Re: [HACKERS] Do we need the gcc feature "__builtin_expect" topromote the branches prediction?

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/CAKJS1f8yqRW3qx2CO9r4bqqvA2Vx68=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