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Define exported C++ thunks in the LLVM function Clang uses for them (#8060)

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Carbon gives the C++ declaration of an exported Carbon thunk an asm
label that
includes the target's global symbol prefix (`_` on Mach-O). Clang names
a
declaration with an asm label `\01<label>` in the IR, so that LLVM uses
the
label verbatim as the symbol name, while Carbon lowered the thunk
definition
under its unprefixed name. Both name the same symbol, but they were
distinct
globals in the IR, so LLVM couldn't see that the C++ calls refer to the
Carbon
definition. That only works while the definition is never discarded.

Mark the Carbon thunk as a `CppThunk` (as described in
`toolchain/docs/check/cpp/thunks.md`), record its C++ declaration in
`clang_decls`, and when lowering the thunk, ask Clang for its function
(creating it if necessary) and define the thunk in that function. This
means
that the thunk and the C++ calls to it always use the same IR global,
with
Clang's name and signature.

The thunk takes all of its parameters by reference, so Clang lowers its
parameters the same way Carbon does, but some C++ ABIs (such as Apple
arm64)
return `this` from constructors. In that case, return the parameter that
Clang
marks `returned`.

On targets without a global prefix, the IR names already matched, so the
only
change is that the thunk definitions now carry Clang's parameter and
function
attributes.
This commit is contained in:
David Blaikie authored and GitHub committed 2026-10-30 01:22:12 +00:00
1 parent 9192ee0032
commit 4abb6100e7
17 files changed
+577 -91
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toolchain/lower/testdata/interop/cpp/function/import/function_ptr.carbon
Vendored
+16 -12
View File
@@ -196,7 +196,7 @@ fn F() {
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define void @_CMyCallback__carbon_thunk.Main(ptr %_, ptr %_1) #0 !dbg !36 {
// CHECK:STDOUT: define void @_CMyCallback__carbon_thunk.Main(ptr noundef nonnull align 4 dereferenceable(4) %_, ptr noundef nonnull align 4 dereferenceable(4) %_1) #1 !dbg !36 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %.loc8 = load float, ptr %_, align 4, !dbg !39
// CHECK:STDOUT: %MyCallback.call = call i32 @_CMyCallback.Main(float %.loc8), !dbg !40
@@ -205,7 +205,7 @@ fn F() {
// 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)) #1
// CHECK:STDOUT: declare void @llvm.lifetime.start.p0(ptr captures(none)) #2
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define linkonce_odr void @_CSelfDestruct.Core.9d0db8a57c58fabc(ptr %t) #0 !dbg !43 {
@@ -213,7 +213,8 @@ fn F() {
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: attributes #0 = { nounwind }
// CHECK:STDOUT: attributes #1 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
// CHECK:STDOUT: attributes #1 = { nounwind "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 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
// CHECK:STDOUT:
// CHECK:STDOUT: !llvm.dbg.cu = !{!26}
// CHECK:STDOUT: !llvm.module.flags = !{!17, !18, !19, !2, !3}
@@ -270,7 +271,7 @@ fn F() {
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define void @_CMyCallback__carbon_thunk.Main(ptr %_, ptr %_1) #0 !dbg !55 {
// CHECK:STDOUT: define void @_CMyCallback__carbon_thunk.Main(ptr noundef nonnull align 4 dereferenceable(4) %_, ptr noundef nonnull align 4 dereferenceable(4) %_1) #1 !dbg !55 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %.loc5 = load float, ptr %_, align 4, !dbg !58
// CHECK:STDOUT: %MyCallback.call = call i32 @_CMyCallback.Main(float %.loc5), !dbg !58
@@ -278,10 +279,11 @@ fn F() {
// CHECK:STDOUT: ret void, !dbg !58
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: declare void @_Z12TakeCallbackPFifE(ptr noundef) #1
// CHECK:STDOUT: declare void @_Z12TakeCallbackPFifE(ptr noundef) #2
// CHECK:STDOUT:
// CHECK:STDOUT: attributes #0 = { nounwind }
// CHECK:STDOUT: attributes #1 = { "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 #1 = { nounwind "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 = { "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.dbg.cu = !{!51}
// CHECK:STDOUT: !llvm.module.flags = !{!17, !18, !19, !2, !3}
@@ -482,14 +484,14 @@ fn F() {
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define void @_CMyCallback__carbon_thunk.Main(ptr %_, ptr %_1) #0 !dbg !108 {
// CHECK:STDOUT: define void @_CMyCallback__carbon_thunk.Main(ptr noundef nonnull align 4 dereferenceable(4) %_, ptr noundef nonnull align 4 dereferenceable(4) %_1) #1 !dbg !108 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: call void @_CMyCallback.Main(ptr %_1, ptr %_), !dbg !112
// CHECK:STDOUT: ret void, !dbg !112
// 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)) #1
// CHECK:STDOUT: declare void @llvm.lifetime.start.p0(ptr captures(none)) #2
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define linkonce_odr void @_CSelfDestruct.Core.9d0db8a57c58fabc(ptr %t) #0 !dbg !114 {
