Re: [HACKERS] Do we need the gcc feature "__builtin_expect" topromote the branches prediction?
| От | Hao Lee |
|---|---|
| Тема | Re: [HACKERS] Do we need the gcc feature "__builtin_expect" topromote the branches prediction? |
| Дата | |
| Msg-id | CAGoxFiH7wOhTFVuM2-kWxSAPvbgfKCxr3Nm5ETRWcPmyVfGgTA@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: [HACKERS] Do we need the gcc feature "__builtin_expect" topromote the branches prediction? (Julien Rouhaud <julien.rouhaud@dalibo.com>) |
| Список | pgsql-hackers |
Okay. Thanks
On Fri, Jun 2, 2017 at 7:27 PM, Julien Rouhaud <julien.rouhaud@dalibo.com> wrote:
On 02/06/2017 12:50, Craig Ringer wrote:
>
>
> On 2 Jun. 2017 16:42, "Hao Lee" <mixtrue@gmail.com
> <mailto:mixtrue@gmail.com>> wrote:
>
> Hi all,
> There is a lot of "if statement" in system, and GCC provides
> a feature,"__builtin_expect", which let compilers know which branch
> is mostly run.
>
>
> Compilers and CPUs are really good at guessing this.
>
> Humans are wrong about it more than we'd like too.
+1
>
> It's surprisingly rarely s good idea to use branch prediction hints.
>
> See the vsrious Linux kernel discussions about this.
>
> If you find concrete sites of frequent or costly branch mis-prediction
> please point them out, with benchmarks.
And also see this thread:
https://www.postgresql.org/message-id/ CAKJS1f8yqRW3qx2CO9r4bqqvA2Vx6 8=3awbh8CJWTP9zXeoHMw@mail. gmail.com
BTW Andres added support for likely/unlikely in aa3ca5e3dd6, but its
usage is still really limited.
--
Julien Rouhaud
http://dalibo.com - http://dalibo.org
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