[clang] [X86_32] fix weird edge case in vaarg. (PR #86388)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Longsheng Mou (CoTinker)

<details>
<summary>Changes</summary>

struct SuperEmpty { struct{ int a[0];} b;};
Such 0 sized structs in c++ mode can not be ignored in i386 for that c++ fields are never empty.But when EmitVAArg, its size is 0, so that va_list not increase.Maybe we can use direct in this case.

https://github.com/llvm/llvm-project/blob/691b97c884a15a7eac641ddf67c9f2f30fb4e747/clang/lib/CodeGen/ABIInfoImpl.cpp#L202-L216

This case use indirect before and `DirectSize == 0`, now we use direct and `DirectSize == 4`.
Or we can just Ignore this kind of arguments, like X86_64 did. 

---
Full diff: https://github.com/llvm/llvm-project/pull/86388.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/Targets/X86.cpp (+3) 
- (modified) clang/test/CodeGenCXX/regparm.cpp (+1-1) 


``````````diff
diff --git a/clang/lib/CodeGen/Targets/X86.cpp b/clang/lib/CodeGen/Targets/X86.cpp
index 1ec0f159ebcb8a..8e6e7c17452048 100644
--- a/clang/lib/CodeGen/Targets/X86.cpp
+++ b/clang/lib/CodeGen/Targets/X86.cpp
@@ -805,6 +805,9 @@ ABIArgInfo X86_32ABIInfo::classifyArgumentType(QualType Ty, CCState &State,
     if (!IsWin32StructABI && isEmptyRecord(getContext(), Ty, true))
       return ABIArgInfo::getIgnore();
 
+    if (TI.Width == 0 && getContext().getLangOpts().CPlusPlus)
+      return ABIArgInfo::getDirect();
+
     llvm::LLVMContext &LLVMContext = getVMContext();
     llvm::IntegerType *Int32 = llvm::Type::getInt32Ty(LLVMContext);
     bool NeedsPadding = false;
diff --git a/clang/test/CodeGenCXX/regparm.cpp b/clang/test/CodeGenCXX/regparm.cpp
index 1fd471c2d0727b..559702a84c99e1 100644
--- a/clang/test/CodeGenCXX/regparm.cpp
+++ b/clang/test/CodeGenCXX/regparm.cpp
@@ -32,7 +32,7 @@ struct S3 {
   } a;
 };
 __attribute((regparm(2))) void foo4(S3 a, int b);
-// CHECK: declare void @_Z4foo42S3i(ptr noundef byval(%struct.S3) align 4, i32 inreg noundef)
+// CHECK: declare void @_Z4foo42S3i(%struct.anon, i32 inreg noundef)
 void bar3(S3 a, int b) {
   foo4(a, b);
 }

``````````

</details>


https://github.com/llvm/llvm-project/pull/86388