diff --git a/toolchain/diagnostics/diagnostic_kind.def b/toolchain/diagnostics/diagnostic_kind.def
index 832a3535fe15..8fb84b6ef4e9 100644
--- a/toolchain/diagnostics/diagnostic_kind.def
+++ b/toolchain/diagnostics/diagnostic_kind.def
@@ -102,6 +102,7 @@ CARBON_DIAGNOSTIC_KIND(PreviousDefinition)
 CARBON_DIAGNOSTIC_KIND(NoMatchingCall)
 CARBON_DIAGNOSTIC_KIND(CallArgCountMismatch)
 CARBON_DIAGNOSTIC_KIND(CallArgTypeMismatch)
+CARBON_DIAGNOSTIC_KIND(MissingReturnStatement)
 CARBON_DIAGNOSTIC_KIND(ReturnStatementDisallowExpression)
 CARBON_DIAGNOSTIC_KIND(ReturnStatementImplicitNote)
 CARBON_DIAGNOSTIC_KIND(ReturnStatementMissingExpression)
diff --git a/toolchain/lowering/testdata/function/call/empty_struct.carbon b/toolchain/lowering/testdata/function/call/empty_struct.carbon
index 8d83682b0c27..30d8488e84f5 100644
--- a/toolchain/lowering/testdata/function/call/empty_struct.carbon
+++ b/toolchain/lowering/testdata/function/call/empty_struct.carbon
@@ -22,6 +22,7 @@
 // CHECK:STDOUT:   %1 = load %StructLiteralType, ptr %StructLiteralValue1, align 1
 // CHECK:STDOUT:   %Echo = call %StructLiteralType @Echo(%StructLiteralType %1)
 // CHECK:STDOUT:   store %StructLiteralType %Echo, ptr %var, align 1
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Echo(a: {}) -> {} {
diff --git a/toolchain/lowering/testdata/function/call/i32.carbon b/toolchain/lowering/testdata/function/call/i32.carbon
index 79c2bc6a76ef..d3ebf47fa278 100644
--- a/toolchain/lowering/testdata/function/call/i32.carbon
+++ b/toolchain/lowering/testdata/function/call/i32.carbon
@@ -16,6 +16,7 @@
 // CHECK:STDOUT:   %var = alloca i32, align 4
 // CHECK:STDOUT:   %Echo = call i32 @Echo(i32 1)
 // CHECK:STDOUT:   store i32 %Echo, ptr %var, align 4
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Echo(a: i32) -> i32 {
diff --git a/toolchain/lowering/testdata/function/call/params_one.carbon b/toolchain/lowering/testdata/function/call/params_one.carbon
index 87f8392c15b8..a1442c908722 100644
--- a/toolchain/lowering/testdata/function/call/params_one.carbon
+++ b/toolchain/lowering/testdata/function/call/params_one.carbon
@@ -9,10 +9,12 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Foo(i32 %a) {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Main() {
 // CHECK:STDOUT:   %Foo = call %EmptyTupleType @Foo(i32 1)
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Foo(a: i32) {}
diff --git a/toolchain/lowering/testdata/function/call/params_one_comma.carbon b/toolchain/lowering/testdata/function/call/params_one_comma.carbon
index c529c36e118d..5c772dd732ca 100644
--- a/toolchain/lowering/testdata/function/call/params_one_comma.carbon
+++ b/toolchain/lowering/testdata/function/call/params_one_comma.carbon
@@ -9,11 +9,13 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Foo(i32 %a) {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Main() {
 // CHECK:STDOUT:   %Foo = call %EmptyTupleType @Foo(i32 1)
 // CHECK:STDOUT:   %Foo1 = call %EmptyTupleType @Foo(i32 1)
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Foo(a: i32,) {}
diff --git a/toolchain/lowering/testdata/function/call/params_two.carbon b/toolchain/lowering/testdata/function/call/params_two.carbon
index c40e4605459d..6182e7931692 100644
--- a/toolchain/lowering/testdata/function/call/params_two.carbon
+++ b/toolchain/lowering/testdata/function/call/params_two.carbon
@@ -9,10 +9,12 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Foo(i32 %a, i32 %b) {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Main() {
 // CHECK:STDOUT:   %Foo = call %EmptyTupleType @Foo(i32 1, i32 2)
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Foo(a: i32, b: i32) {}
diff --git a/toolchain/lowering/testdata/function/call/params_two_comma.carbon b/toolchain/lowering/testdata/function/call/params_two_comma.carbon
index 883024e23a4c..f65ebd4fb7b5 100644
--- a/toolchain/lowering/testdata/function/call/params_two_comma.carbon
+++ b/toolchain/lowering/testdata/function/call/params_two_comma.carbon
@@ -9,11 +9,13 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Foo(i32 %a, i32 %b) {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Main() {
 // CHECK:STDOUT:   %Foo = call %EmptyTupleType @Foo(i32 1, i32 2)
 // CHECK:STDOUT:   %Foo1 = call %EmptyTupleType @Foo(i32 1, i32 2)
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Foo(a: i32, b: i32,) {}
diff --git a/toolchain/lowering/testdata/function/call/params_zero.carbon b/toolchain/lowering/testdata/function/call/params_zero.carbon
index 7c0a6bd3cb2f..f16003c3833c 100644
--- a/toolchain/lowering/testdata/function/call/params_zero.carbon
+++ b/toolchain/lowering/testdata/function/call/params_zero.carbon
@@ -9,10 +9,12 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Foo() {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Main() {
 // CHECK:STDOUT:   %Foo = call %EmptyTupleType @Foo()
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Foo() {}
diff --git a/toolchain/lowering/testdata/function/call/var_param.carbon b/toolchain/lowering/testdata/function/call/var_param.carbon
index 2cac6168de68..66e765a0b2d5 100644
--- a/toolchain/lowering/testdata/function/call/var_param.carbon
+++ b/toolchain/lowering/testdata/function/call/var_param.carbon
@@ -9,6 +9,7 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @DoNothing(i32 %a) {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Main() {
@@ -16,6 +17,7 @@
 // CHECK:STDOUT:   store i32 0, ptr %var, align 4
 // CHECK:STDOUT:   %1 = load i32, ptr %var, align 4
 // CHECK:STDOUT:   %DoNothing = call %EmptyTupleType @DoNothing(i32 %1)
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn DoNothing(a: i32) {}
diff --git a/toolchain/lowering/testdata/function/definition/empty_struct.carbon b/toolchain/lowering/testdata/function/definition/empty_struct.carbon
index 6661561e27d3..a32ae3ab7489 100644
--- a/toolchain/lowering/testdata/function/definition/empty_struct.carbon
+++ b/toolchain/lowering/testdata/function/definition/empty_struct.carbon
@@ -10,6 +10,7 @@
 // CHECK:STDOUT: %StructLiteralType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Echo(%StructLiteralType %a) {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Echo(a: {}) {
diff --git a/toolchain/lowering/testdata/function/definition/params_one.carbon b/toolchain/lowering/testdata/function/definition/params_one.carbon
index ee5842ebaa0d..22fac8ed45ad 100644
--- a/toolchain/lowering/testdata/function/definition/params_one.carbon
+++ b/toolchain/lowering/testdata/function/definition/params_one.carbon
@@ -9,6 +9,7 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Foo(i32 %a) {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Foo(a: i32) {}
diff --git a/toolchain/lowering/testdata/function/definition/params_two.carbon b/toolchain/lowering/testdata/function/definition/params_two.carbon
index d65d5f612811..6afd53193f5e 100644
--- a/toolchain/lowering/testdata/function/definition/params_two.carbon
+++ b/toolchain/lowering/testdata/function/definition/params_two.carbon
@@ -9,6 +9,7 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Foo(i32 %a, i32 %b) {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Foo(a: i32, b: i32) {}
diff --git a/toolchain/lowering/testdata/function/definition/params_zero.carbon b/toolchain/lowering/testdata/function/definition/params_zero.carbon
index 32c5109a3b0a..a80bdcde059c 100644
--- a/toolchain/lowering/testdata/function/definition/params_zero.carbon
+++ b/toolchain/lowering/testdata/function/definition/params_zero.carbon
@@ -9,6 +9,7 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @Foo() {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn Foo() {}
diff --git a/toolchain/lowering/testdata/if/else.carbon b/toolchain/lowering/testdata/if/else.carbon
index 7b0729f3b276..653679267319 100644
--- a/toolchain/lowering/testdata/if/else.carbon
+++ b/toolchain/lowering/testdata/if/else.carbon
@@ -9,12 +9,15 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @F() {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @G() {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @H() {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @If(i1 %b) {
@@ -30,6 +33,7 @@
 // CHECK:STDOUT:
 // CHECK:STDOUT: 3:                                                ; preds = %2, %1
 // CHECK:STDOUT:   %H = call %EmptyTupleType @H()
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn F() {}
diff --git a/toolchain/lowering/testdata/if/no_else.carbon b/toolchain/lowering/testdata/if/no_else.carbon
index 44e6c4ca4fb1..4c13e480c1db 100644
--- a/toolchain/lowering/testdata/if/no_else.carbon
+++ b/toolchain/lowering/testdata/if/no_else.carbon
@@ -9,9 +9,11 @@
 // CHECK:STDOUT: %EmptyTupleType = type {}
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @F() {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @G() {
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: define %EmptyTupleType @If(i1 %b) {
@@ -23,6 +25,7 @@
 // CHECK:STDOUT:
 // CHECK:STDOUT: 2:                                                ; preds = %1, %0
 // CHECK:STDOUT:   %G = call %EmptyTupleType @G()
+// CHECK:STDOUT:   ret void
 // CHECK:STDOUT: }
 
 fn F() {}
diff --git a/toolchain/lowering/testdata/return/code_after_return.carbon b/toolchain/lowering/testdata/return/code_after_return.carbon
new file mode 100644
index 000000000000..9e7608c3d089
--- /dev/null
+++ b/toolchain/lowering/testdata/return/code_after_return.carbon
@@ -0,0 +1,24 @@
+// 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
+//
+// AUTOUPDATE
+// CHECK:STDOUT: ; ModuleID = 'code_after_return.carbon'
+// CHECK:STDOUT: source_filename = "code_after_return.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: %EmptyTupleType = type {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: define %EmptyTupleType @F() {
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: define %EmptyTupleType @Main() {
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }
+
+fn F() {}
+
+fn Main() {
+  return;
+  F();
+}
diff --git a/toolchain/semantics/BUILD b/toolchain/semantics/BUILD
index c649ebfbfb77..8e56772bfd71 100644
--- a/toolchain/semantics/BUILD
+++ b/toolchain/semantics/BUILD
@@ -32,24 +32,12 @@ cc_library(
         ":semantics_node_kind",
         "//common:check",
         "//common:ostream",
+        "//toolchain/common:index_base",
         "//toolchain/parser:parse_tree",
         "@llvm-project//llvm:Support",
     ],
 )
 
-cc_library(
-    name = "semantics_node_block_stack",
-    srcs = ["semantics_node_block_stack.cpp"],
-    hdrs = ["semantics_node_block_stack.h"],
-    deps = [
-        ":semantics_node",
-        "//common:check",
-        "//common:ostream",
-        "//common:vlog",
-        "@llvm-project//llvm:Support",
-    ],
-)
-
 cc_library(
     name = "semantics_node_stack",
     srcs = ["semantics_node_stack.cpp"],
@@ -70,6 +58,7 @@ cc_library(
     srcs = [
         "semantics_ir.cpp",
         "semantics_context.cpp",
+        "semantics_node_block_stack.cpp",
     ] +
     # Glob handler files to avoid missing anyway.
     glob([
@@ -78,11 +67,11 @@ cc_library(
     hdrs = [
         "semantics_context.h",
         "semantics_ir.h",
+        "semantics_node_block_stack.h",
     ],
     deps = [
         ":semantics_builtin_kind",
         ":semantics_node",
-        ":semantics_node_block_stack",
         ":semantics_node_stack",
         "//common:check",
         "//common:ostream",
diff --git a/toolchain/semantics/semantics_context.cpp b/toolchain/semantics/semantics_context.cpp
index 8d6b6e969fcd..004bfd6d20bc 100644
--- a/toolchain/semantics/semantics_context.cpp
+++ b/toolchain/semantics/semantics_context.cpp
@@ -28,11 +28,9 @@ SemanticsContext::SemanticsContext(const TokenizedBuffer& tokens,
       semantics_ir_(&semantics_ir),
       vlog_stream_(vlog_stream),
       node_stack_(parse_tree, vlog_stream),
-      node_block_stack_("node_block_stack_", semantics_ir.node_blocks(),
-                        vlog_stream),
-      params_or_args_stack_("params_or_args_stack_", semantics_ir.node_blocks(),
-                            vlog_stream),
-      args_type_info_stack_("args_type_info_stack_", semantics_ir.node_blocks(),
+      node_block_stack_("node_block_stack_", semantics_ir, vlog_stream),
+      params_or_args_stack_("params_or_args_stack_", semantics_ir, vlog_stream),
+      args_type_info_stack_("args_type_info_stack_", semantics_ir,
                             vlog_stream) {
   // Inserts the "Invalid" and "Type" types as "used types" so that
   // canonicalization can skip them. We don't emit either for lowering.
@@ -130,11 +128,92 @@ auto SemanticsContext::PopScope() -> void {
   }
 }
 
