Branch/jump target prediction

By: sylt (no.delete@this.thanks.com), August 12, 2016 10:35 pm
Room: Moderated Discussions
Peter Cordes (peter.delete@this.cordes.ca) on August 12, 2016 4:23 pm wrote:
>
> A buffer after the early-decode stage is exactly what I was thinking of.
>
> A jump detected this method without involving the BTB leaves that buffer empty. It
> can refill by being able to pre-scan more instructions per clock than the rest of the
> pipeline consumes. (Or on the next pipeline bubble from an execution hazard.)
>

Ideally you would like to have excess fetch bandwidth so that the front-end can scan ahead quickly after a misprediction but depending on other factors (how often do you stall for data dependencies etc) it may not be absolutely required although you will give up some performance. For variable length machines this tends to work out ok by default since the fetch BW is often sized to fetch at least the worst case instruction width and this gives enough excess to get effective scanning for branches.


> I was thinking you'd still want to use the BTB for conditional branches, instead of combining
> this branch target detection with branch direction prediction from the branch history buffer.
> Did you really mean that this would be used for conditional non-indirect branches?
>

Yes, I mean exactly that. Storing only direction prediction can save a lot of area.

The main benefit of the BTB is that it provides low latency resolution of branch targets. The downside is that it may mispredict and that it is often a costly structure.
Getting target addresses by pre-fetching and scanning the stream is less costly in terms of area and 100% accurate for direct branches (assuming your previous predictions are correct). If the branch density that you care about is not higher than your fetch + target decode latency then a BTB provides almost no performance benefit for direct branches (it may actually be a lot worse if you can't afford a large enough BTB).

There are other factors such as power consumption and critical path propagation delay but these are very implementation and architecture/micro-architecture dependent.
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TopicPosted ByDate
Branch/jump target predictionTravis2016/08/09 09:44 AM
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    Early decode of unconditional jumpsExophase2016/08/09 12:29 PM
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      pipelines are too long, noWilco2016/08/10 01:43 AM
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                            Branch/jump target prediction -8David Kanter2016/08/19 11:23 PM
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                        Branch/jump target prediction -8anon2016/08/26 07:14 AM
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