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[ARM] Empty structs are 1-byte for C++ ABI #124762

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Merged
merged 8 commits into from
Jan 31, 2025
Merged

[ARM] Empty structs are 1-byte for C++ ABI #124762

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4d883f0
[ARM] Empty structs are 1-byte for C++ ABI
ostannard Dec 12, 2024
441052f
Add the -fclang-abi-compat=20 option
ostannard Jan 29, 2025
f0eef1b
Allow reverting ABI change with -fclang-abi-compat=20
ostannard Jan 29, 2025
9bcf7b2
Revert unneeded change
ostannard Jan 29, 2025
e396582
Add release note
ostannard Jan 29, 2025
665877e
Assume this will be backported to llvm-19
ostannard Jan 29, 2025
363adc6
Correct architecture in release notes
ostannard Jan 30, 2025
8833aba
Merge remote-tracking branch 'origin/main' into empty-struct-aarch32
ostannard Jan 31, 2025
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2 changes: 2 additions & 0 deletions clang/include/clang/Basic/LangOptions.h
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Original file line number Diff line number Diff line change
Expand Up @@ -246,6 +246,8 @@ class LangOptionsBase {
/// construction vtable because it hasn't added 'type' as a substitution.
/// - Skip mangling enclosing class templates of member-like friend
/// function templates.
/// - Ignore empty struct arguments in C++ mode for ARM, instead of
/// passing them as if they had a size of 1 byte.
Ver19,

/// Conform to the underlying platform's C and C++ ABIs as closely
Expand Down
45 changes: 40 additions & 5 deletions clang/lib/CodeGen/Targets/ARM.cpp
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Expand Up @@ -71,6 +71,7 @@ class ARMABIInfo : public ABIInfo {
unsigned functionCallConv) const;
ABIArgInfo classifyHomogeneousAggregate(QualType Ty, const Type *Base,
uint64_t Members) const;
bool shouldIgnoreEmptyArg(QualType Ty) const;
ABIArgInfo coerceIllegalVector(QualType Ty) const;
bool isIllegalVectorType(QualType Ty) const;
bool containsAnyFP16Vectors(QualType Ty) const;
Expand Down Expand Up @@ -328,6 +329,31 @@ ABIArgInfo ARMABIInfo::classifyHomogeneousAggregate(QualType Ty,
return ABIArgInfo::getDirect(nullptr, 0, nullptr, false, Align);
}

bool ARMABIInfo::shouldIgnoreEmptyArg(QualType Ty) const {
uint64_t Size = getContext().getTypeSize(Ty);
assert((isEmptyRecord(getContext(), Ty, true) || Size == 0) &&
"Arg is not empty");

// Empty records are ignored in C mode, and in C++ on WatchOS.
if (!getContext().getLangOpts().CPlusPlus ||
getABIKind() == ARMABIKind::AAPCS16_VFP)
return true;

// In C++ mode, arguments which have sizeof() == 0 are ignored. This is not a
// situation which is defined by any C++ standard or ABI, but this matches
// GCC's de facto ABI.
if (Size == 0)
return true;

// Clang 19.0 and earlier always ignored empty struct arguments in C++ mode.
if (getContext().getLangOpts().getClangABICompat() <=
LangOptions::ClangABI::Ver19)
return true;

// Otherwise, they are passed as if they have a size of 1 byte.
return false;
}

ABIArgInfo ARMABIInfo::classifyArgumentType(QualType Ty, bool isVariadic,
unsigned functionCallConv) const {
// 6.1.2.1 The following argument types are VFP CPRCs:
Expand Down Expand Up @@ -366,9 +392,15 @@ ABIArgInfo ARMABIInfo::classifyArgumentType(QualType Ty, bool isVariadic,
return getNaturalAlignIndirect(Ty, RAA == CGCXXABI::RAA_DirectInMemory);
}

// Ignore empty records.
if (isEmptyRecord(getContext(), Ty, true))
return ABIArgInfo::getIgnore();
// Empty records are either ignored completely or passed as if they were a
// 1-byte object, depending on the ABI and language standard.
if (isEmptyRecord(getContext(), Ty, true) ||
getContext().getTypeSize(Ty) == 0) {
if (shouldIgnoreEmptyArg(Ty))
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I think I'd prefer to write this something like:

if (size == zero)
  return getIgnore();
if (isempty() && isWatchOS())
  return getIgnore();

This should be equivalent because an empty struct in C is size zero anyway.