+template <typename BranchNode, typename... Args>
+static auto AddDominatedBlockAndBranchImpl(SemanticsContext& context,
+                                           ParseTree::Node parse_node,
+                                           Args... args)
+    -> SemanticsNodeBlockId {
+  if (!context.node_block_stack().is_current_block_reachable()) {
+    return SemanticsNodeBlockId::Unreachable;
+  }
+  auto block_id = context.semantics_ir().AddNodeBlock();
+  context.AddNode(BranchNode::Make(parse_node, block_id, args...));
+  return block_id;
+}
+
+auto SemanticsContext::AddDominatedBlockAndBranch(ParseTree::Node parse_node)
+    -> SemanticsNodeBlockId {
+  return AddDominatedBlockAndBranchImpl<SemanticsNode::Branch>(*this,
+                                                               parse_node);
+}
+
+auto SemanticsContext::AddDominatedBlockAndBranchWithArg(
+    ParseTree::Node parse_node, SemanticsNodeId arg_id)
+    -> SemanticsNodeBlockId {
+  return AddDominatedBlockAndBranchImpl<SemanticsNode::BranchWithArg>(
+      *this, parse_node, arg_id);
+}
+
+auto SemanticsContext::AddDominatedBlockAndBranchIf(ParseTree::Node parse_node,
+                                                    SemanticsNodeId cond_id)
+    -> SemanticsNodeBlockId {
+  return AddDominatedBlockAndBranchImpl<SemanticsNode::BranchIf>(
+      *this, parse_node, cond_id);
+}
+
+auto SemanticsContext::AddConvergenceBlockAndPush(
+    ParseTree::Node parse_node,
+    std::initializer_list<SemanticsNodeBlockId> blocks) -> void {
+  CARBON_CHECK(blocks.size() >= 2) << "no convergence";
+
+  SemanticsNodeBlockId new_block_id = SemanticsNodeBlockId::Unreachable;
+  for (SemanticsNodeBlockId block_id : blocks) {
+    if (block_id != SemanticsNodeBlockId::Unreachable) {
+      if (new_block_id == SemanticsNodeBlockId::Unreachable) {
+        new_block_id = semantics_ir().AddNodeBlock();
+      }
+      AddNodeToBlock(block_id,
+                     SemanticsNode::Branch::Make(parse_node, new_block_id));
+    }
+  }
+  node_block_stack().Push(new_block_id);
+}
+
+auto SemanticsContext::AddConvergenceBlockWithArgAndPush(
+    ParseTree::Node parse_node,
+    std::initializer_list<std::pair<SemanticsNodeBlockId, SemanticsNodeId>>
+        blocks_and_args) -> SemanticsNodeId {
+  CARBON_CHECK(blocks_and_args.size() >= 2) << "no convergence";
+
+  SemanticsNodeBlockId new_block_id = SemanticsNodeBlockId::Unreachable;
+  for (auto [block_id, arg_id] : blocks_and_args) {
+    if (block_id != SemanticsNodeBlockId::Unreachable) {
+      if (new_block_id == SemanticsNodeBlockId::Unreachable) {
+        new_block_id = semantics_ir().AddNodeBlock();
+      }
+      AddNodeToBlock(block_id, SemanticsNode::BranchWithArg::Make(
+                                   parse_node, new_block_id, arg_id));
+    }
+  }
+  node_block_stack().Push(new_block_id);
+
+  // Acquire the result value.
+  SemanticsTypeId result_type_id =
+      semantics_ir().GetNode(blocks_and_args.begin()->second).type_id();
+  return AddNode(
+      SemanticsNode::BlockArg::Make(parse_node, result_type_id, new_block_id));
+}
+
 // Add the current code block to the enclosing function.
 auto SemanticsContext::AddCurrentCodeBlockToFunction() -> void {
   CARBON_CHECK(!node_block_stack().empty()) << "no current code block";
   CARBON_CHECK(!return_scope_stack().empty()) << "no current function";
 
+  if (!node_block_stack().is_current_block_reachable()) {
+    // Don't include unreachable blocks in the function.
+    return;
+  }
+
   auto function_id = semantics_ir()
                          .GetNode(return_scope_stack().back())
                          .GetAsFunctionDeclaration();
@@ -143,6 +222,28 @@ auto SemanticsContext::AddCurrentCodeBlockToFunction() -> void {
       .body_block_ids.push_back(node_block_stack().PeekForAdd());
 }
 
+auto SemanticsContext::is_current_position_reachable() -> bool {
+  switch (auto block_id = node_block_stack().Peek(); block_id.index) {
+    case SemanticsNodeBlockId::Unreachable.index: {
+      return false;
+    }
+    case SemanticsNodeBlockId::Invalid.index: {
+      return true;
+    }
+    default: {
+      // Our current position is at the end of a real block. That position is
+      // reachable unless the previous instruction is a terminator instruction.
+      const auto& block_contents = semantics_ir().GetNodeBlock(block_id);
+      if (block_contents.empty()) {
+        return true;
+      }
+      const auto& last_node = semantics_ir().GetNode(block_contents.back());
+      return last_node.kind().terminator_kind() !=
+             SemanticsTerminatorKind::Terminator;
+    }
+  }
+}
+
 auto SemanticsContext::ImplicitAsForArgs(
     SemanticsNodeBlockId arg_refs_id, ParseTree::Node param_parse_node,
     SemanticsNodeBlockId param_refs_id,
diff --git a/toolchain/semantics/semantics_context.h b/toolchain/semantics/semantics_context.h
index 922c907cd003..805af9268ad7 100644
--- a/toolchain/semantics/semantics_context.h
+++ b/toolchain/semantics/semantics_context.h
@@ -57,9 +57,48 @@ class SemanticsContext {
   // Pops the top scope from scope_stack_, cleaning up names from name_lookup_.
   auto PopScope() -> void;
 
+  // Adds a `Branch` node branching to a new node block, and returns the ID of
+  // the new block. All paths to the branch target must go through the current
+  // block, though not necessarily through this branch.
+  auto AddDominatedBlockAndBranch(ParseTree::Node parse_node)
+      -> SemanticsNodeBlockId;
+
+  // Adds a `Branch` node branching to a new node block with a value, and
+  // returns the ID of the new block. All paths to the branch target must go
+  // through the current block.
+  auto AddDominatedBlockAndBranchWithArg(ParseTree::Node parse_node,
+                                         SemanticsNodeId arg_id)
+      -> SemanticsNodeBlockId;
+
+  // Adds a `BranchIf` node branching to a new node block, and returns the ID
+  // of the new block. All paths to the branch target must go through the
+  // current block.
+  auto AddDominatedBlockAndBranchIf(ParseTree::Node parse_node,
+                                    SemanticsNodeId cond_id)
+      -> SemanticsNodeBlockId;
+
+  // Adds branches from the given list of blocks to a new block, for
+  // reconvergence of control flow, and pushes the new block onto the node
+  // block stack.
+  auto AddConvergenceBlockAndPush(
+      ParseTree::Node parse_tree,
+      std::initializer_list<SemanticsNodeBlockId> blocks) -> void;
+
+  // Adds branches from the given list of blocks and values to a new block, for
+  // reconvergence of control flow with a result value, and pushes the new
+  // block onto the node block stack. Returns a node referring to the result
+  // value.
+  auto AddConvergenceBlockWithArgAndPush(
+      ParseTree::Node parse_node,
+      std::initializer_list<std::pair<SemanticsNodeBlockId, SemanticsNodeId>>
+          blocks_and_args) -> SemanticsNodeId;
+
   // Add the current code block to the enclosing function.
   auto AddCurrentCodeBlockToFunction() -> void;
 
+  // Returns whether the current position in the current block is reachable.
+  auto is_current_position_reachable() -> bool;
+
   // Runs ImplicitAsImpl for a set of arguments and parameters.
   //
   // This will eventually need to support checking against multiple possible
diff --git a/toolchain/semantics/semantics_handle.cpp b/toolchain/semantics/semantics_handle.cpp
index 2acaa11ad1aa..0de23a1cc491 100644
--- a/toolchain/semantics/semantics_handle.cpp
+++ b/toolchain/semantics/semantics_handle.cpp
@@ -469,6 +469,12 @@ auto SemanticsHandleReturnStatement(SemanticsContext& context,
     context.AddNode(SemanticsNode::ReturnExpression::Make(
         parse_node, context.semantics_ir().GetNode(arg).type_id(), arg));
   }
+
+  // Switch to a new, unreachable, empty node block. This typically won't
+  // contain any semantics IR, but it can do if there are statements following
+  // the `return` statement.
+  context.node_block_stack().Pop();
+  context.node_block_stack().PushUnreachable();
   return true;
 }
 
@@ -531,15 +537,16 @@ auto SemanticsHandleShortCircuitOperand(SemanticsContext& context,
   }
 
   // Create a block for the right-hand side and for the continuation.
-  auto lhs_block_id = context.node_block_stack().PopForAdd();
-  auto end_block_id = context.node_block_stack().PushForAdd();
-  auto rhs_block_id = context.node_block_stack().PushForAdd();
-  context.AddNodeToBlock(
-      lhs_block_id,
-      SemanticsNode::BranchIf::Make(parse_node, rhs_block_id, branch_value_id));
-  context.AddNodeToBlock(
-      lhs_block_id, SemanticsNode::BranchWithArg::Make(
-                        parse_node, end_block_id, short_circuit_result_id));
+  auto rhs_block_id =
+      context.AddDominatedBlockAndBranchIf(parse_node, branch_value_id);
+  auto end_block_id = context.AddDominatedBlockAndBranchWithArg(
+      parse_node, short_circuit_result_id);
+
+  // Push the resumption and the right-hand side blocks, and start emitting the
+  // right-hand operand.
+  context.node_block_stack().Pop();
+  context.node_block_stack().Push(end_block_id);
+  context.node_block_stack().Push(rhs_block_id);
   context.AddCurrentCodeBlockToFunction();
 
   // Put the condition back on the stack for SemanticsHandleInfixOperator.
diff --git a/toolchain/semantics/semantics_handle_function.cpp b/toolchain/semantics/semantics_handle_function.cpp
index d8f392701d08..e1706417ff38 100644
--- a/toolchain/semantics/semantics_handle_function.cpp
+++ b/toolchain/semantics/semantics_handle_function.cpp
@@ -12,8 +12,25 @@ auto SemanticsHandleFunctionDeclaration(SemanticsContext& context,
 }
 
 auto SemanticsHandleFunctionDefinition(SemanticsContext& context,
-                                       ParseTree::Node /*parse_node*/) -> bool {
-  context.node_stack().PopAndDiscardId(ParseNodeKind::FunctionDefinitionStart);
+                                       ParseTree::Node parse_node) -> bool {
+  auto function_id = context.node_stack().Pop<SemanticsFunctionId>(
+      ParseNodeKind::FunctionDefinitionStart);
+
+  // If the `}` of the function is reachable, reject if we need a return value
+  // and otherwise add an implicit `return;`.
+  if (context.is_current_position_reachable()) {
+    if (context.semantics_ir()
+            .GetFunction(function_id)
+            .return_type_id.is_valid()) {
+      CARBON_DIAGNOSTIC(
+          MissingReturnStatement, Error,
+          "Missing `return` at end of function with declared return type.");
+      context.emitter().Emit(parse_node, MissingReturnStatement);
+    } else {
+      context.AddNode(SemanticsNode::Return::Make(parse_node));
+    }
+  }
+
   context.return_scope_stack().pop_back();
   context.PopScope();
   context.node_block_stack().Pop();
diff --git a/toolchain/semantics/semantics_handle_if.cpp b/toolchain/semantics/semantics_handle_if.cpp
index d0c5ebcf27e6..0cb8d938dec1 100644
--- a/toolchain/semantics/semantics_handle_if.cpp
+++ b/toolchain/semantics/semantics_handle_if.cpp
@@ -18,19 +18,17 @@ auto SemanticsHandleIfCondition(SemanticsContext& context,
   auto cond_value_id = context.node_stack().Pop<SemanticsNodeId>();
   cond_value_id = context.ImplicitAsBool(parse_node, cond_value_id);
 
