diff mbox series

Remove unnecessary "& 1" in year_month_day_last::day()

Message ID CAOfgUPhjCZKFuOB+84b-b_0a_uMq=bsT3HUks6Q7eCaJpDrrDQ@mail.gmail.com
State Not Applicable
Headers
Series Remove unnecessary "& 1" in year_month_day_last::day() |

Checks

Context Check Description
snail/gcc-patch-check fail Git am fail log

Commit Message

Cassio Neri Nov. 5, 2023, 3:38 p.m. UTC 
  When year_month_day_last::day() was implemented, Dr. Matthias Kretz realised
that the operation "& 1" wasn't necessary but we did not patch it at that
time. This patch removes the unnecessary operation.

libstdc++-v3/ChangeLog:

* include/std/chrono:

Comments

Marc Glisse Nov. 5, 2023, 3:58 p.m. UTC | #1 
On Sun, 5 Nov 2023, Cassio Neri wrote:

> When year_month_day_last::day() was implemented, Dr. Matthias Kretz realised
> that the operation "& 1" wasn't necessary but we did not patch it at that
> time. This patch removes the unnecessary operation.

Is there an entry in gcc's bugzilla about having the optimizer handle this 
kind of optimization?

unsigned f(unsigned x){
   if(x>=32)__builtin_unreachable();
   return 30|(x&1); // --> 30|x
}

(that optimization would come in addition to your patch, doing the 
optimization by hand is still a good idea)

It looks like the criterion would be a|(b&c) when the possible 1 bits of b 
are included in the certainly 1 bits of a|c.
Cassio Neri Nov. 5, 2023, 4:09 p.m. UTC | #2 
I could not find any entry in gcc's bugzilla for that. Perhaps my search
wasn't good enough.


On Sun, 5 Nov 2023 at 15:58, Marc Glisse <marc.glisse@inria.fr> wrote:

> On Sun, 5 Nov 2023, Cassio Neri wrote:
>
> > When year_month_day_last::day() was implemented, Dr. Matthias Kretz
> realised
> > that the operation "& 1" wasn't necessary but we did not patch it at that
> > time. This patch removes the unnecessary operation.
>
> Is there an entry in gcc's bugzilla about having the optimizer handle this
> kind of optimization?
>
> unsigned f(unsigned x){
>    if(x>=32)__builtin_unreachable();
>    return 30|(x&1); // --> 30|x
> }
>
> (that optimization would come in addition to your patch, doing the
> optimization by hand is still a good idea)
>
> It looks like the criterion would be a|(b&c) when the possible 1 bits of b
> are included in the certainly 1 bits of a|c.
>
> --
> Marc Glisse
>
Andrew Pinski Nov. 5, 2023, 7:54 p.m. UTC | #3 
On Sun, Nov 5, 2023 at 9:13 AM Cassio Neri <cassio.neri@gmail.com> wrote:
>
> I could not find any entry in gcc's bugzilla for that. Perhaps my search wasn't good enough.

I filed https://gcc.gnu.org/PR112395 with a first attempt at the patch
(will double check it soon).

Thanks,
Andrew

>
>
> On Sun, 5 Nov 2023 at 15:58, Marc Glisse <marc.glisse@inria.fr> wrote:
>>
>> On Sun, 5 Nov 2023, Cassio Neri wrote:
>>
>> > When year_month_day_last::day() was implemented, Dr. Matthias Kretz realised
>> > that the operation "& 1" wasn't necessary but we did not patch it at that
>> > time. This patch removes the unnecessary operation.
>>
>> Is there an entry in gcc's bugzilla about having the optimizer handle this
>> kind of optimization?
>>
>> unsigned f(unsigned x){
>>    if(x>=32)__builtin_unreachable();
>>    return 30|(x&1); // --> 30|x
>> }
>>
>> (that optimization would come in addition to your patch, doing the
>> optimization by hand is still a good idea)
>>
>> It looks like the criterion would be a|(b&c) when the possible 1 bits of b
>> are included in the certainly 1 bits of a|c.
>>
>> --
>> Marc Glisse
diff mbox series

Patch

diff --git a/libstdc++-v3/include/std/chrono
b/libstdc++-v3/include/std/chrono
index 10e868e5a03..c979a5d05dd 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -1800,8 +1800,10 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
  const auto __m = static_cast<unsigned>(month());

- // Excluding February, the last day of month __m is either 30 or 31 or,
- // in another words, it is 30 + b = 30 | b, where b is in {0, 1}.
+ // Assume 1 <= __m <= 12, otherwise month().ok() == false and the result
+ // of day() is unspecified. Excluding February, the last day of month __m
+ // m is either 30 or 31 or, in another words, it is 30 | b, where b is in
+ // {0, 1}.

  // If __m in {1, 3, 4, 5, 6, 7}, then b is 1 if, and only if __m is odd.
  // Hence, b = __m & 1 = (__m ^ 0) & 1.
@@ -1812,10 +1814,14 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
  // Therefore, b = (__m ^ c) & 1, where c = 0, if __m < 8, or c = 1 if
  // __m >= 8, that is, c = __m >> 3.

+ // Since 30 = (11110)_2 and __m <= 31 = (11111)_2, we have:
+ // 30 | ((__m ^ c) & 1) == 30 | (__m ^ c), that is, the "& 1" is
+ // unnecessary.
+
  // The above mathematically justifies this implementation whose
  // performance does not depend on look-up tables being on the L1 cache.
- return chrono::day{__m != 2 ? ((__m ^ (__m >> 3)) & 1) | 30
-    : _M_y.is_leap() ? 29 : 28};
+ return chrono::day{__m != 2 ? (__m ^ (__m >> 3)) | 30
+ : _M_y.is_leap() ? 29 : 28};
       }

       constexpr