linux-kernel - Re: fyi: gcc33-hammer crashes when compiling kvm emulate.c

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Message-ID: <20101027170023.GA22171@basil.fritz.box>
Date:	Wed, 27 Oct 2010 19:00:23 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: fyi: gcc33-hammer crashes when compiling kvm emulate.c

> Would be good to have __fastpath and __slowpath function attributes.
> 
> __fastpath would always be optimized for speed, __slowpath always
> for size, and everything else would use the default (determined by
> CONFIG_CC_OPTIMIZE_FOR_SIZE).

> 
> Looks like gcc has support for this, with __attribute__((hot)),
> __attribute__((cold)), and __attribute__((optimize)).

I had patches for a long time, but last time I tried it didn't help too much.
(basically make __init hot/cold and mark a few functions) 
Similar things can be also done with unrolling. The Optimize attribute
is currently quite broken in gcc and shouldn't be used.

The main reason I didn't post them is that I didn't want another
likely/unlikely with people sprinkling them randomly and never
looking at profile logs. I am to blame for likely()/unlikely() originally
 and it got abused so much that I regretted it alot. So I don't
think just exposing that is a good idea right now.

I think there are some other ways to do that better, but they
will need more work.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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