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

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Date:	Sat, 16 Feb 2008 09:42:26 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Willy Tarreau <w@....eu>
Cc:	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 Sat, 16 Feb 2008 18:33:16 +0100
Willy Tarreau <w@....eu> wrote:

> On Sat, Feb 16, 2008 at 09:25:52AM -0800, Arjan van de Ven wrote:
> > On Sat, 16 Feb 2008 17:08:01 +0100
> > Roel Kluin <12o3l@...cali.nl> wrote:
> > 
> > > The patch below was not yet tested. If it's correct as it is,
> > > please comment. ---
> > > Fix Unlikely(x) == y
> > > 
> > 
> > you found a great set of bugs..
> > but to be honest... I suspect it's just best to remove unlikely
> > altogether for these cases; unlikely() is almost a
> > go-faster-stripes thing, and if you don't know how to use it you
> > shouldn't be using it... so just removing it for all wrong cases is
> > actually the best thing to do imo.
> 
> Well, eventhough the author may not know how to use it, "unlikely" at
> least indicates the intention of the author, or his knowledge of what
> should happen here. I'd suggest leaving it where it is because the
> authot of this code is in best position to know that this branch is
> unlikely to happen, eventhough he does not correctly use the macro.
>

you have more faith in the authors knowledge of how his code actually behaves than I think is warranted  :)
Or faith in that he knows what "unlikely" means.
I should write docs about this; but unlikely() means:
1) It happens less than 0.01% of the cases.
2) The compiler couldn't have figured this out by itself
   (NULL pointer checks are compiler done already, same for some other conditions)
3) It's a hot codepath where shaving 0.5 cycles (less even on x86) matters
   (and the author is ok with taking a 500 cycles hit if he's wrong)

If you think unlikely() means something else, we should fix what it maps to towards gcc ;)
(to.. be empty ;)

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