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

[Jonathan Wakely via Gcc-patches]

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.n...@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 <jwak...@redhat.com>

   Co-authored-by: Ulrich Drepper <drep...@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!


commit 4a404f66b09d661bdc60e7e0d5d08f00c4e194db
Author: Jonathan Wakely <jwak...@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