Vineet Gupta <vine...@rivosinc.com> writes: > On 10/29/24 11:51, Wilco Dijkstra wrote: >> Hi Vineet, >>> I agree the NARROW/WIDE stuff is obfuscating things in technicalities. >> Is there evidence this change would make things significantly worse for >> some targets? > > Honestly I don't think this needs to be behind any toggle or made optional at > all. The old algorithm was overly eager in spilling. But per last > discussion with Richard [1] at least back in 2012 for some in-order arm32 > core this was better. And also that's where the wide vs. narrow discussions > came up and that it really mattered, as far as I understood.
Right, that's the key. The current algorithm was tuned on an in-order core for which GCC already had a relatively accurate pipeline model. The question is whether this is better on a core like that: that is, on an in-order core for which GCC has a relatively accurate pipeline model. No amount of benchmarking on out-of-order cores will answer that. Somewhat surprisingly, we don't AFAIK have a target hook for "is the current target out-of-order?". Why not make the target hook that instead? I think everyone agrees (including me in the previous thread) that the current behaviour isn't right for OoO cores. If someone has an OoO core that for some reason prefers the current approach (unlikely), we can decide what to do then. But in the meantime, keying of OoO-ness seems simpler and easier to document. Thanks, Richard
