[PATCH 1/4] libstdc++: More efficient date from days.

[Jonathan Wakely]

On 23/02/21 13:24 +0000, Cassio Neri via Libstdc++ wrote:
>This patch reimplements std::chrono::year_month_day::_S_from_days() which
>retrieves a date from the number of elapsed days since 1970/01/01.  The new
>implementation is based on Proposition 6.3 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 4, indicate the new algorithm is
>2.2 times faster than the current one.
>
>The patch adds a test which loops through all integers in [-12687428, 11248737],
>and for each of them, gets the corresponding date 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 interval used in the test covers the full range of values for which a
>roundtrip must work [time.cal.ymd.members].  Despite its completeness the test
>runs in a matter of seconds.
>
>libstdc++-v3/ChangeLog:
>
>    * include/std/chrono:
>    * testsuite/std/time/year_month_day/3.cc: New test.

Thanks! I'm committing this to trunk (it only changes new C++20
material so OK during stage 4 ... and anyway it's both faster and
better tested than the old code).

I've tweaked it slightly to keep some lines below 80 columns, but no
changes except whitespace.