https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102566
--- Comment #32 from Hongtao.liu <crazylht at gmail dot com> --- (In reply to Marko Mäkelä from comment #31) > 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 31 is sign bit, and c = a & 1U << 31; c == 0 is optimized to (sign int)a >= 0. The optimization we did in optimize_atomic_bit_test_and is supposed to match a & 1U << 31, and it failed. I guess it could be extend to match (sign int)a >= 0 when mask is 1U << 31. 7 <D.2055>: 8 <D.2054>: 9 _1 = __atomic_fetch_or_4 (v, 2147483648, 0); 10 _2 = (signed int) _1; 11 if (_2 >= 0) goto <D.2055>; else goto <D.2053>; 12 <D.2053>: 13 return; 14} 15
