4, !dbg !22
-// CHECK:STDOUT:   call void @B(ptr %.loc18_17.1.temp), !dbg !22
+// CHECK:STDOUT:   call void @_CB.Main(ptr %.loc18_17.1.temp), !dbg !22
 // CHECK:STDOUT:   %.loc18_21.1.array.index = getelementptr inbounds [2 x i32], ptr %.loc18_17.1.temp, i32 0, i32 1, !dbg !22
 // CHECK:STDOUT:   %.loc18_21.2 = load i32, ptr %.loc18_21.1.array.index, align 4, !dbg !22
 // CHECK:STDOUT:   store i32 %.loc18_21.2, ptr %d.var, align 4, !dbg !23
@@ -84,12 +84,12 @@ fn Run() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "array_element_access.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "A", linkageName: "A", scope: null, file: !3, line: 10, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "A", linkageName: "_CA.Main", scope: null, file: !3, line: 10, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 10, column: 31, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 10, column: 24, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "B", linkageName: "B", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "B", linkageName: "_CB.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 12, column: 29, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 12, column: 22, scope: !9)
 // CHECK:STDOUT: !12 = distinct !DISubprogram(name: "Run", linkageName: "main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
diff --git a/toolchain/lower/testdata/interface/assoc.carbon b/toolchain/lower/testdata/interface/assoc.carbon
index 5c4881024cea..b79d1b9c6d9f 100644
--- a/toolchain/lower/testdata/interface/assoc.carbon
+++ b/toolchain/lower/testdata/interface/assoc.carbon
@@ -17,7 +17,7 @@ fn F() { I.Assoc; }
 // CHECK:STDOUT: ; ModuleID = 'assoc.carbon'
 // CHECK:STDOUT: source_filename = "assoc.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
@@ -29,7 +29,7 @@ fn F() { I.Assoc; }
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "assoc.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 15, column: 1, scope: !4)
diff --git a/toolchain/lower/testdata/interface/basic.carbon b/toolchain/lower/testdata/interface/basic.carbon
index 1f9062e2b531..68dc97cabeb0 100644
--- a/toolchain/lower/testdata/interface/basic.carbon
+++ b/toolchain/lower/testdata/interface/basic.carbon
@@ -24,12 +24,12 @@ fn G(T: J) {}
 // CHECK:STDOUT: ; ModuleID = 'basic.carbon'
 // CHECK:STDOUT: source_filename = "basic.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @G() !dbg !8 {
+// CHECK:STDOUT: define void @_CG.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !9
 // CHECK:STDOUT: }
@@ -41,9 +41,9 @@ fn G(T: J) {}
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "basic.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 17, column: 19, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 22, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 22, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 22, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/let/tuple.carbon b/toolchain/lower/testdata/let/tuple.carbon
index 3cb4f9b23b79..d2a5995302e8 100644
--- a/toolchain/lower/testdata/let/tuple.carbon
+++ b/toolchain/lower/testdata/let/tuple.carbon
@@ -21,7 +21,7 @@ fn F() -> i32 {
 // CHECK:STDOUT: @tuple.1.loc12_37 = internal constant { i32, i32, i32 } { i32 1, i32 2, i32 3 }
 // CHECK:STDOUT: @tuple.2.loc13_29 = internal constant { i32, i32 } { i32 4, i32 5 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F() !dbg !4 {
+// CHECK:STDOUT: define i32 @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %a.var = alloca { i32, i32, i32 }, align 8, !dbg !7
 // CHECK:STDOUT:   %.loc12_36.2.tuple.elem = getelementptr inbounds nuw { i32, i32, i32 }, ptr %a.var, i32 0, i32 0, !dbg !8
@@ -81,7 +81,7 @@ fn F() -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "tuple.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/namespace/function.carbon b/toolchain/lower/testdata/namespace/function.carbon
index f8121b0702e4..f8bf1a3410a1 100644
--- a/toolchain/lower/testdata/namespace/function.carbon
+++ b/toolchain/lower/testdata/namespace/function.carbon
@@ -24,19 +24,19 @@ fn Bar() {
 // CHECK:STDOUT: ; ModuleID = 'function.carbon'
 // CHECK:STDOUT: source_filename = "function.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Baz() !dbg !4 {
+// CHECK:STDOUT: define void @_CBaz.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Baz.1() !dbg !8 {
+// CHECK:STDOUT: define void @_CBaz.Foo.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Bar() !dbg !10 {
+// CHECK:STDOUT: define void @_CBar.Main() !dbg !10 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @Baz.1(), !dbg !11
+// CHECK:STDOUT:   call void @_CBaz.Foo.Main(), !dbg !11
 // CHECK:STDOUT:   ret void, !dbg !12
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -47,12 +47,12 @@ fn Bar() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "function.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Baz", linkageName: "Baz", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Baz", linkageName: "_CBaz.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Baz", linkageName: "Baz.1", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Baz", linkageName: "_CBaz.Foo.Main", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 17, column: 1, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Bar", linkageName: "Bar", scope: null, file: !3, line: 20, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Bar", linkageName: "_CBar.Main", scope: null, file: !3, line: 20, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 21, column: 3, scope: !10)
 // CHECK:STDOUT: !12 = !DILocation(line: 20, column: 1, scope: !10)
diff --git a/toolchain/lower/testdata/namespace/nested.carbon b/toolchain/lower/testdata/namespace/nested.carbon
index 5da934315b16..d804bab46087 100644
--- a/toolchain/lower/testdata/namespace/nested.carbon
+++ b/toolchain/lower/testdata/namespace/nested.carbon
@@ -21,14 +21,14 @@ fn Foo.Bar.Baz() {
 // CHECK:STDOUT: ; ModuleID = 'nested.carbon'
 // CHECK:STDOUT: source_filename = "nested.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Wiz() !dbg !4 {
+// CHECK:STDOUT: define void @_CWiz.Bar.Foo.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Baz() !dbg !8 {
+// CHECK:STDOUT: define void @_CBaz.Bar.Foo.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @Wiz(), !dbg !9
