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[C] Disable use of NRVO #101038

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[C] Disable use of NRVO #101038

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6 changes: 6 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -156,6 +156,12 @@ Bug Fixes to Compiler Builtins
Bug Fixes to Attribute Support
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Bug Fixes to C Support
^^^^^^^^^^^^^^^^^^^^^^

- No longer applying Named Return Value Optimization (NRVO) in C as it is a
non-conforming optimization in C. (#GH100902)

Bug Fixes to C++ Support
^^^^^^^^^^^^^^^^^^^^^^^^

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4 changes: 3 additions & 1 deletion clang/lib/Sema/SemaDecl.cpp
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Expand Up @@ -2176,7 +2176,9 @@ static void CheckPoppedLabel(LabelDecl *L, Sema &S,
}

void Sema::ActOnPopScope(SourceLocation Loc, Scope *S) {
S->applyNRVO();
// NRVO is only valid in C++, not in C.
if (getLangOpts().CPlusPlus)
S->applyNRVO();

if (S->decl_empty()) return;
assert((S->getFlags() & (Scope::DeclScope | Scope::TemplateParamScope)) &&
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75 changes: 75 additions & 0 deletions clang/test/CodeGen/nrvo.c
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@@ -0,0 +1,75 @@
// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 5
// RUN: %clang_cc1 -triple i386-unknown-unknown -emit-llvm -disable-llvm-passes -o - %s | FileCheck --check-prefixes=C %s
// RUN: %clang_cc1 -triple i386-unknown-unknown -emit-llvm -disable-llvm-passes -o - -x c++ %s | FileCheck --check-prefixes=CXX %s

// NRVO is not allowed in C as it is in C++, so validate that this results in
// use of NRVO in C++, but not in C.
typedef struct {
int i, j;
double a, b;
} S;

int result;

// C-LABEL: define dso_local void @test(
// C-SAME: ptr dead_on_unwind noalias writable sret([[STRUCT_S:%.*]]) align 4 [[AGG_RESULT:%.*]], ptr noundef [[Q:%.*]]) #[[ATTR0:[0-9]+]] {
// C-NEXT: [[ENTRY:.*:]]
// C-NEXT: [[RESULT_PTR:%.*]] = alloca ptr, align 4
// C-NEXT: [[Q_ADDR:%.*]] = alloca ptr, align 4
// C-NEXT: [[S:%.*]] = alloca [[STRUCT_S]], align 4
// C-NEXT: store ptr [[AGG_RESULT]], ptr [[RESULT_PTR]], align 4
// C-NEXT: store ptr [[Q]], ptr [[Q_ADDR]], align 4
// C-NEXT: call void @llvm.memset.p0.i32(ptr align 4 [[S]], i8 0, i32 24, i1 false)
// C-NEXT: [[TMP0:%.*]] = load ptr, ptr [[Q_ADDR]], align 4
// C-NEXT: [[CMP:%.*]] = icmp eq ptr [[S]], [[TMP0]]
// C-NEXT: [[CONV:%.*]] = zext i1 [[CMP]] to i32
// C-NEXT: store i32 [[CONV]], ptr @result, align 4
// C-NEXT: call void @llvm.memcpy.p0.p0.i32(ptr align 4 [[AGG_RESULT]], ptr align 4 [[S]], i32 24, i1 false)
// C-NEXT: ret void
//
// CXX-LABEL: define dso_local void @_Z4testP1S(
// CXX-SAME: ptr dead_on_unwind noalias writable sret([[STRUCT_S:%.*]]) align 4 [[AGG_RESULT:%.*]], ptr noundef [[Q:%.*]]) #[[ATTR0:[0-9]+]] {
// CXX-NEXT: [[ENTRY:.*:]]
// CXX-NEXT: [[RESULT_PTR:%.*]] = alloca ptr, align 4
// CXX-NEXT: [[Q_ADDR:%.*]] = alloca ptr, align 4
// CXX-NEXT: store ptr [[AGG_RESULT]], ptr [[RESULT_PTR]], align 4
// CXX-NEXT: store ptr [[Q]], ptr [[Q_ADDR]], align 4
// CXX-NEXT: call void @llvm.memset.p0.i32(ptr align 4 [[AGG_RESULT]], i8 0, i32 24, i1 false)
// CXX-NEXT: [[TMP0:%.*]] = load ptr, ptr [[Q_ADDR]], align 4
// CXX-NEXT: [[CMP:%.*]] = icmp eq ptr [[AGG_RESULT]], [[TMP0]]
// CXX-NEXT: [[CONV:%.*]] = zext i1 [[CMP]] to i32
// CXX-NEXT: store i32 [[CONV]], ptr @result, align 4
// CXX-NEXT: ret void
//
S test(S* q) {
S s = {0, 0, 0, 0};
result = &s == q;
return s;
}

// C-LABEL: define dso_local i32 @main(
// C-SAME: ) #[[ATTR0]] {
// C-NEXT: [[ENTRY:.*:]]
// C-NEXT: [[RETVAL:%.*]] = alloca i32, align 4
// C-NEXT: [[T:%.*]] = alloca [[STRUCT_S:%.*]], align 4
// C-NEXT: store i32 0, ptr [[RETVAL]], align 4
// C-NEXT: call void @test(ptr dead_on_unwind writable sret([[STRUCT_S]]) align 4 [[T]], ptr noundef [[T]])
// C-NEXT: [[TMP0:%.*]] = load i32, ptr @result, align 4
// C-NEXT: ret i32 [[TMP0]]
//
// CXX-LABEL: define dso_local noundef i32 @main(
// CXX-SAME: ) #[[ATTR2:[0-9]+]] {
// CXX-NEXT: [[ENTRY:.*:]]
// CXX-NEXT: [[RETVAL:%.*]] = alloca i32, align 4
// CXX-NEXT: [[T:%.*]] = alloca [[STRUCT_S:%.*]], align 4
// CXX-NEXT: store i32 0, ptr [[RETVAL]], align 4
// CXX-NEXT: call void @_Z4testP1S(ptr dead_on_unwind writable sret([[STRUCT_S]]) align 4 [[T]], ptr noundef [[T]])
// CXX-NEXT: [[TMP0:%.*]] = load i32, ptr @result, align 4
// CXX-NEXT: ret i32 [[TMP0]]
//
int main(void)
{
S t = test(&t);
return result;
}

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