https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878
Niall Douglas <s_gccbugzilla at nedprod dot com> changed:
What |Removed |Added
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CC| |s_gccbugzilla at nedprod dot
com
--- Comment #30 from Niall Douglas <s_gccbugzilla at nedprod dot com> ---
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.
