[libcxx-commits] [PATCH] D63744: In the libc++ unstable ABI, use [[no_unique_address]] instead of __compressed_pair when available.

[James Y Knight via Phabricator via libcxx-commits]

jyknight added a comment.

Hm, that might indeed be feasible. We'll need to potentially insert padding both before the first type and after the second type, but we can static_assert the correctness against the old layout, so that's not _too_ scary.

If we go this route, it will mean that libc++ is only usable in an ABI-stable manner on Clang 9+ or GCC 9+. On earlier compilers, and on MSVC, the attribute is ignored, and the layout will thus be incorrect/incompatible. Is it viable to require those compiler versions to both build and use libc++? The LLVM build itself only requires GCC 5+/Clang 3.5 at the moment, so that would mean we'd need to start requiring the use of the 2-stage "runtimes" build in order to build libc++, at the least.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D63744/new/

https://reviews.llvm.org/D63744