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

[Marshall Clow via Phabricator via libcxx-commits]

mclow.lists added a comment.

In D63744#2385750 <https://reviews.llvm.org/D63744#2385750>, @jyknight wrote:

> 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.

At which point, we should be asking ourselves, "is this worth doing?"


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D63744/new/

https://reviews.llvm.org/D63744