-  // Create the else block and the then block, and branch to the right one. If
+  // Create the then block and the else block, and branch to the right one. If
   // there is no `else`, the then block will terminate with a branch to the
   // else block, which will be reused as the resumption block.
-  auto if_block_id = context.node_block_stack().PopForAdd();
-  auto else_block_id = context.node_block_stack().PushForAdd();
-  auto then_block_id = context.node_block_stack().PushForAdd();
+  auto then_block_id =
+      context.AddDominatedBlockAndBranchIf(parse_node, cond_value_id);
+  auto else_block_id = context.AddDominatedBlockAndBranch(parse_node);
 
-  // Branch to the appropriate block.
-  context.AddNodeToBlock(
-      if_block_id,
-      SemanticsNode::BranchIf::Make(parse_node, then_block_id, cond_value_id));
-  context.AddNodeToBlock(
-      if_block_id, SemanticsNode::Branch::Make(parse_node, else_block_id));
+  // Push the else and then blocks, and start emitting code in the then block.
+  context.node_block_stack().Pop();
+  context.node_block_stack().Push(else_block_id);
+  context.node_block_stack().Push(then_block_id);
   context.AddCurrentCodeBlockToFunction();
 
   context.node_stack().Push(parse_node);
@@ -71,11 +69,8 @@ auto SemanticsHandleIfStatement(SemanticsContext& context,
       // Branch from the then and else blocks to a new resumption block.
       auto then_block_id = context.node_stack().Pop<SemanticsNodeBlockId>(
           ParseNodeKind::IfStatementElse);
-      auto resume_block_id = context.node_block_stack().PushForAdd();
-      context.AddNodeToBlock(then_block_id, SemanticsNode::Branch::Make(
-                                                parse_node, resume_block_id));
-      context.AddNodeToBlock(sub_block_id, SemanticsNode::Branch::Make(
-                                               parse_node, resume_block_id));
+      context.AddConvergenceBlockAndPush(parse_node,
+                                         {then_block_id, sub_block_id});
       break;
     }
 
diff --git a/toolchain/semantics/semantics_handle_if_expression.cpp b/toolchain/semantics/semantics_handle_if_expression.cpp
index a86dcbf9d4aa..25cb8ad8eb8b 100644
--- a/toolchain/semantics/semantics_handle_if_expression.cpp
+++ b/toolchain/semantics/semantics_handle_if_expression.cpp
@@ -12,24 +12,16 @@ auto SemanticsHandleIfExpressionIf(SemanticsContext& context,
 
   context.node_stack().Push(if_node);
 
-  // Convert the condition to `bool`.
+  // Convert the condition to `bool`, and branch on it.
   cond_value_id = context.ImplicitAsBool(if_node, cond_value_id);
+  auto then_block_id =
+      context.AddDominatedBlockAndBranchIf(if_node, cond_value_id);
+  auto else_block_id = context.AddDominatedBlockAndBranch(if_node);
 
-  // Stop emitting the current block. We'll add some branch instructions to it
-  // later, but we don't want it on the stack any more.
-  auto if_block_id = context.node_block_stack().PeekForAdd();
+  // Push the `else` block and `then` block, and start emitting the `then`.
   context.node_block_stack().Pop();
-
-  // Create the resumption block, `else` block, and `then` block, and branches
-  // to them.
-  context.node_block_stack().Push();
-  auto else_block_id = context.node_block_stack().PushForAdd();
-  auto then_block_id = context.node_block_stack().PushForAdd();
-  context.AddNodeToBlock(
-      if_block_id,
-      SemanticsNode::BranchIf::Make(if_node, then_block_id, cond_value_id));
-  context.AddNodeToBlock(if_block_id,
-                         SemanticsNode::Branch::Make(if_node, else_block_id));
+  context.node_block_stack().Push(else_block_id);
+  context.node_block_stack().Push(then_block_id);
   context.AddCurrentCodeBlockToFunction();
   return true;
 }
@@ -59,20 +51,14 @@ auto SemanticsHandleIfExpressionElse(SemanticsContext& context,
       context.ImplicitAsRequired(else_node, else_value_id, result_type_id);
   auto else_end_block_id = context.node_block_stack().Pop();
 
-  // Create branches to the resumption block.
-  auto resume_block_id = context.node_block_stack().PeekForAdd();
-  context.AddNodeToBlock(then_end_block_id,
-                         SemanticsNode::BranchWithArg::Make(
-                             then_node, resume_block_id, then_value_id));
-  context.AddNodeToBlock(else_end_block_id,
-                         SemanticsNode::BranchWithArg::Make(
-                             else_node, resume_block_id, else_value_id));
+  // Create a resumption block and branches to it.
+  auto chosen_value_id = context.AddConvergenceBlockWithArgAndPush(
+      if_node,
+      {{then_end_block_id, then_value_id}, {else_end_block_id, else_value_id}});
   context.AddCurrentCodeBlockToFunction();
 
-  // Obtain the value in the resumption block and push it.
-  context.AddNodeAndPush(
-      if_node,
-      SemanticsNode::BlockArg::Make(if_node, result_type_id, resume_block_id));
+  // Push the result value.
+  context.node_stack().Push(if_node, chosen_value_id);
   return true;
 }
 
diff --git a/toolchain/semantics/semantics_ir.cpp b/toolchain/semantics/semantics_ir.cpp
index f708899b301f..022361963ad0 100644
--- a/toolchain/semantics/semantics_ir.cpp
+++ b/toolchain/semantics/semantics_ir.cpp
@@ -92,9 +92,55 @@ auto SemanticsIR::MakeFromParseTree(const SemanticsIR& builtin_ir,
   context.VerifyOnFinish();
 
   semantics_ir.has_errors_ = err_tracker.seen_error();
+
+#ifndef NDEBUG
+  if (auto verify = semantics_ir.Verify(); !verify.ok()) {
+    CARBON_FATAL() << semantics_ir
+                   << "Built invalid semantics IR: " << verify.error() << "\n";
+  }
+#endif
+
   return semantics_ir;
 }
 
+auto SemanticsIR::Verify() const -> ErrorOr<Success> {
+  // Invariants don't necessarily hold for invalid IR.
+  if (has_errors_) {
+    return Success();
+  }
+
+  // Check that every code block has a terminator sequence that appears at the
+  // end of the block.
+  for (const SemanticsFunction& function : functions_) {
+    for (SemanticsNodeBlockId block_id : function.body_block_ids) {
+      SemanticsTerminatorKind prior_kind =
+          SemanticsTerminatorKind::NotTerminator;
+      for (SemanticsNodeId node_id : GetNodeBlock(block_id)) {
+        SemanticsTerminatorKind node_kind =
+            GetNode(node_id).kind().terminator_kind();
+        if (prior_kind == SemanticsTerminatorKind::Terminator) {
+          return Error(llvm::formatv("Node {0} in block {1} follows terminator",
+                                     node_id, block_id));
+        }
+        if (prior_kind > node_kind) {
+          return Error(
+              llvm::formatv("Non-terminator node {0} in block {1} follows "
+                            "terminator sequence",
+                            node_id, block_id));
+        }
+        prior_kind = node_kind;
+      }
+      if (prior_kind != SemanticsTerminatorKind::Terminator) {
+        return Error(llvm::formatv("No terminator in block {0}", block_id));
+      }
+    }
+  }
+
+  // TODO: Check that a node only references other nodes that are either global
+  // or that dominate it.
+  return Success();
+}
+
 static constexpr int Indent = 2;
 
 template <typename T>
diff --git a/toolchain/semantics/semantics_ir.h b/toolchain/semantics/semantics_ir.h
index 95f41e04605a..d7ec44f4f56d 100644
--- a/toolchain/semantics/semantics_ir.h
+++ b/toolchain/semantics/semantics_ir.h
@@ -36,9 +36,9 @@ struct SemanticsFunction {
   SemanticsNodeBlockId param_refs_id;
   // The return type. This will be invalid if the return type wasn't specified.
   SemanticsTypeId return_type_id;
-  // A list of the code blocks in the body of the function, in lexical order.
-  // The first block is the entry block. This will be empty for declarations
-  // that don't have a visible definition.
+  // A list of the statically reachable code blocks in the body of the
+  // function, in lexical order. The first block is the entry block. This will
+  // be empty for declarations that don't have a visible definition.
   llvm::SmallVector<SemanticsNodeBlockId> body_block_ids;
 };
 
@@ -69,6 +69,9 @@ class SemanticsIR {
                                 DiagnosticConsumer& consumer,
                                 llvm::raw_ostream* vlog_stream) -> SemanticsIR;
 
+  // Verifies that invariants of the semantics IR hold.
+  auto Verify() const -> ErrorOr<Success>;
+
   // Prints the full IR. Allow omitting builtins so that unrelated changes are
   // less likely to alternate test golden files.
   // TODO: In the future, the things to print may change, for example by adding
@@ -115,7 +118,9 @@ class SemanticsIR {
       -> SemanticsNodeId {
     SemanticsNodeId node_id(nodes_.size());
     nodes_.push_back(node);
-    node_blocks_[block_id.index].push_back(node_id);
+    if (block_id != SemanticsNodeBlockId::Unreachable) {
+      node_blocks_[block_id.index].push_back(node_id);
+    }
     return node_id;
   }
 
@@ -124,15 +129,24 @@ class SemanticsIR {
     return nodes_[node_id.index];
   }
 
+  // Adds an empty node block, returning an ID to reference it.
+  auto AddNodeBlock() -> SemanticsNodeBlockId {
+    SemanticsNodeBlockId id(node_blocks_.size());
+    node_blocks_.push_back({});
+    return id;
+  }
+
   // Returns the requested node block.
   auto GetNodeBlock(SemanticsNodeBlockId block_id) const
       -> const llvm::SmallVector<SemanticsNodeId>& {
+    CARBON_CHECK(block_id != SemanticsNodeBlockId::Unreachable);
     return node_blocks_[block_id.index];
   }
 
   // Returns the requested node block.
   auto GetNodeBlock(SemanticsNodeBlockId block_id)
       -> llvm::SmallVector<SemanticsNodeId>& {
+    CARBON_CHECK(block_id != SemanticsNodeBlockId::Unreachable);
     return node_blocks_[block_id.index];
   }
 
diff --git a/toolchain/semantics/semantics_node.h b/toolchain/semantics/semantics_node.h
index d94ff92035af..925c398a2e58 100644
--- a/toolchain/semantics/semantics_node.h
+++ b/toolchain/semantics/semantics_node.h
@@ -9,6 +9,7 @@
 
 #include "common/check.h"
 #include "common/ostream.h"
+#include "toolchain/common/index_base.h"
 #include "toolchain/parser/parse_tree.h"
 #include "toolchain/semantics/semantics_builtin_kind.h"
 #include "toolchain/semantics/semantics_node_kind.h"
@@ -108,10 +109,17 @@ struct SemanticsNodeBlockId : public IndexBase {
   // An explicitly invalid ID.
   static const SemanticsNodeBlockId Invalid;
 
+  // An ID for unreachable code.
+  static const SemanticsNodeBlockId Unreachable;
+
   using IndexBase::IndexBase;
   auto Print(llvm::raw_ostream& out) const -> void {
-    out << "block";
-    IndexBase::Print(out);
+    if (index == Unreachable.index) {
+      out << "unreachable";
+    } else {
+      out << "block";
+      IndexBase::Print(out);
+    }
   }
 };
 
@@ -119,6 +127,8 @@ constexpr SemanticsNodeBlockId SemanticsNodeBlockId::Empty =
     SemanticsNodeBlockId(0);
 constexpr SemanticsNodeBlockId SemanticsNodeBlockId::Invalid =
     SemanticsNodeBlockId(SemanticsNodeBlockId::InvalidIndex);
+constexpr SemanticsNodeBlockId SemanticsNodeBlockId::Unreachable =
+    SemanticsNodeBlockId(SemanticsNodeBlockId::InvalidIndex - 1);
 
