release/20.x: [ARM] Empty structs are 1-byte for C++ ABI (#124762)

clang/include/clang/Basic/LangOptions.h

@@ -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

clang/lib/CodeGen/Targets/ARM.cpp

@@ -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;
@@ -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:
@@ -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))
+      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
@@ -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)
     return ABIArgInfo::getIgnore();
 
   // Check for homogeneous aggregates with AAPCS-VFP.
@@ -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);

clang/test/CodeGen/arm-empty-args.cpp

@@ -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;
+}