Branch/jump target prediction -8

By: Simon Farnsworth (simon.delete@this.farnz.org.uk), August 19, 2016 7:34 am
Room: Moderated Discussions
anon (anon.delete@this.ymous.com) on August 19, 2016 7:29 am wrote:
>
> > http://www.realworldtech.com/haswell-cpu/3/ says that Haswell's
> > OOO scheduling window is around 300 operations,
> > based on a ROB of 192 µops, but Haswell being able to fuse µops before they enter the OOO engine.
> >
> > The exact window depends on your instruction mix, but hundreds of instructions
> > is the scale you're looking at on a modern desktop or server processor.
> >
> > I'm not entirely clear on what code constructs to avoid with OOO, beyond the obvious of keeping
> > your dependency chains under control - OOO can't help you if, within the OOO window, it can't find
> > enough µops to execute. OTOH, compilers are now pretty good at helping you out here, so...
>
> Haswell's scheduler can contain 60 uops (I'm not clear whether fused uops are still a single uop in the
> scheduler, but I think fused macroops are), so you can pick uops from a pool of 60, not 200-300.
> Skylake is around 90 uops.
>
> It's still a large number but there is a big difference imho.
>

A fused µop is a single µop in the scheduler, AFAICT - once fused, you have one µop forever.

There's 60 µops waiting for a free execution unit (in the scheduler), and 192 µops waiting in the ROB. Thus, depending on how you wish to count it, you can consider the OOO window as 192 µops (the number of µops that have been decoded ready to execute, regardless of whether they're waiting on arguments or execution units), or 60 µops (the number of µops waiting on execution units).
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