Re: [PATCH] libstdc++: More efficient std::chrono::year::leap.

[Jonathan Wakely <jwakely@redhat.com>]

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On 23/06/21 18:51 +0100, Jonathan Wakely wrote:
>Here's what I've committed. Tested x86_64-linux and powerpc64le-linux.
>Pushed to trunk.
>
>
>

>commit b92d12d3fe3f1aa56d190d960e40c62869a6cfbb
>Author: Cassio Neri <cassio.neri@gmail.com>
>Date:   Wed Jun 23 15:32:16 2021
>
>    libstdc++: More efficient std::chrono::year::leap
>    
>    Simple change to std::chrono::year::is_leap. If a year is multiple of 100,
>    then it's divisible by 400 if and only if it's divisible by 16. The latter
>    allows for better code generation.
>    
>    The expression is then either y%16 or y%4 which are both powers of two
>    and so it can be rearranged to use simple bitmask operations.
>    
>    Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
>    Co-authored-by: Ulrich Drepper <drepper@redhat.com>
>    
>    libstdc++-v3/ChangeLog:
>    
>            * include/std/chrono (chrono::year::is_leap()): Optimize.
>
>diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
>index 4631a727d73..863b6a27bdf 100644
>--- a/libstdc++-v3/include/std/chrono
>+++ b/libstdc++-v3/include/std/chrono
>@@ -1606,13 +1606,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> 	// [1] https://github.com/cassioneri/calendar
> 	// [2] https://accu.org/journals/overload/28/155/overload155.pdf#page=16
> 
>+	// Furthermore, if y%100 != 0, then y%400==0 is equivalent to y%16==0,
>+	// so we can rearrange the expression to (mult_100 ? y % 4 : y % 16)==0

But Ulrich pointed out I got my boolean logic all muddled up in the
comment. Fixed with the attached patch!


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commit 4a404f66b09d661bdc60e7e0d5d08f00c4e194db
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jun 23 18:50:03 2021

    libstdc++: Fix comment in chrono::year::is_leap()
    
    libstdc++-v3/ChangeLog:
    
            * include/std/chrono (chrono::year::is_leap()): Fix incorrect
            logic in comment.

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 863b6a27bdf..0b597be1944 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -1606,8 +1606,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	// [1] https://github.com/cassioneri/calendar
 	// [2] https://accu.org/journals/overload/28/155/overload155.pdf#page=16
 
-	// Furthermore, if y%100 != 0, then y%400==0 is equivalent to y%16==0,
-	// so we can rearrange the expression to (mult_100 ? y % 4 : y % 16)==0
+	// Furthermore, if y%100 == 0, then y%400==0 is equivalent to y%16==0,
+	// so we can simplify it to (!mult_100 && y % 4 == 0) || y % 16 == 0,
 	// which is equivalent to (y & (mult_100 ? 15 : 3)) == 0.
 	// See https://gcc.gnu.org/pipermail/libstdc++/2021-June/052815.html