[libcxx-dev] libcxx ABI version 2

[Derek Schuff via libcxx-dev]

Hello libcxx experts,

I work on a toolchain where we've been using the various ABI improvements
which are currently behind the _LIBCPP_ABI_UNSTABLE preprocessor define. We
haven't declared a stable C++ ABI yet but we are considering doing that
soon-ish. I'm wondering what the general thinking is about ABIv2 (and/or
its various parts), e.g. how mature these enhancements are, if there is any
known timeline for declaring ABIv2 stable, would we be totally crazy to
bake any of these into our stable ABI, etc.

We recently discovered a bug in LIBCXX_ABI_OPTIMIZED_FUNCTION (
https://bugs.llvm.org/show_bug.cgi?id=46678) and reverted back to the V1
ABI for now (since we haven't declared stability) but it did come at a
surprising regression in binary size for one of our workloads. Any specific
advice about that bug would also be helpful.

Thanks!
-Derek
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