[Bug target/80878] -mcx16 (enable 128 bit CAS) on x86_64 seems not to work on 7.1.0

pinskia at gcc dot gnu.org gcc-bugzilla@gcc.gnu.org
Thu May 6 19:32:51 GMT 2021


--- Comment #31 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Niall Douglas from comment #30)
> I got bit by this GCC regression today at work. Consider
> https://godbolt.org/z/M9fd7nhdh where std::atomic<__int128> is compare
> exchanged with -march=sandybridge:
> - On GCC 6.4 and earlier, this emits lock cmpxchg16b, as you would expect.
> - From GCC 7 up to trunk (12?), this emits __atomic_compare_exchange_16.
> - On clang, this emits lock cmpxchg16b, as you would expect.
> This is clearly a regression. GCCs before 7 did the right thing. GCCs from 7
> onwards do not. clangs with libstdc++ do do the right thing.
> This isn't just an x64 thing, either. Consider
> https://godbolt.org/z/x6d5GE4o6 where GCC on ARM64 emits
> __atomic_compare_exchange_16, whereas clang on ARM64 emits ldaxp/stlxp, as
> you would expect.
> Please mark this bug as a regression affecting all versions of GCC from 7 to
> trunk, and affecting all 128 bit atomic capable architectures not just x64.

Again the problem is stuff like:
static const _Atomic __int128_t t = 2000;

__int128_t g(void)
  return t;

DOES NOT WORK if you use CAS (or ldaxp/stlxp).

So clang is broken really ....

Also GCC for ARM64 emits calls for all compare and exchange because using the
LSE (from ARMv8.1-a) is useful.

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