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

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Date:	Tue, 19 Feb 2008 10:28:46 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Willy Tarreau <w@....eu>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Andi Kleen <andi@...stfloor.org>,
	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

> 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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