https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878
--- 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.