+// CHECK:STDOUT:   call void @_CWiz.Bar.Foo.Main(), !dbg !9
 // CHECK:STDOUT:   ret void, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -39,10 +39,10 @@ fn Foo.Bar.Baz() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "nested.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Wiz", linkageName: "Wiz", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Wiz", linkageName: "_CWiz.Bar.Foo.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Baz", linkageName: "Baz", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Baz", linkageName: "_CBaz.Bar.Foo.Main", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 18, column: 3, scope: !8)
 // CHECK:STDOUT: !10 = !DILocation(line: 17, column: 1, scope: !8)
diff --git a/toolchain/lower/testdata/operators/and.carbon b/toolchain/lower/testdata/operators/and.carbon
index 9737e3ef7041..4fdcb44ef7f2 100644
--- a/toolchain/lower/testdata/operators/and.carbon
+++ b/toolchain/lower/testdata/operators/and.carbon
@@ -18,23 +18,23 @@ fn And() -> bool {
 // CHECK:STDOUT: ; ModuleID = 'and.carbon'
 // CHECK:STDOUT: source_filename = "and.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @F() !dbg !4 {
+// CHECK:STDOUT: define i1 @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i1 true, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @G() !dbg !8 {
+// CHECK:STDOUT: define i1 @_CG.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i1 true, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @And() !dbg !10 {
+// CHECK:STDOUT: define i1 @_CAnd.Main() !dbg !10 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %F.call = call i1 @F(), !dbg !11
+// CHECK:STDOUT:   %F.call = call i1 @_CF.Main(), !dbg !11
 // CHECK:STDOUT:   br i1 %F.call, label %and.rhs, label %and.result, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: and.rhs:                                          ; preds = %entry
-// CHECK:STDOUT:   %G.call = call i1 @G(), !dbg !12
+// CHECK:STDOUT:   %G.call = call i1 @_CG.Main(), !dbg !12
 // CHECK:STDOUT:   br label %and.result, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: and.result:                                       ; preds = %and.rhs, %entry
@@ -49,13 +49,13 @@ fn And() -> bool {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "and.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 18, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 12, column: 18, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "And", linkageName: "And", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "And", linkageName: "_CAnd.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 10, scope: !10)
 // CHECK:STDOUT: !12 = !DILocation(line: 15, column: 18, scope: !10)
 // CHECK:STDOUT: !13 = !DILocation(line: 15, column: 3, scope: !10)
diff --git a/toolchain/lower/testdata/operators/and_empty_block.carbon b/toolchain/lower/testdata/operators/and_empty_block.carbon
index 4b71ab9fd872..71b421999781 100644
--- a/toolchain/lower/testdata/operators/and_empty_block.carbon
+++ b/toolchain/lower/testdata/operators/and_empty_block.carbon
@@ -17,7 +17,7 @@ fn And(b: bool, c: bool) -> bool {
 // CHECK:STDOUT: ; ModuleID = 'and_empty_block.carbon'
 // CHECK:STDOUT: source_filename = "and_empty_block.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @And(i1 %b, i1 %c) !dbg !4 {
+// CHECK:STDOUT: define i1 @_CAnd.Main(i1 %b, i1 %c) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   br i1 %b, label %and.rhs, label %and.result, !dbg !7
 // CHECK:STDOUT:
@@ -36,7 +36,7 @@ fn And(b: bool, c: bool) -> bool {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "and_empty_block.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "And", linkageName: "And", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "And", linkageName: "_CAnd.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 10, scope: !4)
diff --git a/toolchain/lower/testdata/operators/assignment.carbon b/toolchain/lower/testdata/operators/assignment.carbon
index 29a5e9afe9e2..3d1ecf486977 100644
--- a/toolchain/lower/testdata/operators/assignment.carbon
+++ b/toolchain/lower/testdata/operators/assignment.carbon
@@ -20,7 +20,7 @@ fn Main() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @tuple.loc15_5 = internal constant { i32, i32 } { i32 1, i32 2 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !4 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %a.var = alloca i32, align 4, !dbg !7
 // CHECK:STDOUT:   store i32 12, ptr %a.var, align 4, !dbg !8
@@ -44,7 +44,7 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "assignment.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/operators/not.carbon b/toolchain/lower/testdata/operators/not.carbon
index b9d494fd38d4..5eee885f9133 100644
--- a/toolchain/lower/testdata/operators/not.carbon
+++ b/toolchain/lower/testdata/operators/not.carbon
@@ -15,7 +15,7 @@ fn Not(b: bool) -> bool {
 // CHECK:STDOUT: ; ModuleID = 'not.carbon'
 // CHECK:STDOUT: source_filename = "not.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @Not(i1 %b) !dbg !4 {
+// CHECK:STDOUT: define i1 @_CNot.Main(i1 %b) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc12 = xor i1 %b, true, !dbg !7
 // CHECK:STDOUT:   ret i1 %.loc12, !dbg !8
@@ -28,7 +28,7 @@ fn Not(b: bool) -> bool {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "not.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Not", linkageName: "Not", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Not", linkageName: "_CNot.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 10, scope: !4)
diff --git a/toolchain/lower/testdata/operators/or.carbon b/toolchain/lower/testdata/operators/or.carbon
index cb4c7bd8674e..f3ff4786adef 100644
--- a/toolchain/lower/testdata/operators/or.carbon
+++ b/toolchain/lower/testdata/operators/or.carbon
@@ -18,24 +18,24 @@ fn Or() -> bool {
 // CHECK:STDOUT: ; ModuleID = 'or.carbon'
 // CHECK:STDOUT: source_filename = "or.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @F() !dbg !4 {
+// CHECK:STDOUT: define i1 @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i1 true, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @G() !dbg !8 {
+// CHECK:STDOUT: define i1 @_CG.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i1 true, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @Or() !dbg !10 {
