Segher Boessenkool <seg...@kernel.crashing.org> writes: > On Thu, Jan 11, 2018 at 03:35:37PM +0000, Wilco Dijkstra wrote: >> Segher Boessenkool wrote: >> >> > Of course I see that ldp is useful. I don't think that this particular >> > way of forcing more pairs is a good idea. Needs testing / benchmarking / >> > instrumentation, and we haven't seen any of that. >> >> I wouldn't propose a patch if it caused slowdowns. In fact I am seeing >> speedups, >> particularly benchmarks which benefit from shrinkwrapping (eg. povray). Int >> is >> flat, and there is an overall gain on fp. >> >> > Forcing pairs before separate shrink-wrapping reduces the effectiveness >> > of the latter by a lot. >> >> That doesn't appear to be the case - or at least any reduction in >> effectiveness is >> more than mitigated by the lower number of memory accesses and I-cache >> misses. >> To do better still you'd need to compute an optimal set of pairs, and >> that is quite >> difficult in the current infrastructure. I could dynamically create >> pairs just in the backend >> but that won't be optimal either. > > Right, I certainly believe forming more pairs before sws (as you do) > improves the code -- but I think forming the pairs only after sws will > work even better. > > But yes that is more work to implement, and the benefit (if any) is > unknown. I hoped I could convince you to try ;-)
Where we do stand with this patch? I think it's legitimately a regression fix; like Wilco, I'm seeing benchmarks in which epilogue restores are no longer paired for important functions. Even if things could be improved elsewhere, the patch seems to make sense on its own and is also nicely localised. Thanks, Richard
