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fix: C++ empty record with align lead to va_list out of sync #72197

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fix: C++ empty record with align lead to va_list out of sync #72197

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e683539
fix: C++ empty record with align break aarch64 AAPCS64
hstk30-hw Oct 9, 2025
e5d24bc
Merge branch 'main' into compatible-gcc-align-empty-record
hstk30-hw Oct 10, 2025
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3 changes: 3 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Expand Up @@ -110,6 +110,9 @@ C++ Specific Potentially Breaking Changes
ABI Changes in This Version
---------------------------

- Let C++ empty record fall through to the standard argument-handling path instead of
always pass a single ``i8`` according to aarch64 AAPCS rules.

AST Dumping Potentially Breaking Changes
----------------------------------------
- How nested name specifiers are dumped and printed changes, keeping track of clang AST changes.
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9 changes: 6 additions & 3 deletions clang/lib/CodeGen/Targets/AArch64.cpp
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Expand Up @@ -422,6 +422,12 @@ ABIArgInfo AArch64ABIInfo::classifyArgumentType(QualType Ty, bool IsVariadicFn,
}

// Empty records:
// AAPCS64 does not say that empty records are ignored as arguments,
// but other compilers do so in certain situations, and we copy that behavior.
// Those situations are in fact language-mode-specific, which seems really
// unfortunate, but it's something we just have to accept. If this doesn't
// apply, just fall through to the standard argument-handling path.
// Darwin overrides the psABI here to ignore all empty records in all modes.
uint64_t Size = getContext().getTypeSize(Ty);
bool IsEmpty = isEmptyRecord(getContext(), Ty, true);
if (!Ty->isSVESizelessBuiltinType() && (IsEmpty || Size == 0)) {
Expand All @@ -434,9 +440,6 @@ ABIArgInfo AArch64ABIInfo::classifyArgumentType(QualType Ty, bool IsVariadicFn,
// behaviour here.
if (Size == 0)
return ABIArgInfo::getIgnore();

// Otherwise, they are passed as if they have a size of 1 byte.
return ABIArgInfo::getDirect(llvm::Type::getInt8Ty(getVMContext()));
}

// Homogeneous Floating-point Aggregates (HFAs) need to be expanded.
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20 changes: 19 additions & 1 deletion clang/test/CodeGen/AArch64/args.cpp
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Expand Up @@ -17,11 +17,29 @@ struct Empty {};

// DARWIN: define{{.*}} i32 @empty_arg(i32 noundef %a)
// C: define{{.*}} i32 @empty_arg(i32 noundef %a)
// CXX: define{{.*}} i32 @empty_arg(i8 %e.coerce, i32 noundef %a)
// CXX: define{{.*}} i32 @empty_arg(i64 %e.coerce, i32 noundef %a)
EXTERNC int empty_arg(struct Empty e, int a) {
return a;
}

// CXX: define{{.*}} i32 @empty_align8_arg(i64 %a.coerce, i32 noundef %b)
struct EmptyAlign8 { int __attribute__((aligned(8))) : 0; };
EXTERNC int empty_align8_arg(struct EmptyAlign8 a, int b) {
return b;
}

// CXX: define{{.*}} i32 @empty_align16_arg(i128 %a.coerce, i32 noundef %b)
struct EmptyAlign16 { long long int __attribute__((aligned(16))) : 0; };
EXTERNC int empty_align16_arg(struct EmptyAlign16 a, int b) {
return b;
}

// CXX: define{{.*}} i32 @empty_align32_arg(ptr dead_on_return noundef %a, i32 noundef %b)
struct EmptyAlign32 { long long int __attribute__((aligned(32))) : 0; };
EXTERNC int empty_align32_arg(struct EmptyAlign32 a, int b) {
return b;
}

