[clang] [X86_64] fix empty structure vaarg in c++ (PR #77903)

[via cfe-commits]

https://github.com/hstk30-hw created https://github.com/llvm/llvm-project/pull/77903

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. #77036 

>From ebb2cdbea5e2c29b52f3bd46d31af105cf395e6d 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.
---
 clang/lib/CodeGen/Targets/X86.cpp     |  5 +++++
 clang/test/CodeGen/X86/x86_64-vaarg.c | 15 +++++++++++++++
 2 files changed, 20 insertions(+)
 create mode 100644 clang/test/CodeGen/X86/x86_64-vaarg.c

diff --git a/clang/lib/CodeGen/Targets/X86.cpp b/clang/lib/CodeGen/Targets/X86.cpp
index d053f41ab168f5..fde76ed6709cef 100644
--- a/clang/lib/CodeGen/Targets/X86.cpp
+++ b/clang/lib/CodeGen/Targets/X86.cpp
@@ -2989,6 +2989,11 @@ static Address EmitX86_64VAArgFromMemory(CodeGenFunction &CGF,
   // an 8 byte boundary.
 
   uint64_t SizeInBytes = (CGF.getContext().getTypeSize(Ty) + 7) / 8;
+  
+  if (isEmptyRecord(CGF.getContext(), Ty, true)) {
+    SizeInBytes = 0;  
+  }
+  
   llvm::Value *Offset =
       llvm::ConstantInt::get(CGF.Int32Ty, (SizeInBytes + 7)  & ~7);
   overflow_arg_area = CGF.Builder.CreateGEP(CGF.Int8Ty, overflow_arg_area,
diff --git a/clang/test/CodeGen/X86/x86_64-vaarg.c b/clang/test/CodeGen/X86/x86_64-vaarg.c
new file mode 100644
index 00000000000000..ae201a2d3f3017
--- /dev/null
+++ b/clang/test/CodeGen/X86/x86_64-vaarg.c
@@ -0,0 +1,15 @@
+// RUN: %clang -xc++ -target x86_64-linux-gnu -emit-llvm -S -o - %s | FileCheck %s
+
+struct Empty {};
+
+struct Empty emptyvar;
+
+void take_args(int a, ...) {
+    // CHECK:      %overflow_arg_area = load ptr, ptr %overflow_arg_area_p, align 8
+    // CHECK-NEXT: %overflow_arg_area.next = getelementptr i8, ptr %overflow_arg_area, i32 0
+    // CHECK-NEXT: store ptr %overflow_arg_area.next, ptr %overflow_arg_area_p, align 8
+    __builtin_va_list l;
+    __builtin_va_start(l, a);
+    emptyvar = __builtin_va_arg(l, struct Empty);
+    __builtin_va_end(l);
+}