commit b92d12d3fe3f1aa56d190d960e40c62869a6cfbb Author: Cassio Neri Date: Wed Jun 23 15:32:16 2021 libstdc++: More efficient std::chrono::year::leap 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. The expression is then either y%16 or y%4 which are both powers of two and so it can be rearranged to use simple bitmask operations. Co-authored-by: Jonathan Wakely Co-authored-by: Ulrich Drepper libstdc++-v3/ChangeLog: * include/std/chrono (chrono::year::is_leap()): Optimize. diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono index 4631a727d73..863b6a27bdf 100644 --- a/libstdc++-v3/include/std/chrono +++ b/libstdc++-v3/include/std/chrono @@ -1606,13 +1606,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION // [1] https://github.com/cassioneri/calendar // [2] https://accu.org/journals/overload/28/155/overload155.pdf#page=16 + // Furthermore, if y%100 != 0, then y%400==0 is equivalent to y%16==0, + // so we can rearrange the expression to (mult_100 ? y % 4 : y % 16)==0 + // which is equivalent to (y & (mult_100 ? 15 : 3)) == 0. + // See https://gcc.gnu.org/pipermail/libstdc++/2021-June/052815.html + constexpr uint32_t __multiplier = 42949673; constexpr uint32_t __bound = 42949669; constexpr uint32_t __max_dividend = 1073741799; 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; + return (_M_y & (__is_multiple_of_100 ? 15 : 3)) == 0; } explicit constexpr