+// CHECK:STDOUT: define i1 @_COr.Main() !dbg !10 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   %F.call = call i1 @F(), !dbg !11
+// CHECK:STDOUT:   %F.call = call i1 @_CF.Main(), !dbg !11
 // CHECK:STDOUT:   %.loc15_14.3 = xor i1 %F.call, true, !dbg !11
 // CHECK:STDOUT:   br i1 %.loc15_14.3, label %or.rhs, label %or.result, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: or.rhs:                                           ; preds = %entry
-// CHECK:STDOUT:   %G.call = call i1 @G(), !dbg !12
+// CHECK:STDOUT:   %G.call = call i1 @_CG.Main(), !dbg !12
 // CHECK:STDOUT:   br label %or.result, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: or.result:                                        ; preds = %or.rhs, %entry
@@ -50,13 +50,13 @@ fn Or() -> bool {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "or.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 18, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 12, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 12, column: 18, scope: !8)
-// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Or", linkageName: "Or", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !10 = distinct !DISubprogram(name: "Or", linkageName: "_COr.Main", scope: null, file: !3, line: 14, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !11 = !DILocation(line: 15, column: 10, scope: !10)
 // CHECK:STDOUT: !12 = !DILocation(line: 15, column: 17, scope: !10)
 // CHECK:STDOUT: !13 = !DILocation(line: 15, column: 3, scope: !10)
diff --git a/toolchain/lower/testdata/operators/or_empty_block.carbon b/toolchain/lower/testdata/operators/or_empty_block.carbon
index f56020b91c65..94988174f445 100644
--- a/toolchain/lower/testdata/operators/or_empty_block.carbon
+++ b/toolchain/lower/testdata/operators/or_empty_block.carbon
@@ -17,7 +17,7 @@ fn Or(b: bool, c: bool) -> bool {
 // CHECK:STDOUT: ; ModuleID = 'or_empty_block.carbon'
 // CHECK:STDOUT: source_filename = "or_empty_block.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i1 @Or(i1 %b, i1 %c) !dbg !4 {
+// CHECK:STDOUT: define i1 @_COr.Main(i1 %b, i1 %c) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc14_12.1 = xor i1 %b, true, !dbg !7
 // CHECK:STDOUT:   br i1 %.loc14_12.1, label %or.rhs, label %or.result, !dbg !7
@@ -37,7 +37,7 @@ fn Or(b: bool, c: bool) -> bool {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "or_empty_block.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Or", linkageName: "Or", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Or", linkageName: "_COr.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 10, scope: !4)
diff --git a/toolchain/lower/testdata/operators/overloaded.carbon b/toolchain/lower/testdata/operators/overloaded.carbon
index 9f31913a829e..972aafd7f781 100644
--- a/toolchain/lower/testdata/operators/overloaded.carbon
+++ b/toolchain/lower/testdata/operators/overloaded.carbon
@@ -31,7 +31,7 @@ fn Calculate(a: Number, b: Number) -> Number {
 // CHECK:STDOUT: ; ModuleID = 'overloaded.carbon'
 // CHECK:STDOUT: source_filename = "overloaded.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Op(ptr sret({ i1 }) %return, ptr %self) !dbg !4 {
+// CHECK:STDOUT: define void @"_COp.Number.Main:Negate.Core.Main"(ptr sret({ i1 }) %return, ptr %self) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc17_36.1.is_positive = getelementptr inbounds nuw { i1 }, ptr %self, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc17_36.2 = load i1, ptr %.loc17_36.1.is_positive, align 1, !dbg !7
@@ -41,7 +41,7 @@ fn Calculate(a: Number, b: Number) -> Number {
 // CHECK:STDOUT:   ret void, !dbg !10
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Op.1(ptr sret({ i1 }) %return, ptr %self, ptr %other) !dbg !11 {
+// CHECK:STDOUT: define void @"_COp.Number.Main:Mul.Core.Main"(ptr sret({ i1 }) %return, ptr %self, ptr %other) !dbg !11 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc22_33.1.is_positive = getelementptr inbounds nuw { i1 }, ptr %self, i32 0, i32 0, !dbg !12
 // CHECK:STDOUT:   %.loc22_33.2 = load i1, ptr %.loc22_33.1.is_positive, align 1, !dbg !12
@@ -80,11 +80,11 @@ fn Calculate(a: Number, b: Number) -> Number {
 // CHECK:STDOUT:   ret void, !dbg !20
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Calculate(ptr sret({ i1 }) %return, ptr %a, ptr %b) !dbg !21 {
+// CHECK:STDOUT: define void @_CCalculate.Main(ptr sret({ i1 }) %return, ptr %a, ptr %b) !dbg !21 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc28_10.3.temp = alloca { i1 }, align 8, !dbg !22
-// CHECK:STDOUT:   call void @Op(ptr %.loc28_10.3.temp, ptr %a), !dbg !22
-// CHECK:STDOUT:   call void @Op.1(ptr %return, ptr %.loc28_10.3.temp, ptr %b), !dbg !22
+// CHECK:STDOUT:   call void @"_COp.Number.Main:Negate.Core.Main"(ptr %.loc28_10.3.temp, ptr %a), !dbg !22
+// CHECK:STDOUT:   call void @"_COp.Number.Main:Mul.Core.Main"(ptr %return, ptr %.loc28_10.3.temp, ptr %b), !dbg !22
 // CHECK:STDOUT:   ret void, !dbg !23
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -98,14 +98,14 @@ fn Calculate(a: Number, b: Number) -> Number {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "overloaded.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Op", linkageName: "Op", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Op", linkageName: "_COp.Number.Main:Negate.Core.Main", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 17, column: 32, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 17, column: 28, scope: !4)
 // CHECK:STDOUT: !9 = !DILocation(line: 17, column: 12, scope: !4)
 // CHECK:STDOUT: !10 = !DILocation(line: 17, column: 5, scope: !4)
-// CHECK:STDOUT: !11 = distinct !DISubprogram(name: "Op", linkageName: "Op.1", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !11 = distinct !DISubprogram(name: "Op", linkageName: "_COp.Number.Main:Mul.Core.Main", scope: null, file: !3, line: 21, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !12 = !DILocation(line: 22, column: 29, scope: !11)
 // CHECK:STDOUT: !13 = !DILocation(line: 22, column: 50, scope: !11)
 // CHECK:STDOUT: !14 = !DILocation(line: 22, column: 28, scope: !11)
@@ -115,6 +115,6 @@ fn Calculate(a: Number, b: Number) -> Number {
 // CHECK:STDOUT: !18 = !DILocation(line: 23, column: 55, scope: !11)
