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71ba07239f65b1089ff28fb671a8f6ccacc23ed5
carbon-lang/toolchain/lower/testdata/interop/cpp/thunks.carbon
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Richard Smith 71ba07239f Support pass-by-move when calling a C++ function taking by value. (#7135) 
Previously, we picked a single Carbon parameter pattern for each C++
parameter pattern. This doesn't work well in cases where the Carbon
semantics and the C++ semantics are not perfectly aligned. In
particular, when a parameter is passed by value in C++, that might mean
either pass-by-move (which in Carbon would best be modeled by a `var`
pattern, as no other form of parameter would perform a move) or
pass-by-copy (which in Carbon would best be modeled by a value
parameter, as a `var` parameter would force an extra copy).

After this change, we compute a passing mode for each parameter based on
the implicit conversion sequence from the argument to the parameter as
determined by C++ overload resolution, and use that to determine the
Carbon pattern corresponding to each C++ parameter. This results in
potentially generating multiple different thunks for the same C++
function if it's called in different ways, but we already did that to
handle default arguments and list-initialization. The passing modes are
included in the thunk mangling.

Add a new value store for clang decl signatures, which capture the
information about parameter passing mode as well as the other existing
information about different ways that a C++ function might be imported
to Carbon.

Most of the rules for computing passing modes are the same as before:
const references use pass by value, non-const lvalue references use
pass-by-ref, non-const rvalue references use pass-by-var. But for C++
non-reference parameters, pick between pass-by-value and pass-by-var
based on whether the implicit conversion sequence was effectively
performing a copy. Prefer pass-by-value if either would work and they'd
do the same thing. We still use pass-by-value for const references, even
when the argument is an lvalue and we could pass a reference; we may
want to change this in future.

For virtual functions, we try to pick a worst-case passing mode, as we
can only pick a single signature for what goes in the vtable. Calls to
virtual functions will still use a thunk to C++, allowing variance in
the calling convention at call sites. We don't allow variance in the
overriders as we don't implement support for thunks for virtual
functions yet. We currently use pass-by-value for const reference
parameters here, but that should probably change at some point.