@@ -497,7 +499,8 @@ fn F() {
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: attributes #0 = { nounwind }
// CHECK:STDOUT: attributes #1 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
// CHECK:STDOUT: attributes #1 = { nounwind "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 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
// CHECK:STDOUT:
// CHECK:STDOUT: !llvm.dbg.cu = !{!99}
// CHECK:STDOUT: !llvm.module.flags = !{!17, !18, !19, !2, !3}
@@ -552,16 +555,17 @@ fn F() {
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define void @_CMyCallback__carbon_thunk.Main(ptr %_, ptr %_1) #0 !dbg !126 {
// CHECK:STDOUT: define void @_CMyCallback__carbon_thunk.Main(ptr noundef nonnull align 4 dereferenceable(4) %_, ptr noundef nonnull align 4 dereferenceable(4) %_1) #1 !dbg !126 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: call void @_CMyCallback.Main(ptr %_1, ptr %_), !dbg !129
// CHECK:STDOUT: ret void, !dbg !129
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: declare void @_Z19TakeComplexCallbackPF1CS_E(ptr noundef) #1
// CHECK:STDOUT: declare void @_Z19TakeComplexCallbackPF1CS_E(ptr noundef) #2
// CHECK:STDOUT:
// CHECK:STDOUT: attributes #0 = { nounwind }
// CHECK:STDOUT: attributes #1 = { "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 #1 = { nounwind "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 = { "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.dbg.cu = !{!122}
// CHECK:STDOUT: !llvm.module.flags = !{!17, !18, !19, !2, !3}
toolchain/lower/testdata/interop/cpp/function/import/method_ptr.carbon
Vendored
+12 -10
View File
@@ -194,14 +194,14 @@ fn F() {
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define void @_CMethod__carbon_thunk.C.Main(ptr %self) #0 !dbg !44 {
// CHECK:STDOUT: define void @_CMethod__carbon_thunk.C.Main(ptr noundef nonnull align 1 %self) #2 !dbg !44 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: call void @_CMethod.C.Main(ptr %self), !dbg !47
// CHECK:STDOUT: ret void, !dbg !47
// 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)) #2
// CHECK:STDOUT: declare void @llvm.lifetime.start.p0(ptr captures(none)) #3
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define linkonce_odr void @_CSelfDestruct.Core.9d0db8a57c58fabc(ptr %t) #0 !dbg !49 {
@@ -215,7 +215,8 @@ fn F() {
// CHECK:STDOUT:
// CHECK:STDOUT: attributes #0 = { nounwind }
// CHECK:STDOUT: attributes #1 = { alwaysinline nounwind }
// CHECK:STDOUT: attributes #2 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
// CHECK:STDOUT: attributes #2 = { nounwind "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 #3 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
// CHECK:STDOUT:
// CHECK:STDOUT: !llvm.dbg.cu = !{!35}
// CHECK:STDOUT: !llvm.module.flags = !{!28, !29, !30, !2, !3}
@@ -287,14 +288,14 @@ fn F() {
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define void @_CMethod__carbon_thunk.C.Main(ptr %self) #0 !dbg !73 {
// CHECK:STDOUT: define void @_CMethod__carbon_thunk.C.Main(ptr noundef nonnull align 1 %self) #2 !dbg !73 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: call void @_CMethod.C.Main(ptr %self), !dbg !75
// CHECK:STDOUT: ret void, !dbg !75
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: alwaysinline mustprogress uwtable
// CHECK:STDOUT: define internal void @_Z20TakeGenericMethodPtrIN6Carbon4Main1CEEvMT_KFvvE.carbon_thunk._(ptr noundef %ptr) #2 {
// CHECK:STDOUT: define internal void @_Z20TakeGenericMethodPtrIN6Carbon4Main1CEEvMT_KFvvE.carbon_thunk._(ptr noundef %ptr) #3 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %ptr.addr = alloca ptr, align 8
// CHECK:STDOUT: %coerce = alloca { i64, i64 }, align 8
@@ -311,7 +312,7 @@ fn F() {
// 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: declare void @llvm.lifetime.start.p0(ptr captures(none)) #4
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define linkonce_odr void @_CSelfDestruct.Core.9d0db8a57c58fabc(ptr %t) #0 !dbg !77 {
@@ -323,13 +324,14 @@ fn F() {
// CHECK:STDOUT: ret void, !dbg !89
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: declare void @_Z20TakeGenericMethodPtrIN6Carbon4Main1CEEvMT_KFvvE(i64, i64) #4
// CHECK:STDOUT: declare void @_Z20TakeGenericMethodPtrIN6Carbon4Main1CEEvMT_KFvvE(i64, i64) #5
// CHECK:STDOUT:
// CHECK:STDOUT: attributes #0 = { nounwind }
// CHECK:STDOUT: attributes #1 = { alwaysinline nounwind }
// CHECK:STDOUT: attributes #2 = { 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 #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: attributes #2 = { nounwind "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 #3 = { 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 #4 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
// CHECK:STDOUT: attributes #5 = { "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.dbg.cu = !{!64}
// CHECK:STDOUT: !llvm.module.flags = !{!28, !29, !30, !2, !3}