 // The ID of a real literal.
 struct SemanticsRealLiteralId : public IndexBase {
diff --git a/toolchain/semantics/semantics_node_block_stack.cpp b/toolchain/semantics/semantics_node_block_stack.cpp
index 769425a0b6b2..7a8e75dfbee6 100644
--- a/toolchain/semantics/semantics_node_block_stack.cpp
+++ b/toolchain/semantics/semantics_node_block_stack.cpp
@@ -9,20 +9,18 @@
 
 namespace Carbon {
 
-auto SemanticsNodeBlockStack::Push() -> void {
+auto SemanticsNodeBlockStack::Push(SemanticsNodeBlockId id) -> void {
   CARBON_VLOG() << name_ << " Push " << stack_.size() << "\n";
   CARBON_CHECK(stack_.size() < (1 << 20))
       << "Excessive stack size: likely infinite loop";
-  stack_.push_back(SemanticsNodeBlockId::Invalid);
+  stack_.push_back(id);
 }
 
 auto SemanticsNodeBlockStack::PeekForAdd() -> SemanticsNodeBlockId {
-  CARBON_CHECK(!stack_.empty());
+  CARBON_CHECK(!stack_.empty()) << "no current block";
   auto& back = stack_.back();
   if (!back.is_valid()) {
-    SemanticsNodeBlockId block_id(node_blocks_->size());
-    node_blocks_->resize(block_id.index + 1);
-    back = block_id;
+    back = semantics_ir_->AddNodeBlock();
     CARBON_VLOG() << name_ << " Add " << stack_.size() - 1 << ": " << back
                   << "\n";
   }
@@ -30,6 +28,7 @@ auto SemanticsNodeBlockStack::PeekForAdd() -> SemanticsNodeBlockId {
 }
 
 auto SemanticsNodeBlockStack::Pop() -> SemanticsNodeBlockId {
+  CARBON_CHECK(!stack_.empty()) << "no current block";
   auto back = stack_.pop_back_val();
   CARBON_VLOG() << name_ << " Pop " << stack_.size() << ": " << back << "\n";
   if (!back.is_valid()) {
diff --git a/toolchain/semantics/semantics_node_block_stack.h b/toolchain/semantics/semantics_node_block_stack.h
index 0615fc577ac7..fc972d8e7720 100644
--- a/toolchain/semantics/semantics_node_block_stack.h
+++ b/toolchain/semantics/semantics_node_block_stack.h
@@ -8,6 +8,7 @@
 #include <type_traits>
 
 #include "llvm/ADT/SmallVector.h"
+#include "toolchain/semantics/semantics_ir.h"
 #include "toolchain/semantics/semantics_node.h"
 
 namespace Carbon {
@@ -17,15 +18,20 @@ namespace Carbon {
 // All pushes and pops will be vlogged.
 class SemanticsNodeBlockStack {
  public:
-  explicit SemanticsNodeBlockStack(
-      llvm::StringLiteral name,
-      llvm::SmallVector<llvm::SmallVector<SemanticsNodeId>>& node_blocks,
-      llvm::raw_ostream* vlog_stream)
-      : name_(name), node_blocks_(&node_blocks), vlog_stream_(vlog_stream) {}
+  explicit SemanticsNodeBlockStack(llvm::StringLiteral name,
+                                   SemanticsIR& semantics_ir,
+                                   llvm::raw_ostream* vlog_stream)
+      : name_(name), semantics_ir_(&semantics_ir), vlog_stream_(vlog_stream) {}
+
+  // Pushes an existing node block.
+  auto Push(SemanticsNodeBlockId id) -> void;
 
   // Pushes a new node block. It will be invalid unless PeekForAdd is called in
   // order to support lazy allocation.
-  auto Push() -> void;
+  auto Push() -> void { Push(SemanticsNodeBlockId::Invalid); }
+
+  // Pushes a new unreachable code block.
+  auto PushUnreachable() -> void { Push(SemanticsNodeBlockId::Unreachable); }
 
   // Allocates and pushes a new node block.
   auto PushForAdd() -> SemanticsNodeBlockId {
@@ -35,7 +41,10 @@ class SemanticsNodeBlockStack {
 
   // Peeks at the top node block. This does not trigger lazy allocation, so the
   // returned node block may be invalid.
-  auto Peek() -> SemanticsNodeBlockId { return stack_.back(); }
+  auto Peek() -> SemanticsNodeBlockId {
+    CARBON_CHECK(!stack_.empty()) << "no current block";
+    return stack_.back();
+  }
 
   // Returns the top node block, allocating one if it's still invalid.
   auto PeekForAdd() -> SemanticsNodeBlockId;
@@ -51,6 +60,11 @@ class SemanticsNodeBlockStack {
     return Pop();
   }
 
+  // Returns whether the current block is statically reachable.
+  auto is_current_block_reachable() -> bool {
+    return Peek() != SemanticsNodeBlockId::Unreachable;
+  }
+
   // Prints the stack for a stack dump.
   auto PrintForStackDump(llvm::raw_ostream& output) const -> void;
 
@@ -61,8 +75,8 @@ class SemanticsNodeBlockStack {
   // A name for debugging.
   llvm::StringLiteral name_;
 
-  // The underlying node block storage on SemanticsIR. Always non-null.
-  llvm::SmallVector<llvm::SmallVector<SemanticsNodeId>>* const node_blocks_;
+  // The underlying SemanticsIR instance. Always non-null.
+  SemanticsIR* semantics_ir_;
 
   // Whether to print verbose output.
   llvm::raw_ostream* vlog_stream_;
diff --git a/toolchain/semantics/semantics_node_kind.cpp b/toolchain/semantics/semantics_node_kind.cpp
index b8c068bcf2eb..56a5cbc45db4 100644
--- a/toolchain/semantics/semantics_node_kind.cpp
+++ b/toolchain/semantics/semantics_node_kind.cpp
@@ -11,4 +11,16 @@ CARBON_DEFINE_ENUM_CLASS_NAMES(SemanticsNodeKind) = {
 #include "toolchain/semantics/semantics_node_kind.def"
 };
 
+auto SemanticsNodeKind::terminator_kind() const -> SemanticsTerminatorKind {
+  static constexpr SemanticsTerminatorKind Table[] = {
+#define CARBON_SEMANTICS_NODE_KIND(Name) SemanticsTerminatorKind::NotTerminator,
+#define CARBON_SEMANTICS_TERMINATOR_SEQUENCE_KIND(Name) \
+  SemanticsTerminatorKind::TerminatorSequence,
+#define CARBON_SEMANTICS_TERMINATOR_KIND(Name) \
+  SemanticsTerminatorKind::Terminator,
+#include "toolchain/semantics/semantics_node_kind.def"
+  };
+  return Table[AsInt()];
+}
+
 }  // namespace Carbon
diff --git a/toolchain/semantics/semantics_node_kind.def b/toolchain/semantics/semantics_node_kind.def
index 6abeeefb9e32..f3f0e88af4af 100644
--- a/toolchain/semantics/semantics_node_kind.def
+++ b/toolchain/semantics/semantics_node_kind.def
@@ -10,11 +10,27 @@
 // Supported x-macros are:
 // - CARBON_SEMANTICS_NODE_KIND(Name)
 //   Defines a node kind.
+// - CARBON_SEMANTICS_TERMINATOR_SEQUENCE_KIND(Name)
+//   Defines a node kind for a node in the terminator sequence, such as
+//   BranchIf. These nodes can only appear at the end of a block, after all
+//   nodes that are not in the terminator sequence, before the terminator.
+// - CARBON_SEMANTICS_TERMINATOR_KIND(Name)
+//   Defines a node kind for a terminator node.
 
 #ifndef CARBON_SEMANTICS_NODE_KIND
 #error "Must define the x-macro to use this file."
 #endif
 
+#ifndef CARBON_SEMANTICS_TERMINATOR_SEQUENCE_KIND
+#define CARBON_SEMANTICS_TERMINATOR_SEQUENCE_KIND(Name) \
+  CARBON_SEMANTICS_NODE_KIND(Name)
+#endif
+
+#ifndef CARBON_SEMANTICS_TERMINATOR_KIND
+#define CARBON_SEMANTICS_TERMINATOR_KIND(Name) \
+  CARBON_SEMANTICS_TERMINATOR_SEQUENCE_KIND(Name)
+#endif
+
 CARBON_SEMANTICS_NODE_KIND(Invalid)
 
 // A cross-reference between IRs.
@@ -25,16 +41,16 @@ CARBON_SEMANTICS_NODE_KIND(BinaryOperatorAdd)
 CARBON_SEMANTICS_NODE_KIND(BindName)
 CARBON_SEMANTICS_NODE_KIND(BlockArg)
 CARBON_SEMANTICS_NODE_KIND(BoolLiteral)
-CARBON_SEMANTICS_NODE_KIND(Branch)
-CARBON_SEMANTICS_NODE_KIND(BranchIf)
-CARBON_SEMANTICS_NODE_KIND(BranchWithArg)
+CARBON_SEMANTICS_TERMINATOR_KIND(Branch)
+CARBON_SEMANTICS_TERMINATOR_SEQUENCE_KIND(BranchIf)
+CARBON_SEMANTICS_TERMINATOR_KIND(BranchWithArg)
 CARBON_SEMANTICS_NODE_KIND(Builtin)
 CARBON_SEMANTICS_NODE_KIND(Call)
 CARBON_SEMANTICS_NODE_KIND(FunctionDeclaration)
 CARBON_SEMANTICS_NODE_KIND(IntegerLiteral)
 CARBON_SEMANTICS_NODE_KIND(RealLiteral)
-CARBON_SEMANTICS_NODE_KIND(Return)
-CARBON_SEMANTICS_NODE_KIND(ReturnExpression)
+CARBON_SEMANTICS_TERMINATOR_KIND(Return)
+CARBON_SEMANTICS_TERMINATOR_KIND(ReturnExpression)
 CARBON_SEMANTICS_NODE_KIND(StringLiteral)
 CARBON_SEMANTICS_NODE_KIND(StructMemberAccess)
 CARBON_SEMANTICS_NODE_KIND(StructType)
@@ -44,4 +60,6 @@ CARBON_SEMANTICS_NODE_KIND(StubReference)
 CARBON_SEMANTICS_NODE_KIND(UnaryOperatorNot)
 CARBON_SEMANTICS_NODE_KIND(VarStorage)
 
+#undef CARBON_SEMANTICS_TERMINATOR_SEQUENCE_KIND
+#undef CARBON_SEMANTICS_TERMINATOR_KIND
 #undef CARBON_SEMANTICS_NODE_KIND
diff --git a/toolchain/semantics/semantics_node_kind.h b/toolchain/semantics/semantics_node_kind.h
index 7c0f4c57d09e..421c08e19f58 100644
--- a/toolchain/semantics/semantics_node_kind.h
+++ b/toolchain/semantics/semantics_node_kind.h
@@ -16,12 +16,32 @@ CARBON_DEFINE_RAW_ENUM_CLASS(SemanticsNodeKind, uint8_t) {
 #include "toolchain/semantics/semantics_node_kind.def"
 };
 
+// Whether a node is a terminator or part of the terminator sequence. The nodes
+// in a block appear in the order NotTerminator, then TerminatorSequence, then
+// Terminator, which is also the numerical order of these values.
+enum class SemanticsTerminatorKind {
+  // This node is not a terminator.
+  NotTerminator,
+  // This node is not itself a terminator, but forms part of a terminator
+  // sequence.
+  TerminatorSequence,
+  // This node is a terminator.
+  Terminator,
+};
+
 class SemanticsNodeKind : public CARBON_ENUM_BASE(SemanticsNodeKind) {
  public:
 #define CARBON_SEMANTICS_NODE_KIND(Name) CARBON_ENUM_CONSTANT_DECLARATION(Name)
 #include "toolchain/semantics/semantics_node_kind.def"
 
   using EnumBase::Create;
+
+  // Returns whether this node kind is a code block terminator, such as an
+  // unconditional branch instruction, or part of the termination sequence,
+  // such as a conditional branch instruction. The termination sequence of a
+  // code block appears after all other instructions, and ends with a
+  // terminator instruction.
+  [[nodiscard]] auto terminator_kind() const -> SemanticsTerminatorKind;
 };
 
