[clang] fix: compatible C++ empty record with align UB with gcc (PR #72197)

[via cfe-commits]

https://github.com/hstk30-hw updated https://github.com/llvm/llvm-project/pull/72197

>From 76b86014cb4af9fe5b163d61a96597217d6e843c Mon Sep 17 00:00:00 2001
From: hstk30-hw <hanwei62 at huawei.com>
Date: Sat, 18 Nov 2023 11:00:29 +0000
Subject: [PATCH] fix: empty record size > 64 with align let va_list get out of
 sync

---
 clang/lib/CodeGen/Targets/AArch64.cpp |  8 +++++++-
 clang/test/CodeGen/aarch64-args.cpp   | 13 +++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/clang/lib/CodeGen/Targets/AArch64.cpp b/clang/lib/CodeGen/Targets/AArch64.cpp
index be5145daa00b7f5..50e94306d370e6e 100644
--- a/clang/lib/CodeGen/Targets/AArch64.cpp
+++ b/clang/lib/CodeGen/Targets/AArch64.cpp
@@ -307,7 +307,13 @@ AArch64ABIInfo::classifyArgumentType(QualType Ty, bool IsVariadic,
     // 0.
     if (IsEmpty && Size == 0)
       return ABIArgInfo::getIgnore();
-    return ABIArgInfo::getDirect(llvm::Type::getInt8Ty(getVMContext()));
+    // An empty struct can have size greater than one byte if alignment is
+    // involved.
+    // When size <= 64, we still hold it by i8 in IR for compatible,
+    // and it lowering to registers.
+    // When Size > 64, just fall through to avoid va_list out of sync.
+    if (Size <= 64)
+      return ABIArgInfo::getDirect(llvm::Type::getInt8Ty(getVMContext()));
   }
 
   // Homogeneous Floating-point Aggregates (HFAs) need to be expanded.
diff --git a/clang/test/CodeGen/aarch64-args.cpp b/clang/test/CodeGen/aarch64-args.cpp
index fe1298cc683a404..a528156d173571f 100644
--- a/clang/test/CodeGen/aarch64-args.cpp
+++ b/clang/test/CodeGen/aarch64-args.cpp
@@ -65,3 +65,16 @@ EXTERNC struct SortOfEmpty sort_of_empty_ret(void) {
   struct SortOfEmpty e;
   return e;
 }
+
+// CHECK-GNU-CXX: define{{.*}} i32 @empty_align8_arg(i8 %a.coerce, i32 noundef %b)
+struct EmptyAlign8 { int __attribute__((aligned(8))) : 0; };
+EXTERNC int empty_align8_arg(struct EmptyAlign8 a, int b) {
+  return b;
+}
+
+// CHECK-GNU-CXX: define{{.*}} i32 @empty_align16_arg(i128 %a.coerce, i32 noundef %b)
+struct EmptyAlign16 { long long int __attribute__((aligned(16))) : 0; };
+EXTERNC int empty_align16_arg(struct EmptyAlign16 a, int b) {
+  return b;
+}
+