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carbon-lang/toolchain/lower/testdata/interop/cpp/virtual_base.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/int.carbon
//
// AUTOUPDATE
// TIP: To test this file alone, run:
// TIP: bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/lower/testdata/interop/cpp/virtual_base.carbon
// TIP: To dump output, run:
// TIP: bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/lower/testdata/interop/cpp/virtual_base.carbon
// --- diamond.h
struct A {
A();
int a;
};
struct B : virtual A {
B();
int b;
};
struct C : virtual A {
C();
int c;
};
struct D : B, C {
int d;
};
// Layout of D (x86_64):
//
// 0: B | 0: vptr
// 8: | 8: int B::b
// 12: (4 bytes padding)
// 16: C | 0: vptr
// 24: | 8: int C::c
// 28: int D::d
// 32: A | 0: int A::a
// 36: (4 bytes padding)
// 40: (end of D)
// --- use_diamond.carbon
library "[[@TEST_NAME]]";
import Cpp library "diamond.h";
fn Make() {
// This should construct a 40-byte object, using a construtor with a `...C1`
// mangled name for D (a complete object constructor), which should call
// constructors with `...C2` mangled names for A, B, and C (base subobject
// constructors).
var _: Cpp.D = Cpp.D.D();
}
fn AccessD(d: Cpp.D) -> i32 {
// This should read an i32 at offset 28.
return d.d;
}
// CHECK:STDOUT: ; ModuleID = 'use_diamond.carbon'
// CHECK:STDOUT: source_filename = "use_diamond.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: $_ZN1DC1Ev = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTV1D = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTT1D = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTC1D0_1B = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTI1B = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTS1B = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTI1A = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTS1A = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTC1D16_1C = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTI1C = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTS1C = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTI1D = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: $_ZTS1D = comdat any
// CHECK:STDOUT:
// CHECK:STDOUT: @_ZTV1D = linkonce_odr dso_local unnamed_addr constant { [3 x ptr], [3 x ptr] } { [3 x ptr] [ptr inttoptr (i64 32 to ptr), ptr null, ptr @_ZTI1D], [3 x ptr] [ptr inttoptr (i64 16 to ptr), ptr inttoptr (i64 -16 to ptr), ptr @_ZTI1D] }, comdat, align 8
// CHECK:STDOUT: @_ZTT1D = linkonce_odr dso_local unnamed_addr constant [4 x ptr] [ptr getelementptr inbounds inrange(-24, 0) ({ [3 x ptr], [3 x ptr] }, ptr @_ZTV1D, i32 0, i32 0, i32 3), ptr getelementptr inbounds inrange(-24, 0) ({ [3 x ptr] }, ptr @_ZTC1D0_1B, i32 0, i32 0, i32 3), ptr getelementptr inbounds inrange(-24, 0) ({ [3 x ptr] }, ptr @_ZTC1D16_1C, i32 0, i32 0, i32 3), ptr getelementptr inbounds inrange(-24, 0) ({ [3 x ptr], [3 x ptr] }, ptr @_ZTV1D, i32 0, i32 1, i32 3)], comdat, align 8
// CHECK:STDOUT: @_ZTC1D0_1B = linkonce_odr dso_local unnamed_addr constant { [3 x ptr] } { [3 x ptr] [ptr inttoptr (i64 32 to ptr), ptr null, ptr @_ZTI1B] }, comdat, align 8
// CHECK:STDOUT: @_ZTI1B = linkonce_odr dso_local constant { ptr, ptr, i32, i32, ptr, i64 } { ptr getelementptr inbounds (ptr, ptr @_ZTVN10__cxxabiv121__vmi_class_type_infoE, i64 2), ptr @_ZTS1B, i32 0, i32 1, ptr @_ZTI1A, i64 -6141 }, comdat, align 8
