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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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