On Wed, 1 Aug 2018, Richard Biener wrote: > > This rewrites the value-numbering algorithm used for FRE and PRE from > SSA SCC based to RPO based, thus switching from an algorithm that > handles SSA SCCs optimistically to one that handles CFG SCCs > optimistically. > > The main motivation for this besides being more optimistic was that > adding CFG context sensitive info is easier in RPO style. Also > tracking availability and thus making expression simplification not > based on values like with SCCVN is possible which allows us to remove > all the warts that scrap side-info we store on SSA names. It also > fixes PR86554 which is another manifestation of the same issue. > > Another motivation was that we're in the need of applying value-numbering > on regions like when unrolling loops or as part of cleanup on code > generated by other passes like the vectorizer. Thus this rewrite > makes sure that the value-numbering works efficiently on regions > (though in a non-iterative mode), avoiding work and space that is > on the order of the function size rather than the region size to work on. > Sofar the GIMPLE unroller makes use of this, scrapping its own > simple constant propagation engine. I expect that DOM could get rid of > its value-numbering and instead use a non-iterative RPO-VN run as well. > > The patch adds something called predication but it just implements > what I put on top of SCCVN to not regress in that area. > > With more optimistic handling comes compile-time regressions and > without limiting I can observe for example a 8% compile-time regression > on 416.gamess which contains loop depths exceeding 8. The patch now > contains heuristics to selectively value-number backedges optimistically > or not and chooses to do so for the innermost 3 and the outermost loop > of a nest (controlled by --param rpo-vn-max-loop-depth). I have not > yet played with other values of the param nor re-measured compile-time > for SPEC 2k6. > > I've bootstrapped and tested the series on x86_64-unknown-linux-gnu > with bootstrap-O1 and regular bootstrap. > > I plan to go forward with this for GCC 9.
This. I'm momentarily installing 1/4, will re-post 4/4 and install that on Monday. Richard.
