[Openmcl-devel] Quick HW question...

Daniel Weinreb dlw at itasoftware.com
Wed Nov 17 19:29:59 UTC 2010



Gary Byers wrote:
>
>
> Exactly what needs to be done to help the processor predict whether a 
> branch
> is taken tends to be architecture- and implementation-dependent.  
> (It's often
> the case that simple rules - like "backward conditional branches are 
> predicted
> to be taken and forward conditional branches are predicted to not be 
> taken" -
> go a long way.)
Do current architecures do dynamic branch predication, in which
they keep a bit at runtime to keep track of which way the
branch went last time?  I know that such things have,
at the very least, been suggested, although I had no idea
to what extent they have been put into use, and whether
measurements show that they are enough better than
static predication to justify their cost.
>
> Suppose that there were semi-magic macros (I -think- that they could 
> be macros)
> called something like EXPECTED-TRUE and EXPECTED-FALSE 
Yeah, I always imgined "jump-usually" "if-usually" too.




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