 // CHECK:STDOUT: !19 = !DILocation(line: 22, column: 12, scope: !11)
 // CHECK:STDOUT: !20 = !DILocation(line: 22, column: 5, scope: !11)
-// CHECK:STDOUT: !21 = distinct !DISubprogram(name: "Calculate", linkageName: "Calculate", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !21 = distinct !DISubprogram(name: "Calculate", linkageName: "_CCalculate.Main", scope: null, file: !3, line: 27, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !22 = !DILocation(line: 28, column: 10, scope: !21)
 // CHECK:STDOUT: !23 = !DILocation(line: 28, column: 3, scope: !21)
diff --git a/toolchain/lower/testdata/packages/cross_package_call.carbon b/toolchain/lower/testdata/packages/cross_package_call.carbon
index 26141efa34f4..c24f020f2400 100644
--- a/toolchain/lower/testdata/packages/cross_package_call.carbon
+++ b/toolchain/lower/testdata/packages/cross_package_call.carbon
@@ -23,7 +23,7 @@ fn G() { A.F(); }
 // CHECK:STDOUT: ; ModuleID = 'a.carbon'
 // CHECK:STDOUT: source_filename = "a.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.A() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
@@ -35,20 +35,20 @@ fn G() { A.F(); }
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "a.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 4, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.A", scope: null, file: !3, line: 4, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 4, column: 1, scope: !4)
 // CHECK:STDOUT: ; ModuleID = 'b.carbon'
 // CHECK:STDOUT: source_filename = "b.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @G() !dbg !4 {
+// CHECK:STDOUT: define void @_CG.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @F(), !dbg !7
+// CHECK:STDOUT:   call void @_CF.A.Main(), !dbg !7
 // CHECK:STDOUT:   ret void, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @F()
+// CHECK:STDOUT: declare void @_CF.A.Main()
 // CHECK:STDOUT:
 // CHECK:STDOUT: !llvm.module.flags = !{!0, !1}
 // CHECK:STDOUT: !llvm.dbg.cu = !{!2}
@@ -57,7 +57,7 @@ fn G() { A.F(); }
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "b.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 4, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 4, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 4, column: 10, scope: !4)
diff --git a/toolchain/lower/testdata/pointer/address_of_field.carbon b/toolchain/lower/testdata/pointer/address_of_field.carbon
index df3c583a3805..56b7fa5e0161 100644
--- a/toolchain/lower/testdata/pointer/address_of_field.carbon
+++ b/toolchain/lower/testdata/pointer/address_of_field.carbon
@@ -20,16 +20,16 @@ fn F() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @struct.loc14_47 = internal constant { i32, i32 } { i32 1, i32 2 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @G(ptr)
+// CHECK:STDOUT: declare void @_CG.Main(ptr)
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %s.var = alloca { i32, i32 }, align 8, !dbg !7
 // CHECK:STDOUT:   %.loc14_46.2.a = getelementptr inbounds nuw { i32, i32 }, ptr %s.var, i32 0, i32 0, !dbg !8
 // CHECK:STDOUT:   %.loc14_46.4.b = getelementptr inbounds nuw { i32, i32 }, ptr %s.var, i32 0, i32 1, !dbg !8
 // CHECK:STDOUT:   call void @llvm.memcpy.p0.p0.i64(ptr align 4 %s.var, ptr align 4 @struct.loc14_47, i64 8, i1 false), !dbg !9
 // CHECK:STDOUT:   %.loc15_7.b = getelementptr inbounds nuw { i32, i32 }, ptr %s.var, i32 0, i32 1, !dbg !10
-// CHECK:STDOUT:   call void @G(ptr %.loc15_7.b), !dbg !11
+// CHECK:STDOUT:   call void @_CG.Main(ptr %.loc15_7.b), !dbg !11
 // CHECK:STDOUT:   ret void, !dbg !12
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -45,7 +45,7 @@ fn F() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "address_of_field.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/pointer/address_of_unused.carbon b/toolchain/lower/testdata/pointer/address_of_unused.carbon
index a1e32f5ec158..0b0f94d4fee0 100644
--- a/toolchain/lower/testdata/pointer/address_of_unused.carbon
+++ b/toolchain/lower/testdata/pointer/address_of_unused.carbon
@@ -16,7 +16,7 @@ fn F() {
 // CHECK:STDOUT: ; ModuleID = 'address_of_unused.carbon'
 // CHECK:STDOUT: source_filename = "address_of_unused.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %n.var = alloca i32, align 4, !dbg !7
 // CHECK:STDOUT:   store i32 0, ptr %n.var, align 4, !dbg !8
@@ -30,7 +30,7 @@ fn F() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "address_of_unused.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/pointer/basic.carbon b/toolchain/lower/testdata/pointer/basic.carbon
index 78dc095d67bf..3b3aade1fd60 100644
--- a/toolchain/lower/testdata/pointer/basic.carbon
+++ b/toolchain/lower/testdata/pointer/basic.carbon
@@ -20,17 +20,17 @@ fn F() -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'basic.carbon'
 // CHECK:STDOUT: source_filename = "basic.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @G(ptr %p) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CG.Main(ptr %p) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc12_10.2 = load i32, ptr %p, align 4, !dbg !7
 // CHECK:STDOUT:   ret i32 %.loc12_10.2, !dbg !8
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F() !dbg !9 {
+// CHECK:STDOUT: define i32 @_CF.Main() !dbg !9 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %n.var = alloca i32, align 4, !dbg !10
 // CHECK:STDOUT:   store i32 0, ptr %n.var, align 4, !dbg !11
-// CHECK:STDOUT:   %G.call = call i32 @G(ptr %n.var), !dbg !12
+// CHECK:STDOUT:   %G.call = call i32 @_CG.Main(ptr %n.var), !dbg !12
 // CHECK:STDOUT:   ret i32 %G.call, !dbg !13
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -41,12 +41,12 @@ fn F() -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "basic.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 10, scope: !4)
 // CHECK:STDOUT: !8 = !DILocation(line: 12, column: 3, scope: !4)
-// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !9 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !10 = !DILocation(line: 16, column: 7, scope: !9)
 // CHECK:STDOUT: !11 = !DILocation(line: 16, column: 3, scope: !9)
 // CHECK:STDOUT: !12 = !DILocation(line: 17, column: 10, scope: !9)
diff --git a/toolchain/lower/testdata/pointer/pointer_to_pointer.carbon b/toolchain/lower/testdata/pointer/pointer_to_pointer.carbon
index 142ae5745cde..e68f2c18f7c7 100644
--- a/toolchain/lower/testdata/pointer/pointer_to_pointer.carbon
