Re: [linux-ia64] Predication vs. Branching

From: Jonathan Case Nicklin <nicklin_at_missioncriticallinux.com>
Date: 2000-09-20 00:13:38
Dan Pop wrote:

> On Tue, 19 Sep 2000, Jonathan Case Nicklin wrote:
>
> > Dan Pop wrote:
> >
> > > On Tue, 19 Sep 2000, Jonathan Case Nicklin wrote:
> > >
> > > > This might not be the appropriate forum, but what the heck. Does
anyone
> > > > have
> > > > a definitive answer on what happens to predicated instructions
that are
> > > > not
> > > > executed (ie where in the pipeline are they discarded).
> > >
> > > According to the manual, they become NOPs of the appropriate type.

> > >
> > > Dan
> >
> > Dan,
> >     But When? Do they always consume execution resources or are they

> > dropped as soon as their predicate dependency is fulfilled...
assuming their
> >
> > dependent predicate evaluates false and that there are predicate
dependent
> > instructions in the pipeline.
>
> According to the Itanium microarchitecture manual, they do consume
> execution resources, by their mere presence in the bundle.
>
> I.e. the following code still keeps the CPU busy for one cycle:
>
> {.mib
>         nop.m   0
>         nop.i   0
>         nop.b   0
> }
> {.mib
>         nop.m   0
>         nop.i   0
>         nop.b   0
> }
>
> No execution unit can be assigned to a more useful instruction during
> that cycle.
>
> Replacing the NOPs by predicated away instructions of the
> corresponding types shouldn't change anything.
>
> Dan
>
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Dan,

okey-dokey... The predicated instructions that are not executed do
consume
execution resources because they evaluate to NOPs... and NOPs do carry
all
the way through the pipeline until commit (this is what i was curious
about).
Now the question is, at what point does it become more efficient to
branch
rather than predicate? I assume this would dependent on the pipeline
depth
and total Instruction Fetch Bandwidth.

Thanks,
Jonathan Nicklin
Received on Tue Sep 19 09:15:21 2000

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