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This patch reimplements std::chrono::year_month_day::_M_days_since_epoch() which calculates the number of elapsed days since 1970/01/01. The new implementation is based on Proposition 6.2 of Neri and Schneider, "Euclidean Affine Functions and Applications to Calendar Algorithms" available at https://arxiv.org/abs/2102.06959. The aforementioned paper benchmarks the implementation against several counterparts, including libc++'s (which is identical to the current implementation). The results, shown in Figure 3, indicate the new algorithm is 1.7 times faster than the current one. The patch adds a test which loops through all dates in [-32767/01/01, 32767/12/31], and for each of them, gets the number of days and compares the result against its expected value. The latter is calculated using a much simpler and easy to understand algorithm but which is also much slower. The dates used in the test covers the full range of possible values [time.cal.year.members]. Despite its completeness the test runs in matter of seconds. libstdc++-v3/ChangeLog: * include/std/chrono (year_month_day::_M_days_since_epoch): New implementation. * testsuite/std/time/year_month_day/4.cc: New test.
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