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

[Jonathan Wakely <jwakely@redhat.com>]

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On 25/02/21 14:19 +0000, Jonathan Wakely wrote:
>On 25/02/21 14:02 +0000, Jonathan Wakely wrote:
>>On 25/02/21 13:46 +0000, Cassio Neri via Libstdc++ wrote:
>>>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.
>>
>>OK, I'll make that change shortly, thanks.
>
>We still need to cast to int for the return value though, because
>converting from uint32_t to int is still narrowing.

I've committed this now.

Tested powerpc64le-linux, committed to trunk.



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commit a47cec4ca7302e65f63490ad7f251c5a469bc0e0
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Feb 25 16:57:20 2021

    libstdc++: Use uint32_t for all year_month_day::_S_from_days arithmetic
    
    libstdc++-v3/ChangeLog:
    
            * include/std/chrono (year_month_day::_S_from_days): Perform
            all calculations with type uint32_t.

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index eef503af274..fcdaee7328e 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -2455,7 +2455,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr auto __z2    = static_cast<uint32_t>(-1468000);
       constexpr auto __r2_e3 = static_cast<uint32_t>(536895458);
 
-      const auto __r0 = __dp.count() + __r2_e3;
+      const auto __r0 = static_cast<uint32_t>(__dp.count()) + __r2_e3;
 
       const auto __n1 = 4 * __r0 + 3;
       const auto __q1 = __n1 / 146097;