 #define CARBON_SEMANTICS_NODE_KIND(Name) \
diff --git a/toolchain/semantics/testdata/basics/fail_name_lookup.carbon b/toolchain/semantics/testdata/basics/fail_name_lookup.carbon
index 41db925fdc7a..2f235bef5737 100644
--- a/toolchain/semantics/testdata/basics/fail_name_lookup.carbon
+++ b/toolchain/semantics/testdata/basics/fail_name_lookup.carbon
@@ -19,6 +19,7 @@
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: nodes: [
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -27,6 +28,7 @@
 // CHECK:STDOUT:     node+0,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/call/empty_struct.carbon b/toolchain/semantics/testdata/function/call/empty_struct.carbon
index ca60e0850c1c..22d50fc41c53 100644
--- a/toolchain/semantics/testdata/function/call/empty_struct.carbon
+++ b/toolchain/semantics/testdata/function/call/empty_struct.carbon
@@ -33,6 +33,7 @@
 // CHECK:STDOUT:   {kind: StructValue, arg0: block0, type: type0},
 // CHECK:STDOUT:   {kind: StubReference, arg0: node+8, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block6, arg1: function0, type: type0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -58,6 +59,7 @@
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:     node+10,
+// CHECK:STDOUT:     node+11,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+9,
diff --git a/toolchain/semantics/testdata/function/call/fail_param_count.carbon b/toolchain/semantics/testdata/function/call/fail_param_count.carbon
index e2360cd86be2..251879f4130a 100644
--- a/toolchain/semantics/testdata/function/call/fail_param_count.carbon
+++ b/toolchain/semantics/testdata/function/call/fail_param_count.carbon
@@ -34,64 +34,68 @@
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: nodes: [
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
-// CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+1, type: type1},
+// CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+2, type: type1},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
-// CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+4, type: type1},
+// CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+6, type: type1},
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
-// CHECK:STDOUT:   {kind: BindName, arg0: str4, arg1: node+6, type: type1},
+// CHECK:STDOUT:   {kind: BindName, arg0: str4, arg1: node+8, type: type1},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function2},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function3},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type1},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+10, type: type1},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+13, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type1},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+12, type: type1},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+15, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int2, type: type1},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+14, type: type1},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+17, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int3, type: type1},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+16, type: type1},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+19, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int4, type: type1},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+18, type: type1},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+21, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int5, type: type1},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+20, type: type1},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+23, type: type1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+0,
+// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+10,
+// CHECK:STDOUT:     node+12,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+2,
 // CHECK:STDOUT:     node+3,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+3,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+6,
+// CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+1,
-// CHECK:STDOUT:     node+2,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+2,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+4,
-// CHECK:STDOUT:     node+5,
-// CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:     node+7,
+// CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+5,
-// CHECK:STDOUT:     node+7,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+10,
 // CHECK:STDOUT:     node+11,
-// CHECK:STDOUT:     node+12,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+13,
 // CHECK:STDOUT:     node+14,
 // CHECK:STDOUT:     node+15,
@@ -101,20 +105,24 @@
 // CHECK:STDOUT:     node+19,
 // CHECK:STDOUT:     node+20,
 // CHECK:STDOUT:     node+21,
+// CHECK:STDOUT:     node+22,
+// CHECK:STDOUT:     node+23,
+// CHECK:STDOUT:     node+24,
+// CHECK:STDOUT:     node+25,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+11,
+// CHECK:STDOUT:     node+14,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+13,
-// CHECK:STDOUT:     node+15,
+// CHECK:STDOUT:     node+16,
+// CHECK:STDOUT:     node+18,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+17,
-// CHECK:STDOUT:     node+19,
+// CHECK:STDOUT:     node+20,
+// CHECK:STDOUT:     node+22,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+21,
+// CHECK:STDOUT:     node+24,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/call/fail_param_type.carbon b/toolchain/semantics/testdata/function/call/fail_param_type.carbon
index c04184a2e185..43925f736375 100644
--- a/toolchain/semantics/testdata/function/call/fail_param_type.carbon
+++ b/toolchain/semantics/testdata/function/call/fail_param_type.carbon
@@ -27,9 +27,11 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
 // CHECK:STDOUT:   {kind: RealLiteral, arg0: real0, type: type2},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+4, type: type2},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+5, type: type2},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -43,16 +45,18 @@
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:     node+6,
+// CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+4,
-// CHECK:STDOUT:     node+5,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/call/fail_return_type_mismatch.carbon b/toolchain/semantics/testdata/function/call/fail_return_type_mismatch.carbon
index aab17229aa68..2b0812a79c90 100644
--- a/toolchain/semantics/testdata/function/call/fail_return_type_mismatch.carbon
+++ b/toolchain/semantics/testdata/function/call/fail_return_type_mismatch.carbon
@@ -32,6 +32,7 @@
 // CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+4, type: type2},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function0, type: type0},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+4, arg1: nodeInvalidType, type: typeInvalidType},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -49,6 +50,7 @@
 // CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:     node+7,
+// CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/call/i32.carbon b/toolchain/semantics/testdata/function/call/i32.carbon
index 3769769f9291..e3c09a1659d9 100644
--- a/toolchain/semantics/testdata/function/call/i32.carbon
+++ b/toolchain/semantics/testdata/function/call/i32.carbon
@@ -35,6 +35,7 @@
 // CHECK:STDOUT:   {kind: StubReference, arg0: node+7, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block6, arg1: function0, type: type0},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+5, arg1: node+9, type: type0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -60,6 +61,7 @@
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:     node+10,
+// CHECK:STDOUT:     node+11,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+8,
diff --git a/toolchain/semantics/testdata/function/call/more_param_ir.carbon b/toolchain/semantics/testdata/function/call/more_param_ir.carbon
index 4004509f4426..90856688a91b 100644
--- a/toolchain/semantics/testdata/function/call/more_param_ir.carbon
+++ b/toolchain/semantics/testdata/function/call/more_param_ir.carbon
@@ -37,20 +37,22 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str3, arg1: node+4, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type0},
-// CHECK:STDOUT:   {kind: BinaryOperatorAdd, arg0: node+8, arg1: node+9, type: type0},
+// CHECK:STDOUT:   {kind: BinaryOperatorAdd, arg0: node+9, arg1: node+10, type: type0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int2, type: type0},
-// CHECK:STDOUT:   {kind: BinaryOperatorAdd, arg0: node+10, arg1: node+11, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+12, type: type0},
+// CHECK:STDOUT:   {kind: BinaryOperatorAdd, arg0: node+11, arg1: node+12, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+13, type: type0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int3, type: type0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int4, type: type0},
-// CHECK:STDOUT:   {kind: BinaryOperatorAdd, arg0: node+14, arg1: node+15, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+16, type: type0},
+// CHECK:STDOUT:   {kind: BinaryOperatorAdd, arg0: node+15, arg1: node+16, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+17, type: type0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int5, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+18, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+19, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block6, arg1: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -70,12 +72,12 @@
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+6,
+// CHECK:STDOUT:     node+8,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:     node+10,
 // CHECK:STDOUT:     node+11,
@@ -88,11 +90,13 @@
 // CHECK:STDOUT:     node+18,
 // CHECK:STDOUT:     node+19,
 // CHECK:STDOUT:     node+20,
+// CHECK:STDOUT:     node+21,
+// CHECK:STDOUT:     node+22,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+13,
-// CHECK:STDOUT:     node+17,
-// CHECK:STDOUT:     node+19,
+// CHECK:STDOUT:     node+14,
+// CHECK:STDOUT:     node+18,
+// CHECK:STDOUT:     node+20,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/call/params_one.carbon b/toolchain/semantics/testdata/function/call/params_one.carbon
index 634b3c8f36b6..c57e81a21e42 100644
--- a/toolchain/semantics/testdata/function/call/params_one.carbon
+++ b/toolchain/semantics/testdata/function/call/params_one.carbon
@@ -26,10 +26,12 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+4, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+5, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block6, arg1: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -43,17 +45,19 @@
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:     node+6,
+// CHECK:STDOUT:     node+7,
+// CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/call/params_one_comma.carbon b/toolchain/semantics/testdata/function/call/params_one_comma.carbon
index c0f01e6830e3..4caeb0f76d57 100644
--- a/toolchain/semantics/testdata/function/call/params_one_comma.carbon
+++ b/toolchain/semantics/testdata/function/call/params_one_comma.carbon
@@ -27,13 +27,15 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+4, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+5, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block6, arg1: function0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+7, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+8, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block7, arg1: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -47,23 +49,25 @@
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:     node+9,
+// CHECK:STDOUT:     node+10,
+// CHECK:STDOUT:     node+11,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+8,
+// CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/call/params_two.carbon b/toolchain/semantics/testdata/function/call/params_two.carbon
index 893f1db00758..eb4d2f9fda36 100644
--- a/toolchain/semantics/testdata/function/call/params_two.carbon
+++ b/toolchain/semantics/testdata/function/call/params_two.carbon
@@ -30,12 +30,14 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+2, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+6, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+7, type: type0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+8, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+9, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block6, arg1: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -52,20 +54,22 @@
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+6,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:     node+10,
+// CHECK:STDOUT:     node+11,
+// CHECK:STDOUT:     node+12,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+7,
-// CHECK:STDOUT:     node+9,
+// CHECK:STDOUT:     node+8,
+// CHECK:STDOUT:     node+10,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/call/params_two_comma.carbon b/toolchain/semantics/testdata/function/call/params_two_comma.carbon
index 52082dde2a39..64842c3ab69b 100644
--- a/toolchain/semantics/testdata/function/call/params_two_comma.carbon
+++ b/toolchain/semantics/testdata/function/call/params_two_comma.carbon
@@ -32,17 +32,19 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+2, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+6, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+7, type: type0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+8, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+9, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block6, arg1: function0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int2, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+11, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+12, type: type0},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int3, type: type0},
-// CHECK:STDOUT:   {kind: StubReference, arg0: node+13, type: type0},
+// CHECK:STDOUT:   {kind: StubReference, arg0: node+14, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block7, arg1: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -59,12 +61,12 @@
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+6,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:     node+9,
@@ -74,14 +76,16 @@
 // CHECK:STDOUT:     node+13,
 // CHECK:STDOUT:     node+14,
 // CHECK:STDOUT:     node+15,
+// CHECK:STDOUT:     node+16,
+// CHECK:STDOUT:     node+17,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+7,
-// CHECK:STDOUT:     node+9,
+// CHECK:STDOUT:     node+8,
+// CHECK:STDOUT:     node+10,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+12,
-// CHECK:STDOUT:     node+14,
+// CHECK:STDOUT:     node+13,
+// CHECK:STDOUT:     node+15,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/call/params_zero.carbon b/toolchain/semantics/testdata/function/call/params_zero.carbon
index ba47b6cf0223..8a0a6ad15fb5 100644
--- a/toolchain/semantics/testdata/function/call/params_zero.carbon
+++ b/toolchain/semantics/testdata/function/call/params_zero.carbon
@@ -21,20 +21,24 @@
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: nodes: [
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+0,
+// CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:     node+3,
+// CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/definition/fail_param_name_conflict.carbon b/toolchain/semantics/testdata/function/definition/fail_param_name_conflict.carbon
index 2691ad495b3d..1c2c5ad3fc5e 100644
--- a/toolchain/semantics/testdata/function/definition/fail_param_name_conflict.carbon
