Re: Do we need the gcc feature "__builtin_expect" to promote the branches prediction?

Okay. Thanks

On Fri, Jun 2, 2017 at 7:27 PM, Julien Rouhaud <julien(dot)rouhaud(at)dalibo(dot)com>
wrote:

> On 02/06/2017 12:50, Craig Ringer wrote:
> >
> >
> > On 2 Jun. 2017 16:42, "Hao Lee" <mixtrue(at)gmail(dot)com
> > <mailto:mixtrue(at)gmail(dot)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(at)mail(dot)gmail(dot)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
>