[clang] [X86_32] Teach X86_32 va_arg to ignore empty structs. (PR #86075)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-codegen

Author: Longsheng Mou (CoTinker)

<details>
<summary>Changes</summary>

Empty structs are ignored for parameter passing purposes, but va_arg was incrementing the pointer anyway for that the size of empty struct in c++ is 1 byte, which could lead to va_list getting out of sync. Fix #<!-- -->86057.

---
Full diff: https://github.com/llvm/llvm-project/pull/86075.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/Targets/X86.cpp (+4) 
- (added) clang/test/CodeGenCXX/x86_32-vaarg.cpp (+20) 


``````````diff
diff --git a/clang/lib/CodeGen/Targets/X86.cpp b/clang/lib/CodeGen/Targets/X86.cpp
index 1ec0f159ebcb8a..7931f56ad6835f 100644
--- a/clang/lib/CodeGen/Targets/X86.cpp
+++ b/clang/lib/CodeGen/Targets/X86.cpp
@@ -1069,6 +1069,10 @@ Address X86_32ABIInfo::EmitVAArg(CodeGenFunction &CGF,
 
   auto TypeInfo = getContext().getTypeInfoInChars(Ty);
 
+  // Empty records are ignored for parameter passing purposes on non-Windows.
+  if (!IsWin32StructABI && isEmptyRecord(getContext(), Ty, true))
+    return CGF.CreateMemTemp(Ty);
+
   // x86-32 changes the alignment of certain arguments on the stack.
   //
   // Just messing with TypeInfo like this works because we never pass
diff --git a/clang/test/CodeGenCXX/x86_32-vaarg.cpp b/clang/test/CodeGenCXX/x86_32-vaarg.cpp
new file mode 100644
index 00000000000000..23eac1164118c6
--- /dev/null
+++ b/clang/test/CodeGenCXX/x86_32-vaarg.cpp
@@ -0,0 +1,20 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py
+// RUN: %clang_cc1 -triple i386-linux-gnu -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple i386-linux-gnu -emit-llvm -x c -o - %s | FileCheck %s
+
+typedef struct {} empty;
+
+// CHECK-LABEL: @{{.*}}empty_record_test
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[RESULT_PTR:%.*]] = alloca ptr, align 4
+// CHECK-NEXT:    [[Z_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT:    [[LIST:%.*]] = alloca ptr, align 4
+// CHECK-NEXT:    [[TMP:%.*]] = alloca [[STRUCT_EMPTY:%.*]], align 1
+// CHECK-NEXT:    store ptr [[AGG_RESULT:%.*]], ptr [[RESULT_PTR]], align 4
+// CHECK-NEXT:    store i32 [[Z:%.*]], ptr [[Z_ADDR]], align 4
+// CHECK-NEXT:    call void @llvm.va_start(ptr [[LIST]])
+empty empty_record_test(int z, ...) {
+  __builtin_va_list list;
+  __builtin_va_start(list, z);
+  return __builtin_va_arg(list, empty);
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/86075