+++ b/toolchain/semantics/testdata/function/definition/fail_param_name_conflict.carbon
@@ -25,6 +25,7 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+2, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -43,6 +44,7 @@
 // CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/definition/order.carbon b/toolchain/semantics/testdata/function/definition/order.carbon
index b0d4a7c33673..94bd2aa695fe 100644
--- a/toolchain/semantics/testdata/function/definition/order.carbon
+++ b/toolchain/semantics/testdata/function/definition/order.carbon
@@ -23,22 +23,28 @@
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: nodes: [
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function2},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+0,
-// CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/definition/params_one.carbon b/toolchain/semantics/testdata/function/definition/params_one.carbon
index ec93f6453d61..6ad93f8d8e3e 100644
--- a/toolchain/semantics/testdata/function/definition/params_one.carbon
+++ b/toolchain/semantics/testdata/function/definition/params_one.carbon
@@ -23,6 +23,7 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -38,6 +39,7 @@
 // CHECK:STDOUT:     node+2,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/definition/params_one_comma.carbon b/toolchain/semantics/testdata/function/definition/params_one_comma.carbon
index d703aa47c1d8..01fd8fd75b33 100644
--- a/toolchain/semantics/testdata/function/definition/params_one_comma.carbon
+++ b/toolchain/semantics/testdata/function/definition/params_one_comma.carbon
@@ -23,6 +23,7 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -38,6 +39,7 @@
 // CHECK:STDOUT:     node+2,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/definition/params_two.carbon b/toolchain/semantics/testdata/function/definition/params_two.carbon
index f4e84cdb3aa1..ef3a2ef10d10 100644
--- a/toolchain/semantics/testdata/function/definition/params_two.carbon
+++ b/toolchain/semantics/testdata/function/definition/params_two.carbon
@@ -26,6 +26,7 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+2, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -44,6 +45,7 @@
 // CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/definition/params_two_comma.carbon b/toolchain/semantics/testdata/function/definition/params_two_comma.carbon
index 2a876e6953fb..2266ae5d90ff 100644
--- a/toolchain/semantics/testdata/function/definition/params_two_comma.carbon
+++ b/toolchain/semantics/testdata/function/definition/params_two_comma.carbon
@@ -26,6 +26,7 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+2, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -44,6 +45,7 @@
 // CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/definition/params_zero.carbon b/toolchain/semantics/testdata/function/definition/params_zero.carbon
index 93d9c47a51c8..3d563729dfc7 100644
--- a/toolchain/semantics/testdata/function/definition/params_zero.carbon
+++ b/toolchain/semantics/testdata/function/definition/params_zero.carbon
@@ -19,6 +19,7 @@
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: nodes: [
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -27,6 +28,7 @@
 // CHECK:STDOUT:     node+0,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/function/definition/same_param_name.carbon b/toolchain/semantics/testdata/function/definition/same_param_name.carbon
index a6463c5bf990..3244f4099983 100644
--- a/toolchain/semantics/testdata/function/definition/same_param_name.carbon
+++ b/toolchain/semantics/testdata/function/definition/same_param_name.carbon
@@ -25,9 +25,11 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
-// CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+3, type: type0},
+// CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+4, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -41,18 +43,20 @@
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+2,
-// CHECK:STDOUT:     node+5,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+3,
-// CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/if/else.carbon b/toolchain/semantics/testdata/if/else.carbon
index 78fdd85b6426..66f37ede3585 100644
--- a/toolchain/semantics/testdata/if/else.carbon
+++ b/toolchain/semantics/testdata/if/else.carbon
@@ -8,7 +8,7 @@
 // CHECK:STDOUT:   {name: str0, param_refs: block0, body: {block2}}},
 // CHECK:STDOUT:   {name: str1, param_refs: block0, body: {block3}}},
 // CHECK:STDOUT:   {name: str2, param_refs: block0, body: {block4}}},
-// CHECK:STDOUT:   {name: str3, param_refs: block6, body: {block7, block9, block8, block10}}},
+// CHECK:STDOUT:   {name: str3, param_refs: block6, body: {block7, block8, block9, block10}}},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: integer_literals: [
 // CHECK:STDOUT: ]
@@ -27,55 +27,63 @@
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: nodes: [
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function2},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
-// CHECK:STDOUT:   {kind: BindName, arg0: str4, arg1: node+3, type: type1},
+// CHECK:STDOUT:   {kind: BindName, arg0: str4, arg1: node+6, type: type1},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function3},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block9, arg1: node+3},
-// CHECK:STDOUT:   {kind: Branch, arg0: block8},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block8, arg1: node+6},
+// CHECK:STDOUT:   {kind: Branch, arg0: block9},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function0},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function1},
 // CHECK:STDOUT:   {kind: Branch, arg0: block10},
 // CHECK:STDOUT:   {kind: Branch, arg0: block10},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function2},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+0,
-// CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:     node+2,
-// CHECK:STDOUT:     node+5,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+3,
-// CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+9,
-// CHECK:STDOUT:     node+11,
+// CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+8,
+// CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:     node+10,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+11,
+// CHECK:STDOUT:     node+13,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+12,
+// CHECK:STDOUT:     node+14,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+15,
+// CHECK:STDOUT:     node+16,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/if/fail_reachable_fallthrough.carbon b/toolchain/semantics/testdata/if/fail_reachable_fallthrough.carbon
new file mode 100644
index 000000000000..08d642c6c55f
--- /dev/null
+++ b/toolchain/semantics/testdata/if/fail_reachable_fallthrough.carbon
@@ -0,0 +1,143 @@
+// 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
+//
+// AUTOUPDATE
+// CHECK:STDOUT: cross_reference_irs_size: 1
+// CHECK:STDOUT: functions: [
+// CHECK:STDOUT:   {name: str0, param_refs: block2, return_type: type1, body: {block4, block5, block6, block7}}},
+// CHECK:STDOUT:   {name: str2, param_refs: block9, return_type: type1, body: {block10, block11, block12, block13}}},
+// CHECK:STDOUT:   {name: str3, param_refs: block15, return_type: type1, body: {block16, block17, block18}}},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: integer_literals: [
+// CHECK:STDOUT:   1,
+// CHECK:STDOUT:   2,
+// CHECK:STDOUT:   1,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: real_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: strings: [
+// CHECK:STDOUT:   If1,
+// CHECK:STDOUT:   b,
+// CHECK:STDOUT:   If2,
+// CHECK:STDOUT:   If3,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: types: [
+// CHECK:STDOUT:   nodeBoolType,
+// CHECK:STDOUT:   nodeIntegerType,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: nodes: [
+// CHECK:STDOUT:   {kind: VarStorage, type: type0},
+// CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
+// CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block5, arg1: node+0},
+// CHECK:STDOUT:   {kind: Branch, arg0: block6},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type1},
+// CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+5, type: type1},
+// CHECK:STDOUT:   {kind: Branch, arg0: block7},
+// CHECK:STDOUT:   {kind: VarStorage, type: type0},
+// CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+8, type: type0},
+// CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block11, arg1: node+8},
+// CHECK:STDOUT:   {kind: Branch, arg0: block12},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type1},
+// CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+13, type: type1},
+// CHECK:STDOUT:   {kind: Branch, arg0: block13},
+// CHECK:STDOUT:   {kind: VarStorage, type: type0},
+// CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+16, type: type0},
+// CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function2},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block17, arg1: node+16},
+// CHECK:STDOUT:   {kind: Branch, arg0: block18},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int2, type: type1},
+// CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+21, type: type1},
+// CHECK:STDOUT:   {kind: Branch, arg0: block18},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: node_blocks: [
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+0,
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:     node+10,
+// CHECK:STDOUT:     node+18,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+3,
+// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:     node+6,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+7,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+8,
+// CHECK:STDOUT:     node+9,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+9,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+11,
+// CHECK:STDOUT:     node+12,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+15,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+13,
+// CHECK:STDOUT:     node+14,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+16,
+// CHECK:STDOUT:     node+17,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+17,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+19,
+// CHECK:STDOUT:     node+20,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+21,
+// CHECK:STDOUT:     node+22,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT: ]
+
+fn If1(b: bool) -> i32 {
+  if (b) {
+    return 1;
+  } else {
+  }
+// CHECK:STDERR: fail_reachable_fallthrough.carbon:[[@LINE+1]]:1: Missing `return` at end of function with declared return type.
+}
+
+fn If2(b: bool) -> i32 {
+  if (b) {
+  } else {
+    return 2;
+  }
+// CHECK:STDERR: fail_reachable_fallthrough.carbon:[[@LINE+1]]:1: Missing `return` at end of function with declared return type.
+}
+
+fn If3(b: bool) -> i32 {
+  if (b) {
+    return 1;
+  }
+// CHECK:STDERR: fail_reachable_fallthrough.carbon:[[@LINE+1]]:1: Missing `return` at end of function with declared return type.
+}
diff --git a/toolchain/semantics/testdata/if/fail_scope.carbon b/toolchain/semantics/testdata/if/fail_scope.carbon
index 668e4ac5adf0..e54effa722f9 100644
--- a/toolchain/semantics/testdata/if/fail_scope.carbon
+++ b/toolchain/semantics/testdata/if/fail_scope.carbon
@@ -5,7 +5,7 @@
 // AUTOUPDATE
 // CHECK:STDOUT: cross_reference_irs_size: 1
 // CHECK:STDOUT: functions: [
-// CHECK:STDOUT:   {name: str0, param_refs: block2, return_type: type1, body: {block4, block6, block5}}},
+// CHECK:STDOUT:   {name: str0, param_refs: block2, return_type: type1, body: {block4, block5, block6}}},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: integer_literals: [
 // CHECK:STDOUT:   2,
@@ -25,14 +25,14 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block6, arg1: node+0},
-// CHECK:STDOUT:   {kind: Branch, arg0: block5},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block5, arg1: node+0},
+// CHECK:STDOUT:   {kind: Branch, arg0: block6},
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
 // CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+5, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type1},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+5, arg1: node+7, type: type1},
 // CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+5, type: type1},
-// CHECK:STDOUT:   {kind: Branch, arg0: block5},
+// CHECK:STDOUT:   {kind: Branch, arg0: block6},
 // CHECK:STDOUT:   {kind: ReturnExpression, arg0: nodeInvalidType, type: typeInvalidType},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
@@ -53,15 +53,14 @@
 // CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+11,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:     node+9,
-// CHECK:STDOUT:     node+10,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+11,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/if/no_else.carbon b/toolchain/semantics/testdata/if/no_else.carbon
index 1c72ed43418b..3c23ab994d3a 100644
--- a/toolchain/semantics/testdata/if/no_else.carbon
+++ b/toolchain/semantics/testdata/if/no_else.carbon
@@ -7,7 +7,7 @@
 // CHECK:STDOUT: functions: [
 // CHECK:STDOUT:   {name: str0, param_refs: block0, body: {block2}}},
 // CHECK:STDOUT:   {name: str1, param_refs: block0, body: {block3}}},
-// CHECK:STDOUT:   {name: str2, param_refs: block5, body: {block6, block8, block7}}},
+// CHECK:STDOUT:   {name: str2, param_refs: block5, body: {block6, block7, block8}}},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: integer_literals: [
 // CHECK:STDOUT: ]
@@ -25,46 +25,52 @@
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: nodes: [
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
-// CHECK:STDOUT:   {kind: BindName, arg0: str3, arg1: node+2, type: type1},
+// CHECK:STDOUT:   {kind: BindName, arg0: str3, arg1: node+4, type: type1},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function2},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block8, arg1: node+2},
-// CHECK:STDOUT:   {kind: Branch, arg0: block7},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block7, arg1: node+4},
+// CHECK:STDOUT:   {kind: Branch, arg0: block8},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function0},
-// CHECK:STDOUT:   {kind: Branch, arg0: block7},
+// CHECK:STDOUT:   {kind: Branch, arg0: block8},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+0,
-// CHECK:STDOUT:     node+1,
-// CHECK:STDOUT:     node+4,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+2,
-// CHECK:STDOUT:     node+3,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+3,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+9,
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+3,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+9,
+// CHECK:STDOUT:     node+10,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+11,
+// CHECK:STDOUT:     node+12,
+// CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
 fn F() {}
diff --git a/toolchain/semantics/testdata/if/unreachable_fallthrough.carbon b/toolchain/semantics/testdata/if/unreachable_fallthrough.carbon
new file mode 100644
index 000000000000..e81934388b42
--- /dev/null
+++ b/toolchain/semantics/testdata/if/unreachable_fallthrough.carbon
@@ -0,0 +1,69 @@
+// 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
+//
+// AUTOUPDATE
+// CHECK:STDOUT: cross_reference_irs_size: 1
+// CHECK:STDOUT: functions: [
+// CHECK:STDOUT:   {name: str0, param_refs: block2, return_type: type1, body: {block4, block5, block6}}},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: integer_literals: [