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Actually, hmm... do we want to add fclang-abi-compat support for this change, or do we think it's too niche to matter?

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I wasn't aware of the -fclang-abi-compat option, but I think that would be worth adding for this. I'll also add a release note as suggested in #124760.

return ABIArgInfo::getIgnore();
else
return ABIArgInfo::getDirect(llvm::Type::getInt8Ty(getVMContext()));
}

if (IsAAPCS_VFP) {
// Homogeneous Aggregates need to be expanded when we can fit the aggregate
Expand Down Expand Up @@ -588,7 +620,8 @@ ABIArgInfo ARMABIInfo::classifyReturnType(QualType RetTy, bool isVariadic,

// Otherwise this is an AAPCS variant.

if (isEmptyRecord(getContext(), RetTy, true))
if (isEmptyRecord(getContext(), RetTy, true) ||
getContext().getTypeSize(RetTy) == 0)
Comment on lines +623 to +624
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Does this change actually have any effect?

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This is needed for the SortOfEmpty case, which isn't considered empty, but does have size 0.

return ABIArgInfo::getIgnore();

// Check for homogeneous aggregates with AAPCS-VFP.
Expand Down Expand Up @@ -752,7 +785,9 @@ RValue ARMABIInfo::EmitVAArg(CodeGenFunction &CGF, Address VAListAddr,
CharUnits SlotSize = CharUnits::fromQuantity(4);

// Empty records are ignored for parameter passing purposes.
if (isEmptyRecord(getContext(), Ty, true))
uint64_t Size = getContext().getTypeSize(Ty);
bool IsEmpty = isEmptyRecord(getContext(), Ty, true);
if ((IsEmpty || Size == 0) && shouldIgnoreEmptyArg(Ty))
return Slot.asRValue();

CharUnits TySize = getContext().getTypeSizeInChars(Ty);
Expand Down
131 changes: 131 additions & 0 deletions clang/test/CodeGen/arm-empty-args.cpp
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@@ -0,0 +1,131 @@
// RUN: %clang_cc1 -triple armv7a-linux-gnueabi -emit-llvm -o - -x c %s | FileCheck %s --check-prefixes=CHECK,C
// RUN: %clang_cc1 -triple armv7a-linux-gnueabi -emit-llvm -o - %s | FileCheck %s --check-prefixes=CHECK,CXX
// RUN: %clang_cc1 -triple armv7a-linux-gnueabi -emit-llvm -o - %s -fclang-abi-compat=19 | FileCheck %s --check-prefixes=CHECK,CXXCLANG19
// RUN: %clang_cc1 -triple thumbv7k-apple-watchos2.0 -target-abi aapcs16 -emit-llvm -o - %s | FileCheck %s --check-prefixes=CHECK,WATCHOS

// Empty structs are ignored for PCS purposes on WatchOS and in C mode
// elsewhere. In C++ mode they consume a register slot though. Functions are
// slightly bigger than minimal to make confirmation against actual GCC
// behaviour easier.

