[all-commits] [llvm/llvm-project] bd7bb2: [clang] Match MSVC ABI for over-aligned base tail ...

[Daniel Paoliello via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: bd7bb26108bd86eeeef0962577b52d0f435a7ac3
      https://github.com/llvm/llvm-project/commit/bd7bb26108bd86eeeef0962577b52d0f435a7ac3
  Author: Daniel Paoliello <danpao at microsoft.com>
  Date:   2026-07-27 (Mon, 27 Jul 2026)

  Changed paths:
    M clang/docs/ReleaseNotes.md
    M clang/include/clang/AST/RecordLayout.h
    M clang/lib/AST/RecordLayout.cpp
    M clang/lib/AST/RecordLayoutBuilder.cpp
    A clang/test/Layout/ms-arm64-aligned-base-tail-padding.cpp

  Log Message:
  -----------
  [clang] Match MSVC ABI for over-aligned base tail padding on Arm64 (#210461)

When targeting aarch64-pc-windows-msvc, clang laid out a base following
an over-aligned, non-standard-layout base at the wrong offset. MSVC on
Arm64 reuses the over-aligned base's tail padding for the subsequent
base, but clang rounded the base up to a full slot, so the two disagreed
on member offsets, breaking interop between clang- and MSVC-built
binaries.

Fix: store each record's natural non-virtual alignment (excluding
`alignas`/`__declspec(align)` over-alignment) as
`getNonRequiredNVAlignment()`, and on Arm64 fold in a base's natural
alignment rather than its full alignment. This is gated to Arm64 only;
Arm64EC and x64 follow the x64 rule (no reuse) and are unchanged.

Validated against MSVC (Hostx64\arm64\cl.exe
/d1reportSingleClassLayout): clang now matches MSVC Arm64 exactly for
the reduced repro, the original polymorphic case (vftable + empty base +
template), and a range of probe cases covering natural vs.
over-alignment padding and field vs. base reuse. Arm64EC and x64 output
is byte-identical to before.

Fixes #210174



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