+// CHECK:STDOUT:   1,
+// CHECK:STDOUT:   2,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: real_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: strings: [
+// CHECK:STDOUT:   If,
+// CHECK:STDOUT:   b,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: types: [
+// CHECK:STDOUT:   nodeBoolType,
+// CHECK:STDOUT:   nodeIntegerType,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: nodes: [
+// CHECK:STDOUT:   {kind: VarStorage, type: type0},
+// CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
+// CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block5, arg1: node+0},
+// CHECK:STDOUT:   {kind: Branch, arg0: block6},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type1},
+// CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+5, type: type1},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type1},
+// CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+7, type: type1},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: node_blocks: [
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+0,
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+3,
+// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:     node+6,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+7,
+// CHECK:STDOUT:     node+8,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT: ]
+
+fn If(b: bool) -> i32 {
+  if (b) {
+    return 1;
+  } else {
+    return 2;
+  }
+  // Missing return here is OK.
+}
diff --git a/toolchain/semantics/testdata/if_expression/basic.carbon b/toolchain/semantics/testdata/if_expression/basic.carbon
index d06fe0908f2f..cc83b51b48e3 100644
--- a/toolchain/semantics/testdata/if_expression/basic.carbon
+++ b/toolchain/semantics/testdata/if_expression/basic.carbon
@@ -5,7 +5,7 @@
 // AUTOUPDATE
 // CHECK:STDOUT: cross_reference_irs_size: 1
 // CHECK:STDOUT: functions: [
-// CHECK:STDOUT:   {name: str0, param_refs: block2, return_type: type1, body: {block4, block6, block5, block7}}},
+// CHECK:STDOUT:   {name: str0, param_refs: block2, return_type: type1, body: {block4, block5, block6, block7}}},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: integer_literals: [
 // CHECK:STDOUT: ]
@@ -29,8 +29,8 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
 // CHECK:STDOUT:   {kind: BindName, arg0: str3, arg1: node+4, type: type1},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block6, arg1: node+0},
-// CHECK:STDOUT:   {kind: Branch, arg0: block5},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block5, arg1: node+0},
+// CHECK:STDOUT:   {kind: Branch, arg0: block6},
 // CHECK:STDOUT:   {kind: BinaryOperatorAdd, arg0: node+2, arg1: node+4, type: type1},
 // CHECK:STDOUT:   {kind: BinaryOperatorAdd, arg0: node+4, arg1: node+2, type: type1},
 // CHECK:STDOUT:   {kind: BranchWithArg, arg0: block7, arg1: node+9},
@@ -62,14 +62,14 @@
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+10,
-// CHECK:STDOUT:     node+12,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:     node+11,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+10,
+// CHECK:STDOUT:     node+12,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+13,
 // CHECK:STDOUT:     node+14,
 // CHECK:STDOUT:   ],
diff --git a/toolchain/semantics/testdata/if_expression/constant_condition.carbon b/toolchain/semantics/testdata/if_expression/constant_condition.carbon
index 399ea351e142..ac9f9afb9312 100644
--- a/toolchain/semantics/testdata/if_expression/constant_condition.carbon
+++ b/toolchain/semantics/testdata/if_expression/constant_condition.carbon
@@ -7,8 +7,8 @@
 // CHECK:STDOUT: functions: [
 // CHECK:STDOUT:   {name: str0, param_refs: block0, return_type: type0, body: {block2}}},
 // CHECK:STDOUT:   {name: str1, param_refs: block0, return_type: type0, body: {block3}}},
-// CHECK:STDOUT:   {name: str2, param_refs: block0, return_type: type0, body: {block4, block6, block5, block7}}},
-// CHECK:STDOUT:   {name: str3, param_refs: block0, return_type: type0, body: {block8, block10, block9, block11}}},
+// CHECK:STDOUT:   {name: str2, param_refs: block0, return_type: type0, body: {block4, block5, block6, block7}}},
+// CHECK:STDOUT:   {name: str3, param_refs: block0, return_type: type0, body: {block8, block9, block10, block11}}},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: integer_literals: [
 // CHECK:STDOUT:   1,
@@ -35,8 +35,8 @@
 // CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+4, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function2},
 // CHECK:STDOUT:   {kind: BoolLiteral, arg0: true, type: type1},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block6, arg1: node+7},
-// CHECK:STDOUT:   {kind: Branch, arg0: block5},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block5, arg1: node+7},
+// CHECK:STDOUT:   {kind: Branch, arg0: block6},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function0, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function1, type: type0},
 // CHECK:STDOUT:   {kind: BranchWithArg, arg0: block7, arg1: node+10},
@@ -45,8 +45,8 @@
 // CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+14, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function3},
 // CHECK:STDOUT:   {kind: BoolLiteral, arg0: false, type: type1},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block10, arg1: node+17},
-// CHECK:STDOUT:   {kind: Branch, arg0: block9},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block9, arg1: node+17},
+// CHECK:STDOUT:   {kind: Branch, arg0: block10},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function0, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function1, type: type0},
 // CHECK:STDOUT:   {kind: BranchWithArg, arg0: block11, arg1: node+20},
@@ -77,14 +77,14 @@
 // CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+11,
-// CHECK:STDOUT:     node+13,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+10,
 // CHECK:STDOUT:     node+12,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+11,
+// CHECK:STDOUT:     node+13,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+14,
 // CHECK:STDOUT:     node+15,
 // CHECK:STDOUT:   ],
@@ -94,14 +94,14 @@
 // CHECK:STDOUT:     node+19,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+21,
-// CHECK:STDOUT:     node+23,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+20,
 // CHECK:STDOUT:     node+22,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+21,
+// CHECK:STDOUT:     node+23,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+24,
 // CHECK:STDOUT:     node+25,
 // CHECK:STDOUT:   ],
diff --git a/toolchain/semantics/testdata/if_expression/control_flow.carbon b/toolchain/semantics/testdata/if_expression/control_flow.carbon
index 3e1812e05318..28999d77fcb8 100644
--- a/toolchain/semantics/testdata/if_expression/control_flow.carbon
+++ b/toolchain/semantics/testdata/if_expression/control_flow.carbon
@@ -7,7 +7,7 @@
 // CHECK:STDOUT: functions: [
 // CHECK:STDOUT:   {name: str0, param_refs: block0, return_type: type0, body: {block2}}},
 // CHECK:STDOUT:   {name: str1, param_refs: block0, return_type: type0, body: {block3}}},
-// CHECK:STDOUT:   {name: str2, param_refs: block5, return_type: type0, body: {block6, block8, block7, block9}}},
+// CHECK:STDOUT:   {name: str2, param_refs: block5, return_type: type0, body: {block6, block7, block8, block9}}},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: integer_literals: [
 // CHECK:STDOUT:   1,
@@ -35,8 +35,8 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
 // CHECK:STDOUT:   {kind: BindName, arg0: str3, arg1: node+6, type: type1},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function2},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block8, arg1: node+6},
-// CHECK:STDOUT:   {kind: Branch, arg0: block7},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block7, arg1: node+6},
+// CHECK:STDOUT:   {kind: Branch, arg0: block8},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function0, type: type0},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function1, type: type0},
 // CHECK:STDOUT:   {kind: BranchWithArg, arg0: block9, arg1: node+11},
@@ -72,14 +72,14 @@
 // CHECK:STDOUT:     node+10,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+12,
-// CHECK:STDOUT:     node+14,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+11,
 // CHECK:STDOUT:     node+13,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+12,
+// CHECK:STDOUT:     node+14,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+15,
 // CHECK:STDOUT:     node+16,
 // CHECK:STDOUT:   ],
diff --git a/toolchain/semantics/testdata/if_expression/nested.carbon b/toolchain/semantics/testdata/if_expression/nested.carbon
index 22c88841bb07..faf654107ba1 100644
--- a/toolchain/semantics/testdata/if_expression/nested.carbon
+++ b/toolchain/semantics/testdata/if_expression/nested.carbon
@@ -5,7 +5,7 @@
 // AUTOUPDATE
 // CHECK:STDOUT: cross_reference_irs_size: 1
 // CHECK:STDOUT: functions: [
-// CHECK:STDOUT:   {name: str0, param_refs: block2, return_type: type1, body: {block4, block6, block8, block7, block9, block5, block11, block10, block12, block13}}},
+// CHECK:STDOUT:   {name: str0, param_refs: block2, return_type: type1, body: {block4, block5, block7, block8, block9, block6, block10, block11, block12, block13}}},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: integer_literals: [
 // CHECK:STDOUT:   1,
@@ -33,17 +33,17 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str3, arg1: node+4, type: type0},
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block6, arg1: node+0},
-// CHECK:STDOUT:   {kind: Branch, arg0: block5},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block8, arg1: node+2},
-// CHECK:STDOUT:   {kind: Branch, arg0: block7},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block5, arg1: node+0},
+// CHECK:STDOUT:   {kind: Branch, arg0: block6},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block7, arg1: node+2},
+// CHECK:STDOUT:   {kind: Branch, arg0: block8},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type1},
 // CHECK:STDOUT:   {kind: BranchWithArg, arg0: block9, arg1: node+11},
 // CHECK:STDOUT:   {kind: BranchWithArg, arg0: block9, arg1: node+12},
 // CHECK:STDOUT:   {kind: BlockArg, arg0: block9, type: type1},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block11, arg1: node+4},
-// CHECK:STDOUT:   {kind: Branch, arg0: block10},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block10, arg1: node+4},
+// CHECK:STDOUT:   {kind: Branch, arg0: block11},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int2, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int3, type: type1},
 // CHECK:STDOUT:   {kind: BranchWithArg, arg0: block12, arg1: node+18},
@@ -78,34 +78,34 @@
 // CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+16,
-// CHECK:STDOUT:     node+17,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:     node+10,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+12,
-// CHECK:STDOUT:     node+14,
+// CHECK:STDOUT:     node+16,
+// CHECK:STDOUT:     node+17,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+11,
 // CHECK:STDOUT:     node+13,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+12,
+// CHECK:STDOUT:     node+14,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+15,
 // CHECK:STDOUT:     node+23,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+19,
-// CHECK:STDOUT:     node+21,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+18,
 // CHECK:STDOUT:     node+20,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+19,
+// CHECK:STDOUT:     node+21,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+22,
 // CHECK:STDOUT:     node+24,
 // CHECK:STDOUT:   ],
diff --git a/toolchain/semantics/testdata/operators/and.carbon b/toolchain/semantics/testdata/operators/and.carbon
index babe1564c4d1..44e30572a3f6 100644
--- a/toolchain/semantics/testdata/operators/and.carbon
+++ b/toolchain/semantics/testdata/operators/and.carbon
@@ -7,7 +7,7 @@
 // CHECK:STDOUT: functions: [
 // CHECK:STDOUT:   {name: str0, param_refs: block0, return_type: type0, body: {block2}}},
 // CHECK:STDOUT:   {name: str1, param_refs: block0, return_type: type0, body: {block3}}},
-// CHECK:STDOUT:   {name: str2, param_refs: block0, return_type: type0, body: {block4, block6, block5}}},
+// CHECK:STDOUT:   {name: str2, param_refs: block0, return_type: type0, body: {block4, block5, block6}}},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: integer_literals: [
 // CHECK:STDOUT: ]
@@ -31,11 +31,11 @@
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function2},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function0, type: type0},
 // CHECK:STDOUT:   {kind: BoolLiteral, arg0: false, type: type0},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block6, arg1: node+7},
-// CHECK:STDOUT:   {kind: BranchWithArg, arg0: block5, arg1: node+8},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block5, arg1: node+7},
+// CHECK:STDOUT:   {kind: BranchWithArg, arg0: block6, arg1: node+8},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function1, type: type0},
-// CHECK:STDOUT:   {kind: BranchWithArg, arg0: block5, arg1: node+11},
-// CHECK:STDOUT:   {kind: BlockArg, arg0: block5, type: type0},
+// CHECK:STDOUT:   {kind: BranchWithArg, arg0: block6, arg1: node+11},
+// CHECK:STDOUT:   {kind: BlockArg, arg0: block6, type: type0},
 // CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+13, type: type0},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
@@ -61,13 +61,13 @@
 // CHECK:STDOUT:     node+10,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+13,
-// CHECK:STDOUT:     node+14,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+11,
 // CHECK:STDOUT:     node+12,
 // CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+13,
+// CHECK:STDOUT:     node+14,
+// CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
 fn F() -> bool { return true; }
diff --git a/toolchain/semantics/testdata/operators/or.carbon b/toolchain/semantics/testdata/operators/or.carbon
index 8b64ffa3946b..d1814045283e 100644
--- a/toolchain/semantics/testdata/operators/or.carbon
+++ b/toolchain/semantics/testdata/operators/or.carbon
@@ -7,7 +7,7 @@
 // CHECK:STDOUT: functions: [
 // CHECK:STDOUT:   {name: str0, param_refs: block0, return_type: type0, body: {block2}}},
 // CHECK:STDOUT:   {name: str1, param_refs: block0, return_type: type0, body: {block3}}},
-// CHECK:STDOUT:   {name: str2, param_refs: block0, return_type: type0, body: {block4, block6, block5}}},
+// CHECK:STDOUT:   {name: str2, param_refs: block0, return_type: type0, body: {block4, block5, block6}}},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: integer_literals: [
 // CHECK:STDOUT: ]
@@ -32,11 +32,11 @@
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function0, type: type0},
 // CHECK:STDOUT:   {kind: UnaryOperatorNot, arg0: node+7, type: type0},
 // CHECK:STDOUT:   {kind: BoolLiteral, arg0: true, type: type0},
-// CHECK:STDOUT:   {kind: BranchIf, arg0: block6, arg1: node+8},
-// CHECK:STDOUT:   {kind: BranchWithArg, arg0: block5, arg1: node+9},
+// CHECK:STDOUT:   {kind: BranchIf, arg0: block5, arg1: node+8},
+// CHECK:STDOUT:   {kind: BranchWithArg, arg0: block6, arg1: node+9},
 // CHECK:STDOUT:   {kind: Call, arg0: block0, arg1: function1, type: type0},
-// CHECK:STDOUT:   {kind: BranchWithArg, arg0: block5, arg1: node+12},
-// CHECK:STDOUT:   {kind: BlockArg, arg0: block5, type: type0},
+// CHECK:STDOUT:   {kind: BranchWithArg, arg0: block6, arg1: node+12},
