Branch/jump target prediction -8

By: Maynard Handley (name99.delete@this.name99.org), August 19, 2016 10:34 am
Room: Moderated Discussions
anon (anon.delete@this.ymous.com) on August 19, 2016 8:48 am wrote:
>
> >
> > A fused µop is a single µop in the scheduler, AFAICT - once fused, you have one µop forever.
> >
>
> I don't know, I seem to recall Agner Fog's uarch pdf saying that fused uops are still
> scheduled as two uops even though they stay in the ROB as a single entity (that was
> for Conroe though, it may have changed). Fused macroops are a single uop though.
>
> > 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).
>
> I feel I'm nitpicking here, but uops in the 60-entry scheduler do not necessarily have their arguments ready,
> whereas uops in the ROB may in fact have executed. So I would really say that the OoO window is 60 uops (can't
> select between more than 60 uops). The ROB is just there to absorb latency and allow the OoO window to slide ahead
> while keeping things in-order somewhere. Does it make sense or am I just making up my own terminology?
>
>

The ROB is there primarily to provide
- precise executions
- rollback from incorrect speculation

But the phrase "scheduling window" is somewhat nebulous. There are two SEPARATE numbers
- how large a pool of instructions is available for scheduling execution at any time
- how large a "window of speculation" is possible (which is something like "how far can we execute speculatively beyond an instruction that is blocked --- generally a load from DRAM"
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                            Branch/jump target prediction -8David Kanter2016/08/19 11:23 PM
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                  The (apparent) state of trace caches on modern CPUsExophase2016/08/22 07:55 PM
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                          The (apparent) state of trace caches on modern CPUsMaynard Handley2016/08/24 06:38 AM
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                    The (apparent) state of trace caches on modern CPUsMaynard Handley2016/08/23 06:48 AM
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                      The (wrong) state of trace caches on modern CPUsEric Bron2016/08/25 06:12 AM
                      The (wrong) state of trace caches on modern CPUsMaynard Handley2016/08/25 08:50 AM
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                          The (wrong) state of trace caches on modern CPUsMaynard Handley2016/08/25 11:01 AM
                            The (wrong) state of trace caches on modern CPUsEric Bron2016/08/25 11:20 AM
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