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

[Cassio Neri]

Hi Jonathan,

The issue is that I didn't cast __dp.count() to uint32_t:

-      const auto __r0 = __dp.count() + __r2_e3;
+      const auto __r0 = static_cast<uint32_t>(__dp.count()) + __r2_e3;

The above would be a better fix. Indeed, __r0 belongs to [0, 2^32[ which allows
all arithmetics that follow to be performed on uint32_t values. For performance
this is better than using signed integers.

I wrongly assumed __dp.count() was int32_t which would make __r0 also uint32_t.
It turns out, it is int64_t so are __r0 and other expressions including
__y1 + __z2. Hence, we're losing a bit of performance. I'm glad the compiler
warned us. (Although I don't understand why I didn't get the warning.)

Thanks,
Cassio.

On Thu, Feb 25, 2021 at 11:56 AM Jonathan Wakely <jwakely@redhat.com> wrote:
>
> 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.
>