linux-kernel - Re: [patch 9/9] Scheduler profiling

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Date:	Mon, 4 Jun 2007 18:20:10 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Andi Kleen <andi@...stfloor.org>,
	Nicholas Mc Guire <mcguire@....edu.cn>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 9/9] Scheduler profiling - Use conditional calls

* Andi Kleen (andi@...stfloor.org) wrote:
> > I see your point, but there is a level of control on the branch I would
> > lack by doing so: the ability to put the call in either the if or else
> > branch. It is an optimization on i386.
> 
> What does it optimize exactly?
> 

Nicholas McGuire told me that the non common cases should be put in
else branches of if statements for i386. At the time, I did a quick test
that correlated what he said, but I seem to be unable to reproduce this
behavior now (maybe my code snippet is too simple?): I will then assume
that the likely/unlikely (builtin expects) tells everything that is
needed to gcc until further notice. Therefore, we can use the form :

if (cond_call(var)), as you proposed.

> > Also, I live in the expectation that, someday, the gcc guys will be nice
> > enough to add some kind of support for a nop-based jump that would
> > require code patching to put a jump instead. If it ever happens, my
> > macro could evolve into this for newer compiler versions, which I could
> > not do with the if() statement you are proposing.
> 
> If that ever happens we couldn't use it anyways because Linux still
> has to support old compilers for a long time. And when those are dropped the
> code could be updated.
> 

Agreed.

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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