[Bug libstdc++/114865] std::atomic<X>::compare_exchange_strong seems to hang under GCC 13 on Ubuntu 23.04

["pdimov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114865

--- Comment #1 from Peter Dimov <pdimov at gmail dot com> ---
> The problem may well be inside libatomic, of course; I have no way to tell.

Narrator: but he did, in fact, have a way to tell.

This is a GHA run with GCC 9 to 13 tested on both Ubuntu 23.04 and Ubuntu
23.10, and only GCC 13 hangs.

https://github.com/boostorg/uuid/actions/runs/8852038822/job/24309866206

And indeed, the codegen from GCC 12 (https://godbolt.org/z/xc7oT76fn) is
radically different from (and much simpler than) the GCC 13 one
(https://godbolt.org/z/eP48xv3Mr).

I think that the problem has been introduced with this commit:

https://github.com/gcc-mirror/gcc/commit/157236dbd621644b3cec50b6cf38811959f3e78c

which, ironically enough, was supposed to improve the handling of types with
padding bits, but broke them entirely.

(I told the committee there was nothing wrong with compare_exchange as
specified, but did they listen to me? To ask the question is to answer it.)