linux-kernel - Re: [PATCH 1/3] Fix Unlikely(x) == y

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Date:	Wed, 20 Feb 2008 08:32:45 +0100
From:	Willy Tarreau <w@....eu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Arjan van de Ven <arjan@...radead.org>,
	Roel Kluin <12o3l@...cali.nl>, geoffrey.levand@...sony.com,
	linuxppc-dev@...abs.org, cbe-oss-dev@...abs.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] Fix Unlikely(x) == y

On Tue, Feb 19, 2008 at 10:28:46AM +0100, Andi Kleen wrote:
> > Sometimes, for performance critical paths, I would like gcc to be dumb and
> > follow *my* code and not its hard-coded probabilities. 
> 
> If you really want that, simple: just disable optimization @)

already tried. It fixed some difficulties, but create new expected issues
with data being reloaded often from memory instead of being passed along
a few registers. Don't forget that optimizing for x86 requires a lot of
smartness from the compiler because of the very small number of registers!

> > Maybe one thing we would need would be the ability to assign probabilities
> > to each branch based on what we expect, so that gcc could build a better
> > tree keeping most frequently used code tight.
> 
> Just use profile feedback then for user space. I don't think it's a good
> idea for kernel code though because it leads to unreproducible binaries
> which would wreck the development model.

I never found this to be practically usable in fact, because you have to
use it on the *exact* same source. End of game for cross-compilation. It
would be good to be able to use a few pragmas in the code to say "hey, I
want this block optimized like this". This is what I understood the
__builtin_expect() was for, except that it tends to throw unpredicted
branches too far away.

> > Hmm I've just noticed -fno-guess-branch-probability in the man, I never tried
> > it.
> 
> Or -fno-reorder-blocks

Thanks for the hint, I will try it.

Willy

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