From: Adhemerval Zanella <adhemerval.zanella@linaro.org>
To: Noah Goldstein <goldstein.w.n@gmail.com>,
Paul Eggert <eggert@cs.ucla.edu>
Cc: Florian Weimer <fweimer@redhat.com>,
crrodriguez@opensuse.org,
Adhemerval Zanella via Libc-alpha <libc-alpha@sourceware.org>
Subject: Re: [PATCH 1/4] Remove architecture specific sched_cpucount optimizations
Date: Thu, 6 May 2021 09:22:11 -0300 [thread overview]
Message-ID: <fccf5cb7-4cea-0bda-158c-9d417355dafd@linaro.org> (raw)
In-Reply-To: <CAFUsyfJ7SjQrRE9_MD+i0-umoEZb6pb6PxohBBpBX94H0zEdOw@mail.gmail.com>
On 05/05/2021 16:52, Noah Goldstein via Libc-alpha wrote:
> On Wed, May 5, 2021 at 12:32 PM Paul Eggert <eggert@cs.ucla.edu> wrote:
>>
>> On 5/5/21 10:28 AM, Florian Weimer via Libc-alpha wrote:
>>>> diff --git a/posix/sched_cpucount.c b/posix/sched_cpucount.c
>>>> index b0ca4ea7bc..529286e777 100644
>>>> --- a/posix/sched_cpucount.c
>>>> +++ b/posix/sched_cpucount.c
>>>> @@ -22,31 +22,11 @@ int
>>>> __sched_cpucount (size_t setsize, const cpu_set_t *setp)
>>>> {
>>>> int s = 0;
>>>> + for (int i = 0; i < setsize / sizeof (__cpu_mask); i++)
>>>> {
>>>> + __cpu_mask si = setp->__bits[i];
>>>> + /* Clear the least significant bit set. */
>>>> + for (; si != 0; si &= si - 1, s++);
>>>> }
>>>> -
>>>> return s;
>>>> }
>>> Why “si”? It think si &= si - 1 clears the*most* significant bit in
>>> si. If you agree, please update the comment.
>>
>> Better yet, define a static function 'popcount' that uses Kernighan's
>> trick and call that function. As things stand it's not obvious what the
>> code is doing, regardless of which bit it's clearing. The function's
>> comment should explain why it's not using __builtin_popcount.
>
> It looks to me like GCC is still having a bit of trouble with the new
> implementation. With skylake as a target it seems to be throwing
> in a cmovcc in the outer loop:
>
> https://godbolt.org/z/ocn1KWGPx
>
It seems that come from using a signed int as the counter, I got a slight
better version using unsigned: https://godbolt.org/z/4MrMq1zKb
It is not as better as the builtin, but do we really need that
micro-optimization in this specific implementation? This is exactly what
added the current complexity and using a open-coded popcount implementation
is slight better on architectures that do not provided such instruction
(where builtin_popcount will issue a call to __popcountsi2 and gcc seems
to aiming for open code such scenarios [1]).
In any case I would go for simplicity (and the builtin requires know the
underlying type size, which requires the ugly size of check to call the
correct one).
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92135
next prev parent reply other threads:[~2021-05-06 12:22 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-29 18:25 Adhemerval Zanella
2021-03-29 18:25 ` [PATCH 2/4] linux: Use sched_getaffinity for __get_nprocs (BZ #27645) Adhemerval Zanella
2021-04-27 15:38 ` Florian Weimer
2021-03-29 18:25 ` [PATCH 3/4] linux: Use /sys/devices/system/cpu/possible on __get_nprocs_conf Adhemerval Zanella
2021-05-05 16:53 ` Adhemerval Zanella
2021-05-05 17:54 ` Florian Weimer
2021-05-05 18:06 ` Florian Weimer
2021-05-06 13:03 ` Adhemerval Zanella
2021-05-06 13:51 ` Florian Weimer
2021-05-06 20:07 ` Adhemerval Zanella
2021-05-07 11:07 ` Florian Weimer
2021-05-07 12:43 ` Adhemerval Zanella
2021-05-06 13:17 ` Adhemerval Zanella
2021-03-29 18:25 ` [PATCH 4/4] linux: Remove /proc/cpuinfo fallback on alpha and sparc Adhemerval Zanella
2021-05-05 16:53 ` Adhemerval Zanella
2021-05-05 16:52 ` [PATCH 1/4] Remove architecture specific sched_cpucount optimizations Adhemerval Zanella
2021-05-05 17:28 ` Florian Weimer
2021-05-05 18:25 ` Paul Eggert
2021-05-05 19:52 ` Noah Goldstein
2021-05-06 12:22 ` Adhemerval Zanella [this message]
2021-05-06 18:34 ` Noah Goldstein
2021-05-06 13:33 ` Adhemerval Zanella
2021-05-06 13:43 ` Florian Weimer
2021-05-06 16:16 ` Adhemerval Zanella
2021-05-06 16:42 ` Florian Weimer
2021-05-06 16:54 ` Adhemerval Zanella
2021-05-06 17:12 ` Paul Eggert
2021-05-06 17:51 ` Adhemerval Zanella
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