Date: Tue, 19 Feb 2008 10:28:46 +0100
From: Andi Kleen <andi@firstfloor.org>
To: Willy Tarreau <w@1wt.eu>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>, Andi Kleen <andi@firstfloor.org>,
       Arjan van de Ven <arjan@infradead.org>, Roel Kluin <12o3l@tiscali.nl>,
       geoffrey.levand@am.sony.com, linuxppc-dev@ozlabs.org,
       cbe-oss-dev@ozlabs.org, lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/3] Fix Unlikely(x) == y
Message-ID: <20080219092846.GB6485@one.firstfloor.org>
References: <47B70A61.9030306@tiscali.nl> <20080216094226.1e8eede1@laptopd505.fenrus.org> <p731w7axrat.fsf@bingen.suse.de> <200802191333.53607.nickpiggin@yahoo.com.au> <20080219055806.GA8404@1wt.eu>
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> 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 @)

> 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.

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

Or -fno-reorder-blocks

-Andi

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