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libstdc++: More efficient std::chrono::year::leap

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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 <jwakely@redhat.com>
Co-authored-by: Ulrich Drepper <drepper@redhat.com>

libstdc++-v3/ChangeLog:

	* include/std/chrono (chrono::year::is_leap()): Optimize.
This commit is contained in:
Cassio Neri 2021-06-23 15:32:16 +01:00 • committed by Jonathan Wakely
parent 53330b7d68
commit b92d12d3fe
1 changed files with 6 additions and 1 deletions
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libstdc++-v3/include/std/chrono
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@ -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