Re: likely/unlikely

[Jeff Garzik]

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
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