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.
