[clang] [clang][layout] Fix unsigned char overflow in ms_struct bitfield layout (PR #181433)

[Akram Hany via cfe-commits]

https://github.com/akramhany created https://github.com/llvm/llvm-project/pull/181433

Fixes #161511

When using MS-struct layout rules (`#pragma ms_struct`), Clang produces incorrect memory layouts when encountering large `_BitInt` bit-fields. The reason was that `LastBitfieldStorageUnitSize` variable was declared as `unsigned char`, which caused it to overflow when the values used with `_BitInt` exceeded `255`, so I changed it from `unsigned char` to `uint64_t`.

In the added test, we can clearly see that `f2` and `f3` are both packed in the 256-bit unit.

>From 5d76fa5a7d1b94cd4933b52ff7ed0f527ca10c0e Mon Sep 17 00:00:00 2001
From: akramhany <akramhany65 at gmail.com>
Date: Sat, 14 Feb 2026 01:04:13 +0200
Subject: [PATCH] [clang][layout] Fix unsigned char overflow in ms_struct
 bitfield layout

---
 clang/lib/AST/RecordLayoutBuilder.cpp         |  2 +-
 clang/test/Layout/ms-x86-bitfields-overflow.c | 21 +++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/Layout/ms-x86-bitfields-overflow.c

diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index 36b8eea92c0a4..e55ccb50608b8 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -619,7 +619,7 @@ class ItaniumRecordLayoutBuilder {
 
   /// LastBitfieldStorageUnitSize - If IsMsStruct, represents the size of the
   /// storage unit of the previous field if it was a bitfield.
-  unsigned char LastBitfieldStorageUnitSize;
+  uint64_t LastBitfieldStorageUnitSize;
 
   /// MaxFieldAlignment - The maximum allowed field alignment. This is set by
   /// #pragma pack.
diff --git a/clang/test/Layout/ms-x86-bitfields-overflow.c b/clang/test/Layout/ms-x86-bitfields-overflow.c
new file mode 100644
index 0000000000000..d8964f32cabad
--- /dev/null
+++ b/clang/test/Layout/ms-x86-bitfields-overflow.c
@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fms-extensions -fdump-record-layouts -fsyntax-only %s 2>/dev/null \
+// RUN:            | FileCheck %s
+
+// Test that large _BitInt fields do not overflow the internal storage unit tracker (previously unsigned char).
+// If the bug exists, this struct splits into two 256-bit units.
+// If fixed, f2 and f3 are packed into a single 256-bit unit.
+
+#pragma ms_struct on
+
+struct __attribute__((packed, aligned(1))) A {
+  _BitInt(250) f2 : 2;
+  _BitInt(250) f3 : 2;
+};
+
+// CHECK-LABEL:   0 | struct A{{$}}
+// CHECK-NEXT:0:0-1 |   _BitInt(250) f2
+// CHECK-NEXT:0:2-3 |   _BitInt(250) f3
+// CHECK-NEXT:      | [sizeof=32, align=32]
+
+// Force the compiler to layout the struct
+int x[sizeof(struct A)];