+// CHECK:STDOUT:   {kind: BlockArg, arg0: block6, type: type0},
 // CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+14, type: type0},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
@@ -63,13 +63,13 @@
 // CHECK:STDOUT:     node+11,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
-// CHECK:STDOUT:     node+14,
-// CHECK:STDOUT:     node+15,
-// CHECK:STDOUT:   ],
-// CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+12,
 // CHECK:STDOUT:     node+13,
 // CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+14,
+// CHECK:STDOUT:     node+15,
+// CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
 fn F() -> bool { return true; }
diff --git a/toolchain/semantics/testdata/return/code_after_return.carbon b/toolchain/semantics/testdata/return/code_after_return.carbon
new file mode 100644
index 000000000000..b4f4c292c392
--- /dev/null
+++ b/toolchain/semantics/testdata/return/code_after_return.carbon
@@ -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
+//
+// AUTOUPDATE
+// CHECK:STDOUT: cross_reference_irs_size: 1
+// CHECK:STDOUT: functions: [
+// CHECK:STDOUT:   {name: str0, param_refs: block0, body: {block2}}},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: integer_literals: [
+// CHECK:STDOUT:   1,
+// CHECK:STDOUT:   1,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: real_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: strings: [
+// CHECK:STDOUT:   Main,
+// CHECK:STDOUT:   n,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: types: [
+// CHECK:STDOUT:   nodeEmptyTupleType,
+// CHECK:STDOUT:   nodeIntegerType,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: nodes: [
+// CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
+// CHECK:STDOUT:   {kind: VarStorage, type: type1},
+// CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+2, type: type1},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type1},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type1},
+// CHECK:STDOUT:   {kind: BinaryOperatorAdd, arg0: node+4, arg1: node+5, type: type1},
+// CHECK:STDOUT:   {kind: Assign, arg0: node+2, arg1: node+6, type: type1},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: node_blocks: [
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+0,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT: ]
+
+fn Main() {
+  return;
+  var n: i32 = 1 + 1;
+}
diff --git a/toolchain/semantics/testdata/return/code_after_return_value.carbon b/toolchain/semantics/testdata/return/code_after_return_value.carbon
new file mode 100644
index 000000000000..b550d4f3f5c3
--- /dev/null
+++ b/toolchain/semantics/testdata/return/code_after_return_value.carbon
@@ -0,0 +1,76 @@
+// 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
+//
+// AUTOUPDATE
+// CHECK:STDOUT: cross_reference_irs_size: 1
+// CHECK:STDOUT: functions: [
+// CHECK:STDOUT:   {name: str0, param_refs: block2, return_type: type1, body: {block4}}},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: integer_literals: [
+// CHECK:STDOUT:   0,
+// CHECK:STDOUT:   1,
+// CHECK:STDOUT:   2,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: real_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: strings: [
+// CHECK:STDOUT:   F,
+// CHECK:STDOUT:   b,
+// CHECK:STDOUT:   n,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: types: [
+// CHECK:STDOUT:   nodeBoolType,
+// CHECK:STDOUT:   nodeIntegerType,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: nodes: [
+// CHECK:STDOUT:   {kind: VarStorage, type: type0},
+// CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+0, type: type0},
+// CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type1},
+// CHECK:STDOUT:   {kind: ReturnExpression, arg0: node+3, type: type1},
+// CHECK:STDOUT:   {kind: VarStorage, type: type1},
+// CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+5, type: type1},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type1},
+// CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int2, type: type1},
+// CHECK:STDOUT:   {kind: BlockArg, arg0: unreachable, type: type1},
+// CHECK:STDOUT:   {kind: Assign, arg0: node+5, arg1: node+9, type: type1},
+// CHECK:STDOUT:   {kind: BoolLiteral, arg0: false, type: type0},
+// CHECK:STDOUT:   {kind: UnaryOperatorNot, arg0: node+0, type: type0},
+// CHECK:STDOUT:   {kind: BoolLiteral, arg0: true, type: type0},
+// CHECK:STDOUT:   {kind: BranchWithArg, arg0: unreachable, arg1: node+0},
+// CHECK:STDOUT:   {kind: BlockArg, arg0: unreachable, type: type0},
+// CHECK:STDOUT:   {kind: BranchWithArg, arg0: unreachable, arg1: node+15},
+// CHECK:STDOUT:   {kind: BlockArg, arg0: unreachable, type: type0},
+// CHECK:STDOUT:   {kind: Branch, arg0: unreachable},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: node_blocks: [
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+0,
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+3,
+// CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT: ]
+
+fn F(b: bool) -> i32 {
+  return 0;
+
+  // Exercise various forms of control flow to check that we still consider the
+  // end of the function unreachable after them.
+  var n: i32 = if b then 1 else 2;
+  if (b and (b or b)) { }
+  if (b) { } else { }
+
+  // Unreachable, no error on missing return.
+}
diff --git a/toolchain/semantics/testdata/return/fail_missing_return.carbon b/toolchain/semantics/testdata/return/fail_missing_return.carbon
new file mode 100644
index 000000000000..f12ae075c595
--- /dev/null
+++ b/toolchain/semantics/testdata/return/fail_missing_return.carbon
@@ -0,0 +1,35 @@
+// 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
+//
+// AUTOUPDATE
+// CHECK:STDOUT: cross_reference_irs_size: 1
+// CHECK:STDOUT: functions: [
+// CHECK:STDOUT:   {name: str0, param_refs: block0, return_type: type0, body: {block2}}},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: integer_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: real_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: strings: [
+// CHECK:STDOUT:   Main,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: types: [
+// CHECK:STDOUT:   nodeIntegerType,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: nodes: [
+// CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: node_blocks: [
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+0,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT: ]
+
+fn Main() -> i32 {
+// CHECK:STDERR: fail_missing_return.carbon:[[@LINE+1]]:1: Missing `return` at end of function with declared return type.
+}
diff --git a/toolchain/semantics/testdata/return/fail_missing_return_empty_tuple.carbon b/toolchain/semantics/testdata/return/fail_missing_return_empty_tuple.carbon
new file mode 100644
index 000000000000..5847198f3a23
--- /dev/null
+++ b/toolchain/semantics/testdata/return/fail_missing_return_empty_tuple.carbon
@@ -0,0 +1,27 @@
+// 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
+//
+// AUTOUPDATE
+// CHECK:STDOUT: cross_reference_irs_size: 1
+// CHECK:STDOUT: functions: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: integer_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: real_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: strings: [
+// CHECK:STDOUT:   F,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: types: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: nodes: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: node_blocks: [
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT: ]
+
+// CHECK:STDERR: fail_missing_return_empty_tuple.carbon:[[@LINE+1]]:12: Semantics TODO: HandleTupleLiteral
+fn F() -> () {
+}
diff --git a/toolchain/semantics/testdata/return/missing_return_no_return_type.carbon b/toolchain/semantics/testdata/return/missing_return_no_return_type.carbon
new file mode 100644
index 000000000000..f94e7771fd86
--- /dev/null
+++ b/toolchain/semantics/testdata/return/missing_return_no_return_type.carbon
@@ -0,0 +1,36 @@
+// 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
+//
+// AUTOUPDATE
+// CHECK:STDOUT: cross_reference_irs_size: 1
+// CHECK:STDOUT: functions: [
+// CHECK:STDOUT:   {name: str0, param_refs: block0, body: {block2}}},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: integer_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: real_literals: [
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: strings: [
+// CHECK:STDOUT:   F,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: types: [
+// CHECK:STDOUT:   nodeEmptyTupleType,
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: nodes: [
+// CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
+// CHECK:STDOUT:   {kind: Return},
+// CHECK:STDOUT: ]
+// CHECK:STDOUT: node_blocks: [
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+0,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT:   [
+// CHECK:STDOUT:     node+1,
+// CHECK:STDOUT:   ],
+// CHECK:STDOUT: ]
+
+fn F() {
+}
diff --git a/toolchain/semantics/testdata/var/decl.carbon b/toolchain/semantics/testdata/var/decl.carbon
index abce74aa048a..df17f24b949e 100644
--- a/toolchain/semantics/testdata/var/decl.carbon
+++ b/toolchain/semantics/testdata/var/decl.carbon
@@ -23,6 +23,7 @@
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+1, type: type1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -33,6 +34,7 @@
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/var/decl_with_init.carbon b/toolchain/semantics/testdata/var/decl_with_init.carbon
index 3d304fd7f423..e393eb4c09cb 100644
--- a/toolchain/semantics/testdata/var/decl_with_init.carbon
+++ b/toolchain/semantics/testdata/var/decl_with_init.carbon
@@ -26,6 +26,7 @@
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+1, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type1},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+1, arg1: node+3, type: type1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -38,6 +39,7 @@
 // CHECK:STDOUT:     node+2,
 // CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/var/fail_duplicate_decl.carbon b/toolchain/semantics/testdata/var/fail_duplicate_decl.carbon
index 4e798a595443..17b9c287ef10 100644
--- a/toolchain/semantics/testdata/var/fail_duplicate_decl.carbon
+++ b/toolchain/semantics/testdata/var/fail_duplicate_decl.carbon
@@ -31,6 +31,7 @@
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+5, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type1},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+5, arg1: node+7, type: type1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -47,6 +48,7 @@
 // CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:     node+7,
 // CHECK:STDOUT:     node+8,
+// CHECK:STDOUT:     node+9,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/var/fail_init_type_mismatch.carbon b/toolchain/semantics/testdata/var/fail_init_type_mismatch.carbon
index 84085dbda003..92d1f9ccf82b 100644
--- a/toolchain/semantics/testdata/var/fail_init_type_mismatch.carbon
+++ b/toolchain/semantics/testdata/var/fail_init_type_mismatch.carbon
@@ -27,6 +27,7 @@
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+1, type: type1},
 // CHECK:STDOUT:   {kind: RealLiteral, arg0: real0, type: type2},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+1, arg1: nodeInvalidType, type: typeInvalidType},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -39,6 +40,7 @@
 // CHECK:STDOUT:     node+2,
 // CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/var/fail_init_with_self.carbon b/toolchain/semantics/testdata/var/fail_init_with_self.carbon
index e412dae4b5a0..6d48d4ce8af9 100644
--- a/toolchain/semantics/testdata/var/fail_init_with_self.carbon
+++ b/toolchain/semantics/testdata/var/fail_init_with_self.carbon
@@ -24,6 +24,7 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+1, type: type1},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+1, arg1: nodeInvalidType, type: typeInvalidType},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -35,6 +36,7 @@
 // CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:     node+2,
 // CHECK:STDOUT:     node+3,
+// CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/var/fail_lookup_outside_scope.carbon b/toolchain/semantics/testdata/var/fail_lookup_outside_scope.carbon
index d2627d2a83a7..7be851a90eea 100644
--- a/toolchain/semantics/testdata/var/fail_lookup_outside_scope.carbon
+++ b/toolchain/semantics/testdata/var/fail_lookup_outside_scope.carbon
@@ -24,22 +24,24 @@
 // CHECK:STDOUT:   {kind: FunctionDeclaration, arg0: function0},
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+1, type: type1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT:   {kind: VarStorage, type: type1},
-// CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+3, type: type1},
-// CHECK:STDOUT:   {kind: Assign, arg0: node+3, arg1: nodeInvalidType, type: typeInvalidType},
+// CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+4, type: type1},
+// CHECK:STDOUT:   {kind: Assign, arg0: node+4, arg1: nodeInvalidType, type: typeInvalidType},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+0,
-// CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT:   [
 // CHECK:STDOUT:     node+1,
 // CHECK:STDOUT:     node+2,
+// CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/var/fail_storage_is_literal.carbon b/toolchain/semantics/testdata/var/fail_storage_is_literal.carbon
index 6319595e33c5..bde1601a1b1e 100644
--- a/toolchain/semantics/testdata/var/fail_storage_is_literal.carbon
+++ b/toolchain/semantics/testdata/var/fail_storage_is_literal.carbon
@@ -28,6 +28,7 @@
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+2, type: typeInvalidType},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int1, type: type1},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+2, arg1: nodeInvalidType, type: typeInvalidType},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -41,6 +42,7 @@
 // CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:     node+4,
 // CHECK:STDOUT:     node+5,
+// CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/var/global_lookup_in_scope.carbon b/toolchain/semantics/testdata/var/global_lookup_in_scope.carbon
index 567752b4fbb5..dfc8903ed538 100644
--- a/toolchain/semantics/testdata/var/global_lookup_in_scope.carbon
+++ b/toolchain/semantics/testdata/var/global_lookup_in_scope.carbon
@@ -30,6 +30,7 @@
 // CHECK:STDOUT:   {kind: VarStorage, type: type0},
 // CHECK:STDOUT:   {kind: BindName, arg0: str2, arg1: node+5, type: type0},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+5, arg1: node+0, type: type0},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -45,6 +46,7 @@
 // CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:     node+6,
 // CHECK:STDOUT:     node+7,
+// CHECK:STDOUT:     node+8,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]
 
diff --git a/toolchain/semantics/testdata/var/lookup.carbon b/toolchain/semantics/testdata/var/lookup.carbon
index 8b14ffab2d69..59ba687f10e5 100644
--- a/toolchain/semantics/testdata/var/lookup.carbon
+++ b/toolchain/semantics/testdata/var/lookup.carbon
@@ -26,6 +26,7 @@
 // CHECK:STDOUT:   {kind: BindName, arg0: str1, arg1: node+1, type: type1},
 // CHECK:STDOUT:   {kind: IntegerLiteral, arg0: int0, type: type1},
 // CHECK:STDOUT:   {kind: Assign, arg0: node+1, arg1: node+3, type: type1},
+// CHECK:STDOUT:   {kind: Return},
 // CHECK:STDOUT: ]
 // CHECK:STDOUT: node_blocks: [
 // CHECK:STDOUT:   [
@@ -38,6 +39,7 @@
 // CHECK:STDOUT:     node+2,
 // CHECK:STDOUT:     node+3,
 // CHECK:STDOUT:     node+4,
+// CHECK:STDOUT:     node+5,
 // CHECK:STDOUT:   ],
 // CHECK:STDOUT: ]