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* [gcc r12-1753] libstdc++: More efficient std::chrono::year::leap
@ 2021-06-23 17:29 Jonathan Wakely
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From: Jonathan Wakely @ 2021-06-23 17:29 UTC (permalink / raw)
To: gcc-cvs, libstdc++-cvs
https://gcc.gnu.org/g:b92d12d3fe3f1aa56d190d960e40c62869a6cfbb
commit r12-1753-gb92d12d3fe3f1aa56d190d960e40c62869a6cfbb
Author: Cassio Neri <cassio.neri@gmail.com>
Date: Wed Jun 23 15:32:16 2021 +0100
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 <jwakely@redhat.com>
Co-authored-by: Ulrich Drepper <drepper@redhat.com>
libstdc++-v3/ChangeLog:
* include/std/chrono (chrono::year::is_leap()): Optimize.
Diff:
---
libstdc++-v3/include/std/chrono | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
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
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