[Bug middle-end/102566] [i386] GCC should emit LOCK BTS for simple bit-test-and-set operations with std::atomic

[marko.makela at mariadb dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102566

Marko Mäkelä <marko.makela at mariadb dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |marko.makela at mariadb dot com

--- Comment #31 from Marko Mäkelä <marko.makela at mariadb dot com> ---
Much of this seems to work in GCC 12.2.0 as well as in clang++-15. For clang
there is a related ticket https://github.com/llvm/llvm-project/issues/37322

I noticed a missed optimization in both g++-12 and clang++-15: Some operations
involving bit 31 degrade to loops around lock cmpxchg. I compiled it with "-c
-O2" (AMD64) or "-c -O2 -m32 -march=i686" (IA-32).

#include <atomic>
template<uint32_t b>
void lock_bts(std::atomic<uint32_t> &a) { while (!(a.fetch_or(b) & b)); }
template<uint32_t b>
void lock_btr(std::atomic<uint32_t> &a) { while (a.fetch_and(~b) & b); }
template<uint32_t b>
void lock_btc(std::atomic<uint32_t> &a) { while (a.fetch_xor(b) & b); }
template void lock_bts<1U<<30>(std::atomic<uint32_t> &a);
template void lock_btr<1U<<30>(std::atomic<uint32_t> &a);
template void lock_btc<1U<<30>(std::atomic<uint32_t> &a);
// bug: uses lock cmpxchg
template void lock_bts<1U<<31>(std::atomic<uint32_t> &a);
template void lock_btr<1U<<31>(std::atomic<uint32_t> &a);
template void lock_btc<1U<<31>(std::atomic<uint32_t> &a);