I've checked the generated code and the compiler doesn't figure out the logic. I added a comment to explain. (Revised patch below and attached.) Best wishes, Cassio. --- Simple change to std::chrono::year::is_leap. If a year is multiple of 100, then it's divisible by 400 if and only if it's divisible by 16. The latter allows for better code generation. Tested on x86_64-pc-linux-gnu. libstdc++-v3/ChangeLog: libstdc++-v3/ChangeLog: * include/std/chrono: diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index 4631a727d73..85aa0379432 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -1612,7 +1612,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION constexpr uint32_t __offset = __max_dividend / 2 / 100 * 100; const bool __is_multiple_of_100 = __multiplier * (_M_y + __offset) < __bound; - return (!__is_multiple_of_100 || _M_y % 400 == 0) && _M_y % 4 == 0; + // Usually we test _M_y % 400 == 0 but, when it's already known that + // _M_y%100 == 0, then _M_y % 400==0 is equivalent to _M_y % 16 == 0. + return (!__is_multiple_of_100 || _M_y % 16 == 0) && _M_y % 4 == 0; } explicit constexpr On Fri, May 21, 2021 at 7:05 PM Koning, Paul wrote: > > > > > On May 21, 2021, at 1:46 PM, Cassio Neri via Gcc-patches wrote: > > > > Simple change to std::chrono::year::is_leap. If a year is multiple of 100, > > then it's divisible by 400 if and only if it's divisible by 16. The latter > > allows for better code generation. > > I wonder if the optimizer could be taught to do that. > > The change seems correct but it is very confusing; at the very least the reasoning you gave should be stated in a comment on that check. > > paul > >