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

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.

clang/lib/CodeGen/Targets/X86.cpp

@@ -1069,6 +1069,12 @@ Address X86_32ABIInfo::EmitVAArg(CodeGenFunction &CGF,
 
   auto TypeInfo = getContext().getTypeInfoInChars(Ty);
 
+  CCState State(*const_cast<CGFunctionInfo *>(CGF.CurFnInfo));
+  ABIArgInfo AI = classifyArgumentType(Ty, State, /*ArgIndex*/ 0);
+  // Empty records are ignored for parameter passing purposes.
+  if (AI.isIgnore())
+    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

clang/test/CodeGenCXX/x86_32-vaarg.cpp

@@ -0,0 +1,21 @@
+// 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
+
+typedef struct {} empty;
+
+// CHECK-LABEL: @_Z17empty_record_testiz(
+// 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.p0(ptr [[LIST]])
+// 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);
+}