Branch/jump target prediction -8

By: David Kanter (dkanter.delete@this.realworldtech.com), August 19, 2016 11:23 pm
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.

The ROB tracks all uops in flight - those in the scheduler, those waiting to issue, those executing in various stages (e.g., perhaps a dozen or more FP ops), those stalled (e.g., L1 misses).

So you can choose from ~90 uops to issue.

David
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