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

[Jonathan Wakely <jwakely@redhat.com>]

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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.


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commit 75c74a83acee3f51e6753b8159fa600fe2d86810
Author: Jonathan Wakely <jwakely@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.