linux-kernel - Re: [PATCH 2/5] mm: remove unlikly NULL from kfree

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Message-ID: <49CA935F.8000908@goop.org>
Date:	Wed, 25 Mar 2009 13:26:07 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Matt Mackall <mpm@...enic.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Hua Zhong <hzhong@...il.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Steven Rostedt <srostedt@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 2/5] mm: remove unlikly NULL from kfree

Steven Rostedt wrote:
> Thanks for this info!
>
> Although gcc plays a role too. That is, if we have
>
> 	if (x)
> 		do something small;
>
> 	do something large;
>
>
> this can be broken into:
>
> 	cmp x
> 	beq 1f
> 	do something small
> 1:
> 	do something large
>
> Which plays nice with the cache. But, by adding a unlikely(x), gcc will 
> probably choose to do:
>
> 	cmp x
> 	bne 2f
>
> 1:
> 	do something large
>
> 	ret;
>
> 2:
> 	do something small
> 	b 1b
>
> which hurts in a number of ways.
>   

I think that's probably the dominant effect on x86 systems, because 
Intel doesn't recommend using the branch hint prefixes as far as I can 
tell (their consumption of icache space outweighs any benefit of priming 
the predictor).

    J
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