[X86_32][C++] fix 0 sized struct case in vaarg. (#86388)

struct SuperEmpty { struct{ int a[0];} b;};
Such 0 sized structs in c++ mode can not be ignored in i386 for that c++
fields are never empty.But when EmitVAArg, its size is 0, so that
va_list not increase.Maybe we can just Ignore this kind of arguments,
like X86_64 did. Fix #86385.

Author: CoTinker

clang/lib/CodeGen/Targets/X86.cpp

@@ -795,6 +795,10 @@ ABIArgInfo X86_32ABIInfo::classifyArgumentType(QualType Ty, CCState &State,
     if (!IsWin32StructABI && isEmptyRecord(getContext(), Ty, true))
       return ABIArgInfo::getIgnore();
 
+    // Ignore 0 sized structs.
+    if (TI.Width == 0)
+      return ABIArgInfo::getIgnore();
+
     llvm::LLVMContext &LLVMContext = getVMContext();
     llvm::IntegerType *Int32 = llvm::Type::getInt32Ty(LLVMContext);
     bool NeedsPadding = false;

clang/test/CodeGenCXX/regparm.cpp

@@ -32,7 +32,7 @@ struct S3 {
   } a;
 };
 __attribute((regparm(2))) void foo4(S3 a, int b);
-// CHECK: declare void @_Z4foo42S3i(ptr noundef byval(%struct.S3) align 4, i32 inreg noundef)
+// CHECK: declare void @_Z4foo42S3i(i32 inreg noundef)
 void bar3(S3 a, int b) {
   foo4(a, b);
 }

clang/test/CodeGenCXX/x86_32-vaarg.cpp

@@ -18,3 +18,25 @@ empty empty_record_test(int z, ...) {
   __builtin_va_start(list, z);
   return __builtin_va_arg(list, empty);
 }
+
+typedef struct {
+  struct {
+    int a[0];
+  } b;
+} SortOfEmpty;
+
+// CHECK-LABEL: @_Z18test_sort_of_emptyiz(
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[RESULT_PTR:%.*]] = alloca ptr, align 4
+// CHECK-NEXT:    [[Z_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT:    [[LIST:%.*]] = alloca ptr, align 4
+// CHECK-NEXT:    store ptr [[AGG_RESULT:%.*]], ptr [[RESULT_PTR]], align 4
+// CHECK-NEXT:    store i32 [[Z:%.*]], ptr [[Z_ADDR]], align 4
+// CHECK-NEXT:    call void @llvm.va_start.p0(ptr [[LIST]])
+// CHECK-NEXT:    ret void
+//
+SortOfEmpty test_sort_of_empty(int z, ...) {
+  __builtin_va_list list;
+  __builtin_va_start(list, z);
+  return __builtin_va_arg(list, SortOfEmpty);
+}