[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 +3014,16 @@ 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()) {
+    CharUnits Align = CGF.getContext().getTypeAlignInChars(Ty);
+    VAListAddr = CGF.Builder.CreateStructGEP(VAListAddr, 2);
+    llvm::Value *Load = CGF.Builder.CreateLoad(VAListAddr);
+    llvm::Value *Offset = llvm::ConstantInt::get(CGF.Int32Ty, 0);
+    Load = CGF.Builder.CreateGEP(CGF.Int8Ty, Load, Offset);
+    return Address(Load, CGF.ConvertTypeForMem(Ty), Align);
+  }
+
   // 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 +1,12 @@
+// RUN: %clang_cc1 -triple x86_64-linux-gnu -emit-llvm -o - %s | FileCheck %s
+
+typedef struct { struct {} a; } empty;
+
+// CHECK-LABEL: define{{.*}} void @_Z17empty_record_testv()
+empty empty_record_test(void) {
+// CHECK: [[ADDR0:%[a-z._0-9]+]] = getelementptr inbounds %struct.__va_list_tag, ptr %arraydecay, i32 0, i32 2
+// CHECK-NEXT: [[ADDR1:%[a-z._0-9]+]] = load ptr, ptr [[ADDR0]], align 8
+// CHECK-NEXT: [[ADDR2:%[a-z._0-9]+]] = getelementptr i8, ptr [[ADDR1]], i32 0
+  __builtin_va_list list;
+  return __builtin_va_arg(list, empty);
+}