On 24/02/21 17:28 +0000, Jonathan Wakely wrote:
>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.

I've made the attached tweak, to avoid a narrowing conversion from
long to int. Tested x86_64-linux, committed to trunk.