> > Yes, that is true for OOO execution processors which can rearrange > > insns and execute them speculatively looking through several branches. > > For such processors, software pipelining is more important as the > > processors can look only through a few branches as software pipelining > > could look through any number of branches. That is why Intel compiler > > did not have any insn scheduler (but had software pipelining) until > > Intel Atom introduction which was originally in-order processor. > Correct. Latency scheduling just isn't that important for OOO and instead > you look at scheduling to mitigate costs for large latency operations (ie, > cache miss and transcendental functions). You might also attack secondary > issues like throughput at the retirement stage for example.
Our motivation stems from the fact that even modern, aggressively OOO processors don't have orthogonal resources. Some insns depend on expensive circuitry (area or power wise) that is added only once, making such insns simply scalar, though most of other insns enjoy multiple resources capable of executing them as superscalar. That's why we believe that a hybrid approach might yield good results. We don't have data, for it possibly requires implementing it first. I'd also argue that looking at an OOO pipeline in a steady state is not the only approach. It's also important to consider how quickly the pipeline can be replenished or warmed up to reach a steady state. -- Evandro Menezes Austin, TX