#if __cplusplus
#define EXTERNC extern "C"
#else
#define EXTERNC
#endif

struct Empty {};

// C: define{{.*}} i32 @empty_arg(i32 noundef %a)
// CXX: define{{.*}} i32 @empty_arg(i8 %e.coerce, i32 noundef %a)
// CXXCLANG19: define{{.*}} i32 @empty_arg(i32 noundef %a)
// WATCHOS: define{{.*}} i32 @empty_arg(i32 noundef %a)
EXTERNC int empty_arg(struct Empty e, int a) {
return a;
}

// C: define{{.*}} void @empty_ret()
// CXX: define{{.*}} void @empty_ret()
// CXXCLANG19: define{{.*}} void @empty_ret()
// WATCHOS: define{{.*}} void @empty_ret()
EXTERNC struct Empty empty_ret(void) {
struct Empty e;
return e;
}

// However, what counts as "empty" is a baroque mess. This is super-empty, it's
// ignored even in C++ mode. It also has sizeof == 0, violating C++, but that's
// legacy for you:

struct SuperEmpty {
int arr[0];
};

// C: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
// CXX: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
// CXXCLANG19: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
// WATCHOS: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
EXTERNC int super_empty_arg(struct SuperEmpty e, int a) {
return a;
}

struct SortOfEmpty {
struct SuperEmpty e;
};

// C: define{{.*}} i32 @sort_of_empty_arg(i32 noundef %a)
// CXX: define{{.*}} i32 @sort_of_empty_arg(i8 %e.coerce, i32 noundef %a)
// CXXCLANG19: define{{.*}} i32 @sort_of_empty_arg(i32 noundef %a)
// WATCHOS: define{{.*}} i32 @sort_of_empty_arg(i32 noundef %a)
EXTERNC int sort_of_empty_arg(struct Empty e, int a) {
return a;
}

// C: define{{.*}} void @sort_of_empty_ret()
// CXX: define{{.*}} void @sort_of_empty_ret()
// CXXCLANG19: define{{.*}} void @sort_of_empty_ret()
// WATCHOS: define{{.*}} void @sort_of_empty_ret()
EXTERNC struct SortOfEmpty sort_of_empty_ret(void) {
struct SortOfEmpty e;
return e;
}

#include <stdarg.h>

// va_arg matches the above rules, consuming an incoming argument in cases
// where one would be passed, and not doing so when the argument should be
// ignored.

EXTERNC int empty_arg_variadic(int a, ...) {
// CHECK-LABEL: @empty_arg_variadic(
// C: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// C-NOT: {{ getelementptr }}
// CXX: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// CXX: %argp.next2 = getelementptr inbounds i8, ptr %argp.cur1, i32 4
// CXXCLANG19: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// CXXCLANG19-NOT: {{ getelementptr }}
// WATCHOS: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// WATCHOS-NOT: {{ getelementptr }}
va_list vl;
va_start(vl, a);
struct Empty b = va_arg(vl, struct Empty);
int c = va_arg(vl, int);
va_end(vl);
return c;
}

EXTERNC int super_empty_arg_variadic(int a, ...) {
// CHECK-LABEL: @super_empty_arg_variadic(
// C: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// C-NOT: {{ getelementptr }}
// CXX: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// CXX-NOT: {{ getelementptr }}
// CXXCLANG19: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// CXXCLANG19-NOT: {{ getelementptr }}
// WATCHOS: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// WATCHOS-NOT: {{ getelementptr }}
va_list vl;
va_start(vl, a);
struct SuperEmpty b = va_arg(vl, struct SuperEmpty);
int c = va_arg(vl, int);
va_end(vl);
return c;
}

EXTERNC int sort_of_empty_arg_variadic(int a, ...) {
// CHECK-LABEL: @sort_of_empty_arg_variadic(
// C: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// C-NOT: {{ getelementptr }}
// CXX: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// CXX-NOT: {{ getelementptr }}
// CXXCLANG19: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// CXXCLANG19-NOT: {{ getelementptr }}
// WATCHOS: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
// WATCHOS-NOT: {{ getelementptr }}
va_list vl;
va_start(vl, a);
struct SortOfEmpty b = va_arg(vl, struct SortOfEmpty);
int c = va_arg(vl, int);
va_end(vl);
return c;
}