[X86_64] fix empty structure vaarg in c++

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.So we should just make a
temporary and return its address.

Author: CoTinker

clang/lib/CodeGen/Targets/X86.cpp

@@ -3014,6 +3014,10 @@ 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 +1,13 @@
+// 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: define{{.*}} void @{{.*}}empty_record_test{{.*}}()
+empty empty_record_test(void) {
+// CHECK: [[RET:%[a-z]+]] = alloca %struct.empty, align 1
+// CHECK: [[TMP:%[a-z]+]] = alloca %struct.empty, align 1
+// CHECK: call void @llvm.memcpy{{.*}}(ptr align 1 [[RET]], ptr align 1 [[TMP]]
+  __builtin_va_list list;
+  return __builtin_va_arg(list, empty);
+}