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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <20080220073245.GA28524@1wt.eu>
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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/