+++ b/toolchain/lower/testdata/pointer/pointer_to_pointer.carbon
@@ -18,7 +18,7 @@ fn F(p: i32**) -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'pointer_to_pointer.carbon'
 // CHECK:STDOUT: source_filename = "pointer_to_pointer.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @F(ptr %p) !dbg !4 {
+// CHECK:STDOUT: define i32 @_CF.Main(ptr %p) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %a.var = alloca ptr, align 8, !dbg !7
 // CHECK:STDOUT:   store ptr %p, ptr %a.var, align 8, !dbg !8
@@ -40,7 +40,7 @@ fn F(p: i32**) -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "pointer_to_pointer.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/return/code_after_return.carbon b/toolchain/lower/testdata/return/code_after_return.carbon
index 40a3ce18b2a7..1df8d4b8d464 100644
--- a/toolchain/lower/testdata/return/code_after_return.carbon
+++ b/toolchain/lower/testdata/return/code_after_return.carbon
@@ -18,12 +18,12 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'code_after_return.carbon'
 // CHECK:STDOUT: source_filename = "code_after_return.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !8 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !8 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !9
 // CHECK:STDOUT: }
@@ -35,9 +35,9 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "code_after_return.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 1, scope: !4)
-// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !8 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !9 = !DILocation(line: 14, column: 3, scope: !8)
diff --git a/toolchain/lower/testdata/return/no_value.carbon b/toolchain/lower/testdata/return/no_value.carbon
index a4e43cf18441..7c9572b421cb 100644
--- a/toolchain/lower/testdata/return/no_value.carbon
+++ b/toolchain/lower/testdata/return/no_value.carbon
@@ -15,7 +15,7 @@ fn Main() {
 // CHECK:STDOUT: ; ModuleID = 'no_value.carbon'
 // CHECK:STDOUT: source_filename = "no_value.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Main() !dbg !4 {
+// CHECK:STDOUT: define void @_CMain.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret void, !dbg !7
 // CHECK:STDOUT: }
@@ -27,7 +27,7 @@ fn Main() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "no_value.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 3, scope: !4)
diff --git a/toolchain/lower/testdata/return/return_var.carbon b/toolchain/lower/testdata/return/return_var.carbon
index eb0ec185576e..3063aa012c0f 100644
--- a/toolchain/lower/testdata/return/return_var.carbon
+++ b/toolchain/lower/testdata/return/return_var.carbon
@@ -22,7 +22,7 @@ fn Make() -> C {
 // CHECK:STDOUT: ; ModuleID = 'return_var.carbon'
 // CHECK:STDOUT: source_filename = "return_var.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @Make(ptr sret({ i32, ptr }) %return) !dbg !4 {
+// CHECK:STDOUT: define void @_CMake.Main(ptr sret({ i32, ptr }) %return) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc18_30.2.data = getelementptr inbounds nuw { i32, ptr }, ptr %return, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc18_30.4.next = getelementptr inbounds nuw { i32, ptr }, ptr %return, i32 0, i32 1, !dbg !7
@@ -37,7 +37,7 @@ fn Make() -> C {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "return_var.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Make", linkageName: "Make", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Make", linkageName: "_CMake.Main", scope: null, file: !3, line: 16, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 18, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/return/return_var_byval.carbon b/toolchain/lower/testdata/return/return_var_byval.carbon
index 8a51a51c0d01..55c420829238 100644
--- a/toolchain/lower/testdata/return/return_var_byval.carbon
+++ b/toolchain/lower/testdata/return/return_var_byval.carbon
@@ -16,7 +16,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'return_var_byval.carbon'
 // CHECK:STDOUT: source_filename = "return_var_byval.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Main() !dbg !4 {
+// CHECK:STDOUT: define i32 @_CMain.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %x.var = alloca i32, align 4, !dbg !7
 // CHECK:STDOUT:   store i32 0, ptr %x.var, align 4, !dbg !8
@@ -31,7 +31,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "return_var_byval.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 16, scope: !4)
diff --git a/toolchain/lower/testdata/return/value.carbon b/toolchain/lower/testdata/return/value.carbon
index 9acf6aab27e8..0a772d6970f5 100644
--- a/toolchain/lower/testdata/return/value.carbon
+++ b/toolchain/lower/testdata/return/value.carbon
@@ -15,7 +15,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'value.carbon'
 // CHECK:STDOUT: source_filename = "value.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Main() !dbg !4 {
+// CHECK:STDOUT: define i32 @_CMain.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   ret i32 0, !dbg !7
 // CHECK:STDOUT: }
@@ -27,7 +27,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "value.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 3, scope: !4)
diff --git a/toolchain/lower/testdata/return/var.carbon b/toolchain/lower/testdata/return/var.carbon
index ff723860a62a..271544d634ef 100644
--- a/toolchain/lower/testdata/return/var.carbon
+++ b/toolchain/lower/testdata/return/var.carbon
@@ -16,7 +16,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT: ; ModuleID = 'var.carbon'
 // CHECK:STDOUT: source_filename = "var.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: define i32 @Main() !dbg !4 {
+// CHECK:STDOUT: define i32 @_CMain.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %x.var = alloca i32, align 4, !dbg !7
 // CHECK:STDOUT:   store i32 0, ptr %x.var, align 4, !dbg !8
@@ -31,7 +31,7 @@ fn Main() -> i32 {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "var.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "Main", linkageName: "_CMain.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 12, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/struct/nested_struct_in_place.carbon b/toolchain/lower/testdata/struct/nested_struct_in_place.carbon
index 71ce1ebbdf46..b110ded86f14 100644
--- a/toolchain/lower/testdata/struct/nested_struct_in_place.carbon