// CHECK:STDOUT: @_ZTVN10__cxxabiv121__vmi_class_type_infoE = external global [0 x ptr]
// CHECK:STDOUT: @_ZTS1B = linkonce_odr dso_local constant [3 x i8] c"1B\00", comdat, align 1
// CHECK:STDOUT: @_ZTI1A = linkonce_odr dso_local constant { ptr, ptr } { ptr getelementptr inbounds (ptr, ptr @_ZTVN10__cxxabiv117__class_type_infoE, i64 2), ptr @_ZTS1A }, comdat, align 8
// CHECK:STDOUT: @_ZTVN10__cxxabiv117__class_type_infoE = external global [0 x ptr]
// CHECK:STDOUT: @_ZTS1A = linkonce_odr dso_local constant [3 x i8] c"1A\00", comdat, align 1
// CHECK:STDOUT: @_ZTC1D16_1C = linkonce_odr dso_local unnamed_addr constant { [3 x ptr] } { [3 x ptr] [ptr inttoptr (i64 16 to ptr), ptr null, ptr @_ZTI1C] }, comdat, align 8
// CHECK:STDOUT: @_ZTI1C = linkonce_odr dso_local constant { ptr, ptr, i32, i32, ptr, i64 } { ptr getelementptr inbounds (ptr, ptr @_ZTVN10__cxxabiv121__vmi_class_type_infoE, i64 2), ptr @_ZTS1C, i32 0, i32 1, ptr @_ZTI1A, i64 -6141 }, comdat, align 8
// CHECK:STDOUT: @_ZTS1C = linkonce_odr dso_local constant [3 x i8] c"1C\00", comdat, align 1
// CHECK:STDOUT: @_ZTI1D = linkonce_odr dso_local constant { ptr, ptr, i32, i32, ptr, i64, ptr, i64 } { ptr getelementptr inbounds (ptr, ptr @_ZTVN10__cxxabiv121__vmi_class_type_infoE, i64 2), ptr @_ZTS1D, i32 2, i32 2, ptr @_ZTI1B, i64 2, ptr @_ZTI1C, i64 4098 }, comdat, align 8
// CHECK:STDOUT: @_ZTS1D = linkonce_odr dso_local constant [3 x i8] c"1D\00", comdat, align 1
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define void @_CMake.Main() #0 !dbg !11 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %_.var = alloca [40 x i8], align 1, !dbg !14
// CHECK:STDOUT: call void @llvm.lifetime.start.p0(ptr %_.var), !dbg !14
// CHECK:STDOUT: call void @_ZN1DC1Ev.carbon_thunk.(ptr %_.var), !dbg !15
// CHECK:STDOUT: ret void, !dbg !16
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: alwaysinline mustprogress uwtable
// CHECK:STDOUT: define internal void @_ZN1DC1Ev.carbon_thunk.(ptr noundef %return) #1 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %return.addr = alloca ptr, align 8
// CHECK:STDOUT: store ptr %return, ptr %return.addr, align 8, !tbaa !17
// CHECK:STDOUT: %0 = load ptr, ptr %return.addr, align 8, !tbaa !17
// CHECK:STDOUT: call void @_ZN1DC1Ev(ptr noundef nonnull align 8 dereferenceable(32) %0)
// CHECK:STDOUT: ret void
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: ; Function Attrs: nounwind
// CHECK:STDOUT: define i32 @_CAccessD.Main(ptr %d) #0 !dbg !20 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %.loc16_11.1.d = getelementptr inbounds nuw [40 x i8], ptr %d, i32 0, i32 28, !dbg !27
// CHECK:STDOUT: %.loc16_11.2 = load i32, ptr %.loc16_11.1.d, align 4, !dbg !27
// CHECK:STDOUT: ret i32 %.loc16_11.2, !dbg !28
// 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:
// CHECK:STDOUT: ; Function Attrs: inlinehint mustprogress uwtable
// CHECK:STDOUT: define linkonce_odr dso_local void @_ZN1DC1Ev(ptr noundef nonnull align 8 dereferenceable(32) %this) unnamed_addr #3 comdat align 2 {
// CHECK:STDOUT: entry:
// CHECK:STDOUT: %this.addr = alloca ptr, align 8
// CHECK:STDOUT: store ptr %this, ptr %this.addr, align 8, !tbaa !17
// CHECK:STDOUT: %this1 = load ptr, ptr %this.addr, align 8
// CHECK:STDOUT: %0 = getelementptr inbounds i8, ptr %this1, i64 32
// CHECK:STDOUT: call void @_ZN1AC2Ev(ptr noundef nonnull align 4 dereferenceable(4) %0)
