[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 9f8515c020ba509ddd305d49883b8c622bfc21b5 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 | 23 +++++++++++++++++++++++
 2 files changed, 27 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..f0177906a09a81
--- /dev/null
+++ b/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);
+}