[clang] Properly check for -mms-bitfields in Sema/struct-packed-align.c

[DanShaders]

Before #71148, providing only -triple=x86_64-windows-gnu to cc1 did not set -mms-bitfields (-fms-layout-compatibility=microsoft). Therefore, MS-compatible layout was only triggered in true MSVC targets. This is not the case now, so we should only check if we are compiling for Windows to test to determine if MS layout will be used.

The change of behavior is harmless as it only affects direct invocations of cc1.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Dan Klishch (DanShaders)

Changes

Before #71148, providing only -triple=x86_64-windows-gnu to cc1 did not set -mms-bitfields (-fms-layout-compatibility=microsoft). Therefore, MS-compatible layout was only triggered in true MSVC targets. This is not the case now, so we should only check if we are compiling for Windows to test to determine if MS layout will be used.

The change of behavior is harmless as it only affects direct invocations of cc1.

---

Full diff: https://github.com/llvm/llvm-project/pull/172337.diff

1 Files Affected:

• (modified) clang/test/Sema/struct-packed-align.c (+3-2)

diff --git a/clang/test/Sema/struct-packed-align.c b/clang/test/Sema/struct-packed-align.c
index d6d0724da49f8..dec4398a2578d 100644
--- a/clang/test/Sema/struct-packed-align.c
+++ b/clang/test/Sema/struct-packed-align.c
@@ -1,5 +1,6 @@
 // RUN: %clang_cc1 %s -fsyntax-only -verify
 // RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-windows-coff -verify
+// RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-windows-gnu -verify
 // RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-scei-ps4 -verify
 // RUN: %clang_cc1 %s -fsyntax-only -triple=x86_64-sie-ps5 -verify
 
@@ -131,7 +132,7 @@ struct nS {
   nt start_lba;
 };
 
-#if defined(_WIN32) && !defined(__declspec) // _MSC_VER is unavailable in cc1.
+#if defined(_WIN32)
 // Alignment doesn't affect packing in MS mode.
 extern int n1[sizeof(struct nS) == 16 ? 1 : -1];
 extern int n2[__alignof(struct nS) == 8 ? 1 : -1];
@@ -159,7 +160,7 @@ struct packed_chars {
   char f : 4, g : 8, h : 8, i : 8;
 };
 
-#if (defined(_WIN32) || defined(__SCE__)) && !defined(__declspec) // _MSC_VER is unavailable in cc1.
+#if defined(_WIN32) || defined(__SCE__)
 // On Windows clang uses MSVC compatible layout in this case.
 //
 // Additionally, test for pre-r254596 clang behavior on the PS4/PS5 targets.

[mstorsjo]

Thanks for looking into this!

I can confirm, that before 1760eff, bin/clang -target x86_64-windows-gnu -c ../../clang/test/Sema/struct-packed-align.c (without -cc1) also did fail, triggering these static asserts. So the actual user-facing behaviour is retained, and only the test with a plain -cc1 has changed.

[DanShaders]

Someone will have to merge this as I don't have write access

[mstorsjo]

Thanks! Btw @DanShaders, for future commits - please turn off Keep my email addresses private setting in your account. See LLVM Developer Policy and LLVM Discourse for more information.

(We have CI that is meant to point this out already when PRs are opened, but it doesn't really work as we'd want in all cases.)