// CHECK:STDOUT: call void @_ZN1BC2Ev(ptr noundef nonnull align 8 dereferenceable(12) %this1, ptr noundef getelementptr inbounds nuw (i8, ptr @_ZTT1D, i64 8))
// CHECK:STDOUT: %1 = getelementptr inbounds i8, ptr %this1, i64 16
// CHECK:STDOUT: call void @_ZN1CC2Ev(ptr noundef nonnull align 8 dereferenceable(12) %1, ptr noundef getelementptr inbounds nuw (i8, ptr @_ZTT1D, i64 16))
// CHECK:STDOUT: store ptr getelementptr inbounds inrange(-24, 0) ({ [3 x ptr], [3 x ptr] }, ptr @_ZTV1D, i32 0, i32 0, i32 3), ptr %this1, align 8, !tbaa !29
// CHECK:STDOUT: %add.ptr = getelementptr inbounds i8, ptr %this1, i64 16
// CHECK:STDOUT: store ptr getelementptr inbounds inrange(-24, 0) ({ [3 x ptr], [3 x ptr] }, ptr @_ZTV1D, i32 0, i32 1, i32 3), ptr %add.ptr, align 8, !tbaa !29
// CHECK:STDOUT: ret void
// CHECK:STDOUT: }
// CHECK:STDOUT:
// CHECK:STDOUT: declare void @_ZN1AC2Ev(ptr noundef nonnull align 4 dereferenceable(4)) unnamed_addr #4
// CHECK:STDOUT:
// CHECK:STDOUT: declare void @_ZN1BC2Ev(ptr noundef nonnull align 8 dereferenceable(12), ptr noundef) unnamed_addr #4
// CHECK:STDOUT:
// CHECK:STDOUT: declare void @_ZN1CC2Ev(ptr noundef nonnull align 8 dereferenceable(12), ptr noundef) unnamed_addr #4
// CHECK:STDOUT:
// CHECK:STDOUT: ; uselistorder directives
// CHECK:STDOUT: uselistorder ptr inttoptr (i64 32 to ptr), { 1, 0 }
// CHECK:STDOUT: uselistorder ptr inttoptr (i64 16 to ptr), { 1, 0 }
// CHECK:STDOUT: uselistorder ptr getelementptr inbounds inrange(-24, 0) ({ [3 x ptr], [3 x ptr] }, ptr @_ZTV1D, i32 0, i32 0, i32 3), { 1, 0 }
// CHECK:STDOUT: uselistorder ptr getelementptr inbounds inrange(-24, 0) ({ [3 x ptr], [3 x ptr] }, ptr @_ZTV1D, i32 0, i32 1, i32 3), { 1, 0 }
// CHECK:STDOUT: uselistorder ptr @_ZTI1D, { 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 = { nocallback nofree nosync nounwind willreturn memory(argmem: readwrite) }
// CHECK:STDOUT: attributes #3 = { inlinehint 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 = { "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: "use_diamond.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: "Make", linkageName: "_CMake.Main", scope: null, file: !6, line: 6, type: !12, spFlags: DISPFlagDefinition, unit: !5)
// CHECK:STDOUT: !12 = !DISubroutineType(types: !13)
// CHECK:STDOUT: !13 = !{null}
// CHECK:STDOUT: !14 = !DILocation(line: 11, column: 3, scope: !11)
// CHECK:STDOUT: !15 = !DILocation(line: 11, column: 18, scope: !11)
// CHECK:STDOUT: !16 = !DILocation(line: 6, column: 1, scope: !11)
// CHECK:STDOUT: !17 = !{!18, !18, i64 0}
// CHECK:STDOUT: !18 = !{!"p1 _ZTS1D", !19, i64 0}
// CHECK:STDOUT: !19 = !{!"any pointer", !9, i64 0}
// CHECK:STDOUT: !20 = distinct !DISubprogram(name: "AccessD", linkageName: "_CAccessD.Main", scope: null, file: !6, line: 14, type: !21, spFlags: DISPFlagDefinition, unit: !5, retainedNodes: !25)
// CHECK:STDOUT: !21 = !DISubroutineType(types: !22)
// CHECK:STDOUT: !22 = !{!23, !24}
// CHECK:STDOUT: !23 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
// CHECK:STDOUT: !24 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 64)
// CHECK:STDOUT: !25 = !{!26}
// CHECK:STDOUT: !26 = !DILocalVariable(arg: 1, scope: !20, type: !24)
// CHECK:STDOUT: !27 = !DILocation(line: 16, column: 10, scope: !20)
// CHECK:STDOUT: !28 = !DILocation(line: 16, column: 3, scope: !20)
// CHECK:STDOUT: !29 = !{!30, !30, i64 0}
// CHECK:STDOUT: !30 = !{!"vtable pointer", !10, i64 0}
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