// DARWIN: define{{.*}} void @empty_ret()
// C: define{{.*}} void @empty_ret()
// CXX: define{{.*}} void @empty_ret()
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10 changes: 6 additions & 4 deletions clang/test/CodeGen/AArch64/struct-coerce-using-ptr.cpp
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Expand Up @@ -575,19 +575,21 @@ void TSpp_align16(SSpp_align16 s) { *s.a.x = 1; }
struct Sempty {
};
// CHECK-A64-LABEL: define dso_local void @_Z6Tempty6Sempty(
// CHECK-A64-SAME: i8 [[S_COERCE:%.*]]) #[[ATTR0]] {
// CHECK-A64-SAME: i64 [[S_COERCE:%.*]]) #[[ATTR0]] {
// CHECK-A64-NEXT: [[ENTRY:.*:]]
// CHECK-A64-NEXT: [[S:%.*]] = alloca [[STRUCT_SEMPTY:%.*]], align 1
// CHECK-A64-NEXT: [[COERCE_DIVE:%.*]] = getelementptr inbounds nuw [[STRUCT_SEMPTY]], ptr [[S]], i32 0, i32 0
// CHECK-A64-NEXT: store i8 [[S_COERCE]], ptr [[COERCE_DIVE]], align 1
// CHECK-A64-NEXT: [[COERCE_VAL_II:%.*]] = trunc i64 [[S_COERCE]] to i8
// CHECK-A64-NEXT: store i8 [[COERCE_VAL_II]], ptr [[COERCE_DIVE]], align 1
// CHECK-A64-NEXT: ret void
//
// CHECK-A64_32-LABEL: define void @_Z6Tempty6Sempty(
// CHECK-A64_32-SAME: i8 [[S_COERCE:%.*]]) #[[ATTR0]] {
// CHECK-A64_32-SAME: i64 [[S_COERCE:%.*]]) #[[ATTR0]] {
// CHECK-A64_32-NEXT: [[ENTRY:.*:]]
// CHECK-A64_32-NEXT: [[S:%.*]] = alloca [[STRUCT_SEMPTY:%.*]], align 1
// CHECK-A64_32-NEXT: [[COERCE_DIVE:%.*]] = getelementptr inbounds nuw [[STRUCT_SEMPTY]], ptr [[S]], i32 0, i32 0
// CHECK-A64_32-NEXT: store i8 [[S_COERCE]], ptr [[COERCE_DIVE]], align 1
// CHECK-A64_32-NEXT: [[COERCE_VAL_II:%.*]] = trunc i64 [[S_COERCE]] to i8
// CHECK-A64_32-NEXT: store i8 [[COERCE_VAL_II]], ptr [[COERCE_DIVE]], align 1
// CHECK-A64_32-NEXT: ret void
//
void Tempty(Sempty s) { }
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4 changes: 2 additions & 2 deletions clang/test/CodeGen/arm64-microsoft-arguments.cpp
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Expand Up @@ -57,7 +57,7 @@ S4 f4() {

// Pass and return from instance method called from instance method.
// CHECK: define {{.*}} void @{{.*}}bar@Q1{{.*}}(ptr {{[^,]*}} %this, ptr dead_on_unwind inreg noalias writable sret(%class.P1) align 1 %agg.result)
// CHECK: call void {{.*}}foo@P1{{.*}}(ptr noundef{{[^,]*}} %ref.tmp, ptr dead_on_unwind inreg writable sret(%class.P1) align 1 %agg.result, i8 %0)
// CHECK: call void {{.*}}foo@P1{{.*}}(ptr noundef{{[^,]*}} %ref.tmp, ptr dead_on_unwind inreg writable sret(%class.P1) align 1 %agg.result, i64 %coerce.val.ii)

class P1 {
public:
Expand All @@ -76,7 +76,7 @@ P1 Q1::bar() {

// Pass and return from instance method called from free function.
// CHECK: define {{.*}} void {{.*}}bar{{.*}}()
// CHECK: call void {{.*}}foo@P2{{.*}}(ptr noundef{{[^,]*}} %ref.tmp, ptr dead_on_unwind inreg writable sret(%class.P2) align 1 %retval, i8 %0)
// CHECK: call void {{.*}}foo@P2{{.*}}(ptr noundef{{[^,]*}} %ref.tmp, ptr dead_on_unwind inreg writable sret(%class.P2) align 1 %retval, i64 %coerce.val.ii)
class P2 {
public:
P2 foo(P2 x);
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2 changes: 1 addition & 1 deletion clang/test/CodeGenCXX/aarch64-arguments.cpp
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@@ -1,5 +1,5 @@
// RUN: %clang_cc1 -triple arm64-none-linux -emit-llvm -w -o - %s | FileCheck -check-prefix=PCS %s

// PCS: define{{.*}} void @{{.*}}(i8 %a
// PCS: define{{.*}} void @{{.*}}(i64 %a.coerce)
struct s0 {};
void f0(s0 a) {}
2 changes: 1 addition & 1 deletion clang/test/CodeGenCXX/arm64-darwinpcs.cpp
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Expand Up @@ -7,7 +7,7 @@ void test_extensions(bool a, char b, short c) {}

struct Empty {};
void test_empty(Empty e) {}
// CHECK: define{{.*}} void @_Z10test_empty5Empty(i8
// CHECK: define{{.*}} void @_Z10test_empty5Empty(i64 %e.coerce)
// CHECK-DARWIN: define{{.*}} void @_Z10test_empty5Empty()

struct HFA {
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