+++ b/toolchain/lower/testdata/struct/nested_struct_in_place.carbon
@@ -17,15 +17,15 @@ fn G() {
 // CHECK:STDOUT: ; ModuleID = 'nested_struct_in_place.carbon'
 // CHECK:STDOUT: source_filename = "nested_struct_in_place.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @F(ptr sret({ i32, i32, i32 }))
+// CHECK:STDOUT: declare void @_CF.Main(ptr sret({ i32, i32, i32 }))
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @G() !dbg !4 {
+// CHECK:STDOUT: define void @_CG.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %v.var = alloca { { i32, i32, i32 }, { i32, i32, i32 } }, align 8, !dbg !7
 // CHECK:STDOUT:   %.loc14_74.1.a = getelementptr inbounds nuw { { i32, i32, i32 }, { i32, i32, i32 } }, ptr %v.var, i32 0, i32 0, !dbg !8
-// CHECK:STDOUT:   call void @F(ptr %.loc14_74.1.a), !dbg !9
+// CHECK:STDOUT:   call void @_CF.Main(ptr %.loc14_74.1.a), !dbg !9
 // CHECK:STDOUT:   %.loc14_74.2.b = getelementptr inbounds nuw { { i32, i32, i32 }, { i32, i32, i32 } }, ptr %v.var, i32 0, i32 1, !dbg !8
-// CHECK:STDOUT:   call void @F(ptr %.loc14_74.2.b), !dbg !10
+// CHECK:STDOUT:   call void @_CF.Main(ptr %.loc14_74.2.b), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -36,7 +36,7 @@ fn G() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "nested_struct_in_place.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/tuple/access/return_value_access.carbon b/toolchain/lower/testdata/tuple/access/return_value_access.carbon
index 112baa51e642..8bada903f65b 100644
--- a/toolchain/lower/testdata/tuple/access/return_value_access.carbon
+++ b/toolchain/lower/testdata/tuple/access/return_value_access.carbon
@@ -19,7 +19,7 @@ fn Run() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: @tuple.loc11_39 = internal constant { i32, i32 } { i32 12, i32 24 }
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F(ptr sret({ i32, i32 }) %return) !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main(ptr sret({ i32, i32 }) %return) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %.loc11_38.2.tuple.elem = getelementptr inbounds nuw { i32, i32 }, ptr %return, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   %.loc11_38.4.tuple.elem = getelementptr inbounds nuw { i32, i32 }, ptr %return, i32 0, i32 1, !dbg !7
@@ -31,7 +31,7 @@ fn Run() {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %t.var = alloca i32, align 4, !dbg !10
 // CHECK:STDOUT:   %.loc14_17.1.temp = alloca { i32, i32 }, align 8, !dbg !11
-// CHECK:STDOUT:   call void @F(ptr %.loc14_17.1.temp), !dbg !11
+// CHECK:STDOUT:   call void @_CF.Main(ptr %.loc14_17.1.temp), !dbg !11
 // CHECK:STDOUT:   %.loc14_19.1.tuple.index = getelementptr inbounds nuw { i32, i32 }, ptr %.loc14_17.1.temp, i32 0, i32 1, !dbg !11
 // CHECK:STDOUT:   %.loc14_19.2 = load i32, ptr %.loc14_19.1.tuple.index, align 4, !dbg !11
 // CHECK:STDOUT:   store i32 %.loc14_19.2, ptr %t.var, align 4, !dbg !12
@@ -50,7 +50,7 @@ fn Run() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "return_value_access.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 11, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 11, column: 31, scope: !4)
diff --git a/toolchain/lower/testdata/tuple/nested_tuple_in_place.carbon b/toolchain/lower/testdata/tuple/nested_tuple_in_place.carbon
index 2384cb65bfcf..f832080472fc 100644
--- a/toolchain/lower/testdata/tuple/nested_tuple_in_place.carbon
+++ b/toolchain/lower/testdata/tuple/nested_tuple_in_place.carbon
@@ -17,15 +17,15 @@ fn G() {
 // CHECK:STDOUT: ; ModuleID = 'nested_tuple_in_place.carbon'
 // CHECK:STDOUT: source_filename = "nested_tuple_in_place.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @F(ptr sret({ i32, i32, i32 }))
+// CHECK:STDOUT: declare void @_CF.Main(ptr sret({ i32, i32, i32 }))
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @G() !dbg !4 {
+// CHECK:STDOUT: define void @_CG.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %v.var = alloca { { i32, i32, i32 }, { i32, i32, i32 } }, align 8, !dbg !7
 // CHECK:STDOUT:   %.loc14_56.1.tuple.elem = getelementptr inbounds nuw { { i32, i32, i32 }, { i32, i32, i32 } }, ptr %v.var, i32 0, i32 0, !dbg !8
-// CHECK:STDOUT:   call void @F(ptr %.loc14_56.1.tuple.elem), !dbg !9
+// CHECK:STDOUT:   call void @_CF.Main(ptr %.loc14_56.1.tuple.elem), !dbg !9
 // CHECK:STDOUT:   %.loc14_56.2.tuple.elem = getelementptr inbounds nuw { { i32, i32, i32 }, { i32, i32, i32 } }, ptr %v.var, i32 0, i32 1, !dbg !8
-// CHECK:STDOUT:   call void @F(ptr %.loc14_56.2.tuple.elem), !dbg !10
+// CHECK:STDOUT:   call void @_CF.Main(ptr %.loc14_56.2.tuple.elem), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -36,7 +36,7 @@ fn G() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "nested_tuple_in_place.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "G", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "G", linkageName: "_CG.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/tuple/value_formation.carbon b/toolchain/lower/testdata/tuple/value_formation.carbon
index d99f713376b0..228a6f108229 100644
--- a/toolchain/lower/testdata/tuple/value_formation.carbon
+++ b/toolchain/lower/testdata/tuple/value_formation.carbon
@@ -19,9 +19,9 @@ fn F() {
 // CHECK:STDOUT: ; ModuleID = 'value_formation.carbon'
 // CHECK:STDOUT: source_filename = "value_formation.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @G(ptr)
+// CHECK:STDOUT: declare void @_CG.Main(ptr)
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F() !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %a.var = alloca { i32, i32, i32 }, align 8, !dbg !7
 // CHECK:STDOUT:   %b.var = alloca { i32, i32, i32 }, align 8, !dbg !8
@@ -56,7 +56,7 @@ fn F() {
 // CHECK:STDOUT:   store ptr %tuple.loc16_6, ptr %tuple.loc16_107, align 8, !dbg !11
 // CHECK:STDOUT:   %tuple.loc16_108 = getelementptr inbounds nuw { ptr, ptr }, ptr %tuple.loc16_10, i32 0, i32 1, !dbg !11
 // CHECK:STDOUT:   store ptr %tuple.loc16_9, ptr %tuple.loc16_108, align 8, !dbg !11
-// CHECK:STDOUT:   call void @G(ptr %tuple.loc16_10), !dbg !12
+// CHECK:STDOUT:   call void @_CG.Main(ptr %tuple.loc16_10), !dbg !12
 // CHECK:STDOUT:   ret void, !dbg !13
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -67,7 +67,7 @@ fn F() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "value_formation.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 7, scope: !4)
