Allow reverting ABI change with -fclang-abi-compat=20

Author: ostannard

clang/include/clang/Basic/LangOptions.h

@@ -249,7 +249,8 @@ class LangOptionsBase {
 
     /// Attempt to be ABI-compatible with code generated by Clang 20.x.
     /// This causes clang to:
-    ///   - Nothing yet
+    /// - Ignore empty struct arguments in C++ mode for AArch64, instead of
+    ///   passing them as if they had a size of 1 byte.
     Ver20,
 
     /// Conform to the underlying platform's C and C++ ABIs as closely

clang/lib/CodeGen/Targets/ARM.cpp

@@ -345,6 +345,11 @@ bool ARMABIInfo::shouldIgnoreEmptyArg(QualType Ty) const {
   if (Size == 0)
     return true;
 
+  // Clang 20.0 and earlier always ignored empty struct arguments in C++ mode.
+  if (getContext().getLangOpts().getClangABICompat() <=
+      LangOptions::ClangABI::Ver20)
+    return true;
+
   // Otherwise, they are passed as if they have a size of 1 byte.
   return false;
 }

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

@@ -1,5 +1,6 @@
 // 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=20 | FileCheck %s --check-prefixes=CHECK,CXXCLANG20
 // 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
@@ -17,13 +18,15 @@ struct Empty {};
 
 // C: define{{.*}} i32 @empty_arg(i32 noundef %a)
 // CXX: define{{.*}} i32 @empty_arg(i8 %e.coerce, i32 noundef %a)
+// CXXCLANG20: 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()
+// CXXCLANG20: define{{.*}} void @empty_ret()
 // WATCHOS: define{{.*}} void @empty_ret()
 EXTERNC struct Empty empty_ret(void) {
   struct Empty e;
@@ -40,6 +43,7 @@ struct SuperEmpty {
 
 // C: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
 // CXX: define{{.*}} i32 @super_empty_arg(i32 noundef %a)
+// CXXCLANG20: 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;
@@ -51,13 +55,15 @@ struct SortOfEmpty {
 
 // C: define{{.*}} i32 @sort_of_empty_arg(i32 noundef %a)
 // CXX: define{{.*}} i32 @sort_of_empty_arg(i8 %e.coerce, i32 noundef %a)
+// CXXCLANG20: 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()
+// CXXCLANG20: define{{.*}} void @sort_of_empty_ret()
 // WATCHOS: define{{.*}} void @sort_of_empty_ret()
 EXTERNC struct SortOfEmpty sort_of_empty_ret(void) {
   struct SortOfEmpty e;
@@ -76,6 +82,8 @@ EXTERNC int empty_arg_variadic(int a, ...) {
 // 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
+// CXXCLANG20: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
+// CXXCLANG20-NOT: {{ getelementptr }}
 // WATCHOS: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
 // WATCHOS-NOT: {{ getelementptr }}
   va_list vl;
@@ -92,6 +100,8 @@ EXTERNC int super_empty_arg_variadic(int a, ...) {
 // C-NOT: {{ getelementptr }}
 // CXX: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
 // CXX-NOT: {{ getelementptr }}
+// CXXCLANG20: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
+// CXXCLANG20-NOT: {{ getelementptr }}
 // WATCHOS: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
 // WATCHOS-NOT: {{ getelementptr }}
   va_list vl;
@@ -108,6 +118,8 @@ EXTERNC int sort_of_empty_arg_variadic(int a, ...) {
 // C-NOT: {{ getelementptr }}
 // CXX: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
 // CXX-NOT: {{ getelementptr }}
+// CXXCLANG20: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
+// CXXCLANG20-NOT: {{ getelementptr }}
 // WATCHOS: %argp.next = getelementptr inbounds i8, ptr %argp.cur, i32 4
 // WATCHOS-NOT: {{ getelementptr }}
   va_list vl;