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

[via cfe-commits]

https://github.com/hstk30-hw updated https://github.com/llvm/llvm-project/pull/77907

>From 95e44553a83f92876c1489f43efad57daf960965 Mon Sep 17 00:00:00 2001
From: Longsheng Mou <moulongsheng at huawei.com>
Date: Fri, 12 Jan 2024 18:24:08 +0800
Subject: [PATCH] [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.
---
 clang/lib/CodeGen/Targets/X86.cpp      |  4 ++++
 clang/test/CodeGenCXX/x86_64-vaarg.cpp | 13 +++++++++++++
 2 files changed, 17 insertions(+)
 create mode 100644 clang/test/CodeGenCXX/x86_64-vaarg.cpp

diff --git a/clang/lib/CodeGen/Targets/X86.cpp b/clang/lib/CodeGen/Targets/X86.cpp
index d053f41ab168f5..936ca16f1565fa 100644
--- a/clang/lib/CodeGen/Targets/X86.cpp
+++ b/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)
diff --git a/clang/test/CodeGenCXX/x86_64-vaarg.cpp b/clang/test/CodeGenCXX/x86_64-vaarg.cpp
new file mode 100644
index 00000000000000..849afd5fdd53b8
--- /dev/null
+++ b/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);
+}