[clang][docs] Add note about -Wms-bitfield-padding in MSVC compatibility docs

[ojhunt]

No description provided.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Oliver Hunt (ojhunt)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/155770.diff

1 Files Affected:

• (modified) clang/docs/MSVCCompatibility.rst (+6)

diff --git a/clang/docs/MSVCCompatibility.rst b/clang/docs/MSVCCompatibility.rst
index b4a7d23e1b2c6..63c8d8e70a997 100644
--- a/clang/docs/MSVCCompatibility.rst
+++ b/clang/docs/MSVCCompatibility.rst
@@ -284,3 +284,9 @@ Since `__m128` is not a class type in clang any overloads after a template defin
 With MSVC ``foo(__m128)`` will be selected but with clang ``foo<__m128>()`` will be selected since on clang `__m128` is a builtin type.
 
 In general the takeaway is `__m128` is a builtin type on clang while a class type on MSVC.
+
+Warnings
+========
+
+* ``-Wms-bitfield-padding``
+  When generating code compatible with MSVC, clang applies the bit-field padding ABI used by MSVC. The padding behavior of sequential bit-fields in a record's layout is dependent on whether the underlying storage type of those bit-fields are the same. To help diagnose unexpected padding the ``-Wms-bitfield-padding`` warning can be used to diagnose cases where the MSVC ABI will not pad bit-fields (even if not targeting relevant platforms).

[ojhunt]

I wasn't sure if this was warranted, but given little effort to add the note I figured that I could create it an ping the last person to touch it :D

[ojhunt]

@cor3ntin prodding you in case you know who should review, or are ok signing off on it :D

[MaxEW707]

Is it worth linking to, https://clang.llvm.org/docs/DiagnosticsReference.html#wms-bitfield-padding, since the warning explanation on that page has really good examples.

Something like, See :doc:`diagnostics reference <DiagnosticsReference>` for more information on the -Wms-bitfield-padding warning.