On Mon, Aug 4, 2014 at 10:17 AM, Eric Anholt <e...@anholt.net> wrote: > Connor Abbott <cwabbo...@gmail.com> writes: > >> On Mon, Aug 4, 2014 at 9:46 AM, Eric Anholt <e...@anholt.net> wrote: >>> Connor Abbott <cwabbo...@gmail.com> writes: >>> >>>> On Fri, Aug 1, 2014 at 12:25 PM, Eric Anholt <e...@anholt.net> wrote: >>>>> Connor Abbott <cwabbo...@gmail.com> writes: >>>>> >>>>>> On Fri, Aug 1, 2014 at 11:51 AM, Eric Anholt <e...@anholt.net> wrote: >>>>>>> Connor Abbott <cwabbo...@gmail.com> writes: >>>>>>> >>>>>>>> On Thu, Jul 31, 2014 at 1:05 PM, Eric Anholt <e...@anholt.net> wrote: >>>>>>>>> Connor Abbott <cwabbo...@gmail.com> writes: >>>>>>>>>> That's not necessarily true - you could want to spill a trivially >>>>>>>>>> colored register that interferes with a non trivially colored >>>>>>>>>> register, especially if the spill cost of the non trivially colored >>>>>>>>>> register is higher than that of the trivially colored register >>>>>>>>>> because >>>>>>>>>> e.g. the non trivially colored register is used in a loop. >>>>>>>>>> Especially, >>>>>>>>>> I ran into trouble with the varying packing tests in piglit which >>>>>>>>>> after a few rounds of spilling looked something like: >>>>>>>>> >>>>>>>>> If it's trivially colorable, then when you're trying to get a >>>>>>>>> non-conflicting color for a difficult neighbor (an optimistic-coloring >>>>>>>>> one near the top of the stack that's triggering the need for >>>>>>>>> spilling), >>>>>>>>> it's out consideration because it's deeper in the stack. >>>>>>>> >>>>>>>> Right... so in that case, can't we just ignore everything on the stack >>>>>>>> below the node that couldn't be colored in ra_select()? >>>>>>> >>>>>>> Yep, that's what the code's doing currently. >>>>>> >>>>>> No, it's also considering other optimistically colored nodes below the >>>>>> one that failed on the stack... see patch 3 in v3 of my series, which >>>>>> changes the code to actually do that. >>>>> >>>>> Oh, I misread you as saying s/couldn't be colored/first couldn't be >>>>> trivially colored/. It seems like obviously the behavior we want, to >>>>> me. >>>> >>>> Cool, then can you put your r-b on the last two patches of the v3 >>>> series so we can push it? >>> >>> Not really -- I'm saying that we *do* want to consider the other >>> optimistically colored nodes. Those ones lower in the stack might >>> actually be useful to spill, since if spilled they could potentially >>> make other values trivially colorable, which would change the order of >>> pushes and might make our node colorable. >>> >> >> Well, you could apply that same argument to *any* spillable node in >> the graph, which brings us right back to where we started. I think >> what we have now is a rather arbitrary compromise between "consider >> everything for spilling because it might make other things trivially >> colorable" and "only spill the things we know are likely to help." I >> still think we should go with the former, since it produces numbers >> equivalent to what we have right now instead of slightly worse >> numbers. And besides, as I noted in the commit message, considering >> those nodes doesn't seem to be useful in practice since we get the >> exact same code anyways - even with reducing the number of registers >> to make more things spill. > > I don't believe that's true. The argument before was about > trivially-colorable things -- things that always appear in the stack > before the optimistic things. They can't have an effect on the things > higher in the stack, becuase they're already in the stack and you know > you can color them regardless of choices higher in the stack, so you > don't have to spill them ever. > > For your optimistic nodes, though, even though you've put them in the > stack, you want to consider them for spilling because even once you > resolve the spills and manage to color things high in the stack (in your > scheme), you may have to spill that lower-down optimistic node anyway. > So you failed to consider an important node for spilling, that might > have also resolved your higher-in-the-stack troubles, and got extra > spilling as a result.
How, exactly, could spilling in this case ever resolve the "higher-in-the-stack troubles"? Say we have an optimistically colored node A, that is lower on the stack than the optimistically colored node B that is currently failing to be colored. There are two effects of spilling A: 1) Some of the nodes that we split A into may become trivially colorable, which will push them down on the stack - but this is all happening below B on the stack anyways, so it doesn't make any difference for now. 2) Some of the nodes that were interfering with A may no longer interfere with any of the nodes we split A into - but again, we only care about B or nodes above B on the stack, so we don't count that interference anyways, and therefore we still haven't gained anything. So although spilling A may help in the future, I still fail to see how it could help now - we still have to spill nodes above B (or B itself) before we can make progress. _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
