https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98863
--- Comment #40 from rguenther at suse dot de <rguenther at suse dot de> --- On Mon, 8 Feb 2021, rsandifo at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98863 > > --- Comment #39 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot > gnu.org> --- > Just to give an update on this: I have a patch that reduces the > amount of memory consumed by fwprop so that it no longer seems > to be outlier. However, it involves doing more bitmap operations. > In this testcase we have a larger number of registers that > seem to be live but unused across a large region of code, > so bitmap ANDs with the live in sets are expensive and hit > the worst-case O(nblocks×nregisters). I'm still trying to find > a way of reducing the effect of that. But isn't this what the RD problem does as well (yeah, DF shows up as quite compile-time expensive here), and thus all UD/DU chain users suffer from the same issue? What I didn't explore further is re-doing the way RD numbers defs in the bitmaps with the idea that all defs just used inside a single BB are not necessary to be represented (the local problems take care of them). But that of course only helps if there are a significant number of such defs (shadowed by later defs of the same reg in the same BB) - which usually should be the case. There's extra overhead for re-numbering things of course (but my hope was to make the RD problem fit in the cache for a nice speedup...)