diff --git a/toolchain/lower/testdata/tuple/value_forwarding.carbon b/toolchain/lower/testdata/tuple/value_forwarding.carbon
index 059fcaee7fd9..3eec759e1aa5 100644
--- a/toolchain/lower/testdata/tuple/value_forwarding.carbon
+++ b/toolchain/lower/testdata/tuple/value_forwarding.carbon
@@ -17,16 +17,16 @@ fn F(a: (i32, i32, i32), b: (i32, i32, i32)) {
 // CHECK:STDOUT: ; ModuleID = 'value_forwarding.carbon'
 // CHECK:STDOUT: source_filename = "value_forwarding.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @G(ptr)
+// CHECK:STDOUT: declare void @_CG.Main(ptr)
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @F(ptr %a, ptr %b) !dbg !4 {
+// CHECK:STDOUT: define void @_CF.Main(ptr %a, ptr %b) !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   %tuple = alloca { ptr, ptr }, align 8, !dbg !7
 // CHECK:STDOUT:   %tuple1 = getelementptr inbounds nuw { ptr, ptr }, ptr %tuple, i32 0, i32 0, !dbg !7
 // CHECK:STDOUT:   store ptr %a, ptr %tuple1, align 8, !dbg !7
 // CHECK:STDOUT:   %tuple2 = getelementptr inbounds nuw { ptr, ptr }, ptr %tuple, i32 0, i32 1, !dbg !7
 // CHECK:STDOUT:   store ptr %b, ptr %tuple2, align 8, !dbg !7
-// CHECK:STDOUT:   call void @G(ptr %tuple), !dbg !8
+// CHECK:STDOUT:   call void @_CG.Main(ptr %tuple), !dbg !8
 // CHECK:STDOUT:   ret void, !dbg !9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -37,7 +37,7 @@ fn F(a: (i32, i32, i32), b: (i32, i32, i32)) {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "value_forwarding.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "F", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "F", linkageName: "_CF.Main", scope: null, file: !3, line: 13, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 14, column: 5, scope: !4)
diff --git a/toolchain/lower/testdata/while/break_continue.carbon b/toolchain/lower/testdata/while/break_continue.carbon
index 2d7cb52accc0..99f76bb98644 100644
--- a/toolchain/lower/testdata/while/break_continue.carbon
+++ b/toolchain/lower/testdata/while/break_continue.carbon
@@ -22,29 +22,29 @@ fn While() {
 // CHECK:STDOUT: ; ModuleID = 'break_continue.carbon'
 // CHECK:STDOUT: source_filename = "break_continue.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare i1 @A()
+// CHECK:STDOUT: declare i1 @_CA.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare i1 @B()
+// CHECK:STDOUT: declare i1 @_CB.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare i1 @C()
+// CHECK:STDOUT: declare i1 @_CC.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @While() !dbg !4 {
+// CHECK:STDOUT: define void @_CWhile.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   br label %while.cond
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.cond:                                       ; preds = %entry, %if.else.loc18, %if.then.loc17
-// CHECK:STDOUT:   %A.call = call i1 @A(), !dbg !7
+// CHECK:STDOUT:   %A.call = call i1 @_CA.Main(), !dbg !7
 // CHECK:STDOUT:   br i1 %A.call, label %while.body, label %while.done, !dbg !8
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.body:                                       ; preds = %while.cond
-// CHECK:STDOUT:   %B.call = call i1 @B(), !dbg !9
+// CHECK:STDOUT:   %B.call = call i1 @_CB.Main(), !dbg !9
 // CHECK:STDOUT:   br i1 %B.call, label %if.then.loc17, label %if.else.loc17, !dbg !10
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.then.loc17:                                    ; preds = %while.body
 // CHECK:STDOUT:   br label %while.cond, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.else.loc17:                                    ; preds = %while.body
-// CHECK:STDOUT:   %C.call = call i1 @C(), !dbg !12
+// CHECK:STDOUT:   %C.call = call i1 @_CC.Main(), !dbg !12
 // CHECK:STDOUT:   br i1 %C.call, label %if.then.loc18, label %if.else.loc18, !dbg !13
 // CHECK:STDOUT:
 // CHECK:STDOUT: if.then.loc18:                                    ; preds = %if.else.loc17
@@ -67,7 +67,7 @@ fn While() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "break_continue.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "While", linkageName: "While", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "While", linkageName: "_CWhile.Main", scope: null, file: !3, line: 15, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 16, column: 10, scope: !4)
diff --git a/toolchain/lower/testdata/while/preheader.carbon b/toolchain/lower/testdata/while/preheader.carbon
index ba9d51a05fe9..b0ebec653b4a 100644
--- a/toolchain/lower/testdata/while/preheader.carbon
+++ b/toolchain/lower/testdata/while/preheader.carbon
@@ -31,34 +31,34 @@ fn While() {
 // CHECK:STDOUT: ; ModuleID = 'preheader.carbon'
 // CHECK:STDOUT: source_filename = "preheader.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare i1 @Cond()
+// CHECK:STDOUT: declare i1 @_CCond.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @F()
+// CHECK:STDOUT: declare void @_CF.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @G()
+// CHECK:STDOUT: declare void @_CG.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @While() !dbg !4 {
+// CHECK:STDOUT: define void @_CWhile.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
 // CHECK:STDOUT:   br label %while.cond.loc20
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.cond.loc20:                                 ; preds = %entry, %while.body.loc20
-// CHECK:STDOUT:   %Cond.call.loc20 = call i1 @Cond(), !dbg !7
+// CHECK:STDOUT:   %Cond.call.loc20 = call i1 @_CCond.Main(), !dbg !7
 // CHECK:STDOUT:   br i1 %Cond.call.loc20, label %while.body.loc20, label %while.done.loc20, !dbg !8
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.body.loc20:                                 ; preds = %while.cond.loc20
-// CHECK:STDOUT:   call void @F(), !dbg !9
+// CHECK:STDOUT:   call void @_CF.Main(), !dbg !9
 // CHECK:STDOUT:   br label %while.cond.loc20, !dbg !10
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.done.loc20:                                 ; preds = %while.cond.loc20
-// CHECK:STDOUT:   %Cond.call.loc24 = call i1 @Cond(), !dbg !11
+// CHECK:STDOUT:   %Cond.call.loc24 = call i1 @_CCond.Main(), !dbg !11
 // CHECK:STDOUT:   br i1 %Cond.call.loc24, label %while.cond.loc25, label %if.else, !dbg !12
