Paul A. Clayton (paaronclayton.delete@this.gmail.com) on February 17, 2013 5:28 pm wrote:
> I find multiple condition registers attractive partially because such can be less expensive front-end
> registers (future file for an OoO implementation) allowing early branch resolution. In theory,
> branches whose conditions are set early could be resolved just after branch prediction (the predictor
> could indicate that the branch is a candidate for early resolution and what condition register+condition
> is tested), but even just after decode branch resolution could have some benefits.

I agree with you on this. On one hand, you might say "branch prediction is very good", but on the other hand, you have fundamentally unpredictable branches (due to icache or branch prediction capacity limits, and due to branches without trivial patterns).

Prediction is one way to assist with the latter, but early resolution could help with both. Branch mispredicts seem to be quite a fundamental limiter on OOOE and pipeline depth, and of course even before you hit performance limits, you are already paying an efficiency cost.

On the other hand, I suspect quite a lot of branches will be unsuitable. And shuffling code around so that "something" gets done in between the comparison and the branch has the obvious flaw that it's effectively just putting the same kind of speculative work into the bubble.

Are you aware of any papers that study this in isolation?