Re: [PATCH] malloc: remove __builtin_expect

[Yury Gribov <y dot gribov at samsung dot com>]

On 01/26/2016 11:59 AM, Jörn Engel wrote:
> On Tue, Jan 26, 2016 at 10:56:49AM +0300, Yury Gribov wrote:
> > On 01/26/2016 03:24 AM, Joern Engel wrote:
> > > From: Joern Engel <joern@purestorage.org>
> > >
> > > It was disabled anyway and only served as obfuscation.  No change
> > > post-compilation.
> >
> > FYI I've witnessed significant improvements from (real) __builtin_expect in
> > other projects.
>
> Interesting.  I tried to find any effect and couldn't.  Michael Kerrisk
> managed to write an example program that demonstrated the advantage:
> http://blog.man7.org/2012/10/how-much-do-builtinexpect-likely-and.html
>
> I tried his program and again couldn't demonstrate any effect.  Not that
> I question his numbers, there likely was an effect using his compiler
> and machine.  But with a newer compiler and/or cpu, the effect was gone.
>
> So if you have a reference to the project and can replicate the
> improvement, I would be interested.

Ok, it took time to dig the archives: 
http://comments.gmane.org/gmane.comp.debugging.address-sanitizer/1288 . 
Basically adding __builtin_expect in an important hot-path saved 20% of 
performance (that's on Clang though).

> Jörn
>
> --
> It's worth remembering that one reason the Internet succeeded was that
> it did not need the permission of the local telcos  in order to get up
> and going.
> -- iang