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.cond.loc25:                                 ; preds = %while.body.loc25, %while.done.loc20
-// CHECK:STDOUT:   %Cond.call.loc25 = call i1 @Cond(), !dbg !13
+// CHECK:STDOUT:   %Cond.call.loc25 = call i1 @_CCond.Main(), !dbg !13
 // CHECK:STDOUT:   br i1 %Cond.call.loc25, label %while.body.loc25, label %while.done.loc25, !dbg !14
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.body.loc25:                                 ; preds = %while.cond.loc25
-// CHECK:STDOUT:   call void @G(), !dbg !15
+// CHECK:STDOUT:   call void @_CG.Main(), !dbg !15
 // CHECK:STDOUT:   br label %while.cond.loc25, !dbg !16
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.done.loc25:                                 ; preds = %while.cond.loc25
@@ -78,7 +78,7 @@ fn While() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "preheader.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "While", linkageName: "While", scope: null, file: !3, line: 19, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "While", linkageName: "_CWhile.Main", scope: null, file: !3, line: 19, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 20, column: 10, scope: !4)
diff --git a/toolchain/lower/testdata/while/unreachable_end.carbon b/toolchain/lower/testdata/while/unreachable_end.carbon
index 8a50e4bb4e78..abd175daba7e 100644
--- a/toolchain/lower/testdata/while/unreachable_end.carbon
+++ b/toolchain/lower/testdata/while/unreachable_end.carbon
@@ -26,29 +26,29 @@ fn While() {
 // CHECK:STDOUT: ; ModuleID = 'unreachable_end.carbon'
 // CHECK:STDOUT: source_filename = "unreachable_end.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare i1 @Cond()
+// CHECK:STDOUT: declare i1 @_CCond.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @F()
+// CHECK:STDOUT: declare void @_CF.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @G()
+// CHECK:STDOUT: declare void @_CG.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @H()
+// CHECK:STDOUT: declare void @_CH.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @While() !dbg !4 {
+// CHECK:STDOUT: define void @_CWhile.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @F(), !dbg !7
+// CHECK:STDOUT:   call void @_CF.Main(), !dbg !7
 // CHECK:STDOUT:   br label %while.cond, !dbg !8
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.cond:                                       ; preds = %entry
-// CHECK:STDOUT:   %Cond.call = call i1 @Cond(), !dbg !9
+// CHECK:STDOUT:   %Cond.call = call i1 @_CCond.Main(), !dbg !9
 // CHECK:STDOUT:   br i1 %Cond.call, label %while.body, label %while.done, !dbg !8
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.body:                                       ; preds = %while.cond
-// CHECK:STDOUT:   call void @G(), !dbg !10
+// CHECK:STDOUT:   call void @_CG.Main(), !dbg !10
 // CHECK:STDOUT:   ret void, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.done:                                       ; preds = %while.cond
-// CHECK:STDOUT:   call void @H(), !dbg !12
+// CHECK:STDOUT:   call void @_CH.Main(), !dbg !12
 // CHECK:STDOUT:   ret void, !dbg !13
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -59,7 +59,7 @@ fn While() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "unreachable_end.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "While", linkageName: "While", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "While", linkageName: "_CWhile.Main", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 18, column: 3, scope: !4)
diff --git a/toolchain/lower/testdata/while/while.carbon b/toolchain/lower/testdata/while/while.carbon
index 8708aacd45b4..c00fe82f5d26 100644
--- a/toolchain/lower/testdata/while/while.carbon
+++ b/toolchain/lower/testdata/while/while.carbon
@@ -25,29 +25,29 @@ fn While() {
 // CHECK:STDOUT: ; ModuleID = 'while.carbon'
 // CHECK:STDOUT: source_filename = "while.carbon"
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare i1 @Cond()
+// CHECK:STDOUT: declare i1 @_CCond.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @F()
+// CHECK:STDOUT: declare void @_CF.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @G()
+// CHECK:STDOUT: declare void @_CG.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: declare void @H()
+// CHECK:STDOUT: declare void @_CH.Main()
 // CHECK:STDOUT:
-// CHECK:STDOUT: define void @While() !dbg !4 {
+// CHECK:STDOUT: define void @_CWhile.Main() !dbg !4 {
 // CHECK:STDOUT: entry:
-// CHECK:STDOUT:   call void @F(), !dbg !7
+// CHECK:STDOUT:   call void @_CF.Main(), !dbg !7
 // CHECK:STDOUT:   br label %while.cond, !dbg !8
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.cond:                                       ; preds = %while.body, %entry
-// CHECK:STDOUT:   %Cond.call = call i1 @Cond(), !dbg !9
+// CHECK:STDOUT:   %Cond.call = call i1 @_CCond.Main(), !dbg !9
 // CHECK:STDOUT:   br i1 %Cond.call, label %while.body, label %while.done, !dbg !8
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.body:                                       ; preds = %while.cond
-// CHECK:STDOUT:   call void @G(), !dbg !10
+// CHECK:STDOUT:   call void @_CG.Main(), !dbg !10
 // CHECK:STDOUT:   br label %while.cond, !dbg !11
 // CHECK:STDOUT:
 // CHECK:STDOUT: while.done:                                       ; preds = %while.cond
-// CHECK:STDOUT:   call void @H(), !dbg !12
+// CHECK:STDOUT:   call void @_CH.Main(), !dbg !12
 // CHECK:STDOUT:   ret void, !dbg !13
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -58,7 +58,7 @@ fn While() {
 // CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
 // CHECK:STDOUT: !2 = distinct !DICompileUnit(language: DW_LANG_C, file: !3, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
 // CHECK:STDOUT: !3 = !DIFile(filename: "while.carbon", directory: "")
-// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "While", linkageName: "While", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
+// CHECK:STDOUT: !4 = distinct !DISubprogram(name: "While", linkageName: "_CWhile.Main", scope: null, file: !3, line: 17, type: !5, spFlags: DISPFlagDefinition, unit: !2)
 // CHECK:STDOUT: !5 = !DISubroutineType(types: !6)
 // CHECK:STDOUT: !6 = !{}
 // CHECK:STDOUT: !7 = !DILocation(line: 18, column: 3, scope: !4)