pipelines are too long, no

By: Paul A. Clayton (paaronclayton.delete@this.gmail.com), August 10, 2016 7:44 pm
Room: Moderated Discussions
Wilco (Wilco.Dijkstra.delete@this.ntlworld.com) on August 10, 2016 2:43 am wrote:
> no name (no.delete@this.name.com) on August 9, 2016 7:17 pm wrote:

[snip]
> > Even the RISC-V manual said something this in their ISA manual
> > "another advantage of a fused compare-and-branch
> > instruction is that branches are observed earlier in the front-end instruction stream, and so can be
> > predicted earlier."
>
> That's quite incorrect (and why would you mention such implementation details in an ISA manual???).

Especially for an academic ISA, mentioning the rationales of design choices is appropriate. (I tend to agree that the statement is at least poorly phrased.)

> It is an advantage of delayed branches. Compare&branch actually makes branch prediction harder
> as basic blocks are now smaller and contain fewer non-branch instructions.
>
> > Sure, by not making it 2 instructions it's marginally ahead in the instruction
> > stream, but 90% of the time it will be within the same fetch block and make no difference to the predictor.
> > The BTB will be consulted regardless of the presence of a branch within the block.
>
> Indeed. But the BTB does have to work harder as there are more branches per fetch.

Generally one only needs to predict the address of the next fetch not the addresses of all possible branches. While using global history for prediction makes this a more difficult problem (because the next address would not be dependent only on the previous instruction fetch address), dense branches probably tend to be short so one might allow paired entries (non-contiguous fetch would make this problematic but such is rarely used) though even widening the BTB read is not that problematic (the size/latency tradeoff is an issue).

(Even with only a single entry and global history prediction, the actual penalty may be small for delayed address resolution when the branch direction predictor predicts that a different branch's target is used. Such target mispredictions are probably relatively rare and may also be associated with lower branch prediction accuracy.)
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