https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98226
--- Comment #6 from Oleg Zaikin <zaikin.icc at gmail dot com> --- (In reply to Jonathan Wakely from comment #2) > Oh, but you didn't enable any optimization at all, so who cares about the > performance? Let me give the whole picture. The issue is very close to that from https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97759 where a C-style implementation was replaced by C++20 std::has_single_bit. We have a complex project that of course is compiled with all proper optimization flags. There we have a function firstzero(unsigned x) that returns 2^i, with i the position of the first 0 of x, and 0 iff there is no 0. Its implementation is: unsigned firstzero(const unsigned x) noexcept { #if __cplusplus > 201703L return x == unsigned(-1) ? 0 : unsigned(1) << std::countr_one(x); #else const unsigned y = x+1; return (y ^ x) & y; #endif } When we switched from C++17-based g++ to C++20-based g++, the performance of the whole program decreased by about 7 %. It turned out that the main reason is the firstzero function.
