[X86_64] fix empty structure vaarg in c++ (llvm#77907)

SizeInBytes of empty structure is 0 in C, while 1 in C++. And empty
structure argument of the function is ignored in X86_64 backend.As a
result, the value of variable arguments in C++ is incorrect. fix llvm#77036

Co-authored-by: Longsheng Mou <[email protected]>

Author: hstk30-hw

clang/lib/CodeGen/Targets/X86.cpp

@@ -3019,6 +3019,10 @@ Address X86_64ABIInfo::EmitVAArg(CodeGenFunction &CGF, Address VAListAddr,
   ABIArgInfo AI = classifyArgumentType(Ty, 0, neededInt, neededSSE,
                                        /*isNamedArg*/false);
 
+  // Empty records are ignored for parameter passing purposes.
+  if (AI.isIgnore())
+    return CGF.CreateMemTemp(Ty);
+
   // AMD64-ABI 3.5.7p5: Step 1. Determine whether type may be passed
   // in the registers. If not go to step 7.
   if (!neededInt && !neededSSE)

clang/test/CodeGenCXX/x86_64-vaarg.cpp

@@ -0,0 +1,23 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -emit-llvm -x c -o - %s | FileCheck %s
+
+typedef struct { struct {} a; } empty;
+
+// CHECK-LABEL: @{{.*}}empty_record_test
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[RETVAL:%.*]] = alloca [[STRUCT_EMPTY:%.*]], align 1
+// CHECK-NEXT:    [[Z_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT:    [[LIST:%.*]] = alloca [1 x %struct.__va_list_tag], align 16
+// CHECK-NEXT:    [[TMP:%.*]] = alloca [[STRUCT_EMPTY]], align 1
+// CHECK-NEXT:    store i32 [[Z:%.*]], ptr [[Z_ADDR]], align 4
+// CHECK-NEXT:    [[ARRAYDECAY:%.*]] = getelementptr inbounds [1 x %struct.__va_list_tag], ptr [[LIST]], i64 0, i64 0
+// CHECK-NEXT:    call void @llvm.va_start(ptr [[ARRAYDECAY]])
+// CHECK-NEXT:    [[ARRAYDECAY1:%.*]] = getelementptr inbounds [1 x %struct.__va_list_tag], ptr [[LIST]], i64 0, i64 0
+// CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr align 1 [[RETVAL]], ptr align 1 [[TMP]], i64 {{.*}}, i1 false)
+// CHECK-NEXT:    ret void
+empty empty_record_test(int z, ...) {
+  __builtin_va_list list;
+  __builtin_va_start(list, z);
+  return __builtin_va_arg(list, empty);
+}