Wilco Dijkstra <wilco.dijks...@arm.com> writes: > Hi, > >>>> What do you think about disabling late scheduling as well? >>> >>> I think this would definitely need separate consideration and evaluation >>> given the above. >>> >>> Another thing to consider is the macro fusion machinery. IIRC it works >>> during scheduling so if we don’t run any scheduling we don’t get an >>> opportunity to bring those instructions together? >>> >>> That said, I’m not sure the scheduling actually tries to bring macro fused >>> instructions together rather than simply avoiding moving them apart. >> >> I will run the numbers, but if useful, late scheduling could be disabled >> separately from fusion >> scheduling. However fusion really shouldn't be done as a scheduling hack - >> we should use >> fused RTL patterns like we do for GOT accesses and AES fusion. > > I ran the numbers, late scheduling (using the ancient Cortex-A57 model) gives > 0.23% speedup on SPECINT and 0.73% on SPECFP. I guess the gains come from > scheduling loads and other critical operations earlier and perhaps improved > dispatch > due to increased interleaving of operations.
Thanks. So yeah, I agree we should keep it. I think it was common ground that the benefits are kind-of incidental (in that the scheduler really is trying to fill Cortex-A57 pipeline bubbles rather than find a good frontend mix for modern cores) and that it would be better to try to target the effect that we want directly. But in terms of whether the switch we have should be on or off, I agree it should be on. The patch is OK if there are no objections by Thursday morning UTC. Richard
