On Wed, Jan 18, 2017 at 4:32 PM, Bin.Cheng <amker.ch...@gmail.com> wrote: > On Wed, Jan 18, 2017 at 2:54 PM, Richard Biener > <richard.guent...@gmail.com> wrote: >> On Wed, Jan 18, 2017 at 11:10 AM, Martin Liška <mli...@suse.cz> wrote: >>> Hello. >>> >>> >>> Patch can bootstrap on ppc64le-redhat-linux and survives regression tests. >>> >>> Ready to be installed? >> >> I'm not sure. If we have such zero distance refs in the IL at the >> time pcom runs then not handling >> them will pessimize code-gen for cases where they are part of a larger >> chain. Esp. I don't like > Do you mean different chains distributed because of MAX_DISTANCE by > "larger chain"? With the patch, such chain of refs would still be > pred-commoned, just the arithmetic operation not combined, which could > be handled by later DOM? >> another stmt_dominates_stmt_p call and thus rather not handle length >> == 0 at all... > Not handle length == 0 chains at all may be sub-optimal. As you said, > such chain of refs at the point may simply because previous dom/cse > fail to analyze the references. >> >> We already seem to go great length in associating stuff when combining >> stuff thus isn't this >> maybe an artifact of this association? Maybe we simply need to sort >> the new chain after >> combining it so the root stmt comes last? >> >> Note that there seems to be only a single length per chain but not all >> refs in a chain need to >> have the same distance. This means your fix is likely incomplete? >> What prevents the situation >> to arise for distance != 0? > Yes, it's possible for two refs have the same distance in a chain with > length > 0. But that should not be a problem, because existing uses > are replaced by newly generated PHI variables which always dominate > the uses, right?
I must admit I don't know predcom in such detail but then can we handle distance == 0 by simply inserting a PHI for those as well (a degenerate one of course)? Or can for distance == 0 the ref be not loop invariant? Note that for length == 0 all refs in the chain will have a dependence distance of zero. So my first argument probably doesn't hold and we could simply remove handling of length == 0 chains and rely on CSE? Richard. > Thanks, > bin >> >> Richard. >> >>> Martin >>>
