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Message-ID: <3C4543F0.29CCD025@mandrakesoft.com>
Date: Wed, 16 Jan 2002 04:12:16 -0500
From: Jeff Garzik <jgarzik@mandrakesoft.com>
Organization: MandrakeSoft
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To: Anton Blanchard <anton@samba.org>
CC: David Schwartz <davids@webmaster.com>, linux-kernel@vger.kernel.org
Subject: Re: likely/unlikely
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Anton Blanchard wrote:
> 
> 
> > likely/unlikely set the branch prediction values to 99% or 1%
> > respectively.  If this causes the code generated to perform less
> > optimally than without, I'm sure the gcc guys would be -very- interested
> > to hear that...
> 
> On some ppc64 the branch prediction is quite good and static prediction
> will override the dynamic prediction. I think we avoid predicting a
> branch unless we are quite sure (95%/5%).
> 
> So if likely/unlikely is overused (on more marginal conditionals) then
> it could be a performance loss.

oh agreed... but marginal conditionals should not be getting
likely()/unlikely() as you are then lying to the compiler about the true
branch predictability...

	Jeff


-- 
Jeff Garzik      | Alternate titles for LOTR:
Building 1024    | Fast Times at Uruk-Hai
MandrakeSoft     | The Took, the Elf, His Daughter and Her Lover
                 | Samwise Gamgee: International Hobbit of Mystery
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