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.
commit 75c74a83acee3f51e6753b8159fa600fe2d86810
Author: Jonathan Wakely <jwak...@redhat.com>
Date: Thu Feb 25 11:48:18 2021
libstdc++: Fix narrowing conversion in year_month_day [PR 99265]
libstdc++-v3/ChangeLog:
PR libstdc++/99265
* include/std/chrono (year_month_day::_S_from_days): Cast long
to int explicitly.
diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index feb2c2a1fad..eef503af274 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -2481,8 +2481,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
const auto __m1 = __j ? __m0 - 12 : __m0;
const auto __d1 = __d0 + 1;
- return year_month_day{chrono::year{__y1 + __z2}, chrono::month{__m1},
- chrono::day{__d1}};
+ return year_month_day{chrono::year{static_cast<int>(__y1 + __z2)},
+ chrono::month{__m1}, chrono::day{__d1}};
}
// Days since 1970/01/01.