Assisted-by: Gemini via Antigravity
2026-05-13 01:44:07 +00:00

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// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/primitives.carbon
// EXTRA-ARGS: --clang-arg=-std=c++26
//
// AUTOUPDATE
// TIP: To test this file alone, run:
// TIP: bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/lower/testdata/interop/cpp/thunks.carbon
// TIP: To dump output, run:
// TIP: bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lower/testdata/interop/cpp/thunks.carbon
// --- need_thunk.h
struct NeedThunk {
NeedThunk();
NeedThunk(const NeedThunk&);
~NeedThunk() = default;
};
// --- multiple_thunks_in_one_class.carbon
library "[[@TEST_NAME]]";
import Cpp library "need_thunk.h";
// We intend to create multiple thunks named `Op` here to demonstrate the need
// for the mangling to include the signature function.
fn Call(n: Cpp.NeedThunk) {
var unused n2: Cpp.NeedThunk = n;
}
// CHECK:STDOUT: ; ModuleID = 'multiple_thunks_in_one_class.carbon'
// CHECK:STDOUT: source_filename = "multiple_thunks_in_one_class.carbon"
// CHECK:STDOUT: target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
// CHECK:STDOUT: target triple = "x86_64-unknown-linux-gnu"
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define void @_CCall.Main(ptr %n) #0 !dbg !11 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %n2.var = alloca [1 x i8], align 1, !dbg !17
// CHECK:STDOUT: %.loc9_34.1.temp = alloca [1 x i8], align 1, !dbg !18
// CHECK:STDOUT: call void @llvm.lifetime.start.p0(ptr %n2.var), !dbg !17
// CHECK:STDOUT: call void @llvm.lifetime.start.p0(ptr %.loc9_34.1.temp), !dbg !18
// CHECK:STDOUT: call void @_ZN9NeedThunkC1ERKS_.carbon_thunk._(ptr %n, ptr %n2.var), !dbg !18
// CHECK:STDOUT: ret void, !dbg !19
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: alwaysinline mustprogress uwtable
// CHECK:STDOUT: define internal void @_ZN9NeedThunkC1ERKS_.carbon_thunk._(ptr noundef %0, ptr noundef %return) #1 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %.addr = alloca ptr, align 8
// CHECK:STDOUT: %return.addr = alloca ptr, align 8
// CHECK:STDOUT: store ptr %0, ptr %.addr, align 8, !tbaa !20
// CHECK:STDOUT: store ptr %return, ptr %return.addr, align 8, !tbaa !20
// CHECK:STDOUT: %1 = load ptr, ptr %return.addr, align 8, !tbaa !20
// CHECK:STDOUT: %2 = load ptr, ptr %.addr, align 8, !tbaa !20
// CHECK:STDOUT: call void @_ZN9NeedThunkC1ERKS_(ptr noundef nonnull align 1 dereferenceable(1) %1, ptr noundef nonnull align 1 dereferenceable(1) %2)
// CHECK:STDOUT: ret void
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: alwaysinline nounwind
// CHECK:STDOUT: define void @"_COp:thunk:Copy.fdeb6dbda014bd1f.Core:NeedThunk.Cpp"(ptr sret([1 x i8]) %return, ptr %self) #2 !dbg !23 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: call void @_ZN9NeedThunkC1ERKS_.carbon_thunk._(ptr %self, ptr %return), !dbg !29
// CHECK:STDOUT: ret void, !dbg !29
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: alwaysinline nounwind
// CHECK:STDOUT: define void @"_COp:thunk:Destroy.91afe58d20fe9768.Core:NeedThunk.Cpp"(ptr %self) #2 !dbg !30 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: ret void, !dbg !33
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nocallback nofree nosync nounwind willreturn memory(argmem: readwrite)
// CHECK:STDOUT: declare void @llvm.lifetime.start.p0(ptr captures(none)) #3
// CHECK:STDOUT:
// CHECK:STDOUT: declare void @_ZN9NeedThunkC1ERKS_(ptr noundef nonnull align 1 dereferenceable(1), ptr noundef nonnull align 1 dereferenceable(1)) unnamed_addr #4
// CHECK:STDOUT:
// CHECK:STDOUT: ; uselistorder directives
// CHECK:STDOUT: uselistorder ptr @llvm.lifetime.start.p0, { 1, 0 }
// CHECK:STDOUT:
// CHECK:STDOUT: attributes #0 = { nounwind }
// CHECK:STDOUT: attributes #1 = { alwaysinline mustprogress uwtable "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
// CHECK:STDOUT: attributes #2 = { alwaysinline nounwind }
// CHECK:STDOUT: attributes #3 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
// CHECK:STDOUT: attributes #4 = { "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cmov,+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
// CHECK:STDOUT:
// CHECK:STDOUT: !llvm.module.flags = !{!0, !1, !2, !3, !4}
// CHECK:STDOUT: !llvm.dbg.cu = !{!5}
// CHECK:STDOUT: !llvm.errno.tbaa = !{!7}
// CHECK:STDOUT:
// CHECK:STDOUT: !0 = !{i32 7, !"Dwarf Version", i32 5}
// CHECK:STDOUT: !1 = !{i32 2, !"Debug Info Version", i32 3}
// CHECK:STDOUT: !2 = !{i32 8, !"PIC Level", i32 2}
// CHECK:STDOUT: !3 = !{i32 7, !"PIE Level", i32 2}
// CHECK:STDOUT: !4 = !{i32 7, !"uwtable", i32 2}
// CHECK:STDOUT: !5 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !6, producer: "carbon", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
// CHECK:STDOUT: !6 = !DIFile(filename: "multiple_thunks_in_one_class.carbon", directory: "")
// CHECK:STDOUT: !7 = !{!8, !8, i64 0}
// CHECK:STDOUT: !8 = !{!"int", !9, i64 0}
// CHECK:STDOUT: !9 = !{!"omnipotent char", !10, i64 0}
// CHECK:STDOUT: !10 = !{!"Simple C++ TBAA"}
// CHECK:STDOUT: !11 = distinct !DISubprogram(name: "Call", linkageName: "_CCall.Main", scope: null, file: !6, line: 8, type: !12, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !15)
// CHECK:STDOUT: !12 = !DISubroutineType(types: !13)
// CHECK:STDOUT: !13 = !{null, !14}
// CHECK:STDOUT: !14 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 64)
// CHECK:STDOUT: !15 = !{!16}
// CHECK:STDOUT: !16 = !DILocalVariable(arg: 1, scope: !11, type: !14)
// CHECK:STDOUT: !17 = !DILocation(line: 9, column: 3, scope: !11)
// CHECK:STDOUT: !18 = !DILocation(line: 9, column: 34, scope: !11)
// CHECK:STDOUT: !19 = !DILocation(line: 8, column: 1, scope: !11)
// CHECK:STDOUT: !20 = !{!21, !21, i64 0}
// CHECK:STDOUT: !21 = !{!"p1 _ZTS9NeedThunk", !22, i64 0}
// CHECK:STDOUT: !22 = !{!"any pointer", !9, i64 0}
// CHECK:STDOUT: !23 = distinct !DISubprogram(name: "Op", linkageName: "_COp:thunk:Copy.fdeb6dbda014bd1f.Core:NeedThunk.Cpp", scope: null, file: !24, line: 4, type: !25, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !27)
// CHECK:STDOUT: !24 = !DIFile(filename: "./need_thunk.h", directory: "")
// CHECK:STDOUT: !25 = !DISubroutineType(types: !26)
// CHECK:STDOUT: !26 = !{!14, !14}
// CHECK:STDOUT: !27 = !{!28}
// CHECK:STDOUT: !28 = !DILocalVariable(arg: 1, scope: !23, type: !14)
// CHECK:STDOUT: !29 = !DILocation(line: 4, column: 3, scope: !23)
// CHECK:STDOUT: !30 = distinct !DISubprogram(name: "Op", linkageName: "_COp:thunk:Destroy.91afe58d20fe9768.Core:NeedThunk.Cpp", scope: null, file: !24, line: 5, type: !12, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !31)
// CHECK:STDOUT: !31 = !{!32}
// CHECK:STDOUT: !32 = !DILocalVariable(arg: 1, scope: !30, type: !14)
// CHECK:STDOUT: !33 = !DILocation(line: 5, column: 3, scope: !30)
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