[PATCH] D109760: [GVN] Simple GVN hoist

Wed Sep 15 04:39:15 PDT 2021

chill added a comment.

In D109760#3000216 <https://reviews.llvm.org/D109760#3000216>, @xbolva00 wrote:

> I believe the latest issues with gvnhoist were regressions, what about to take a look at this pass again and possibly enable (some “obviously” profitable) parts of gvnhoist again?
>
> Did you run spec benchmarks with trunk+gvnhoist?

I did and indeed I got even better speedup, but with `-O2 -mllvm -enable-gvnhoist`. With `-O3` there was no improvement (and maybe a regression) (due to unrolling, see thread on the mailing list).

In D109760#3001010 <https://reviews.llvm.org/D109760#3001010>, @junparser wrote:

> As for mcf, I guess it may casued by large register pressure due to hoist some expression from some huge successor.  So simple cost model is necessary.

In D109760#3000184 <https://reviews.llvm.org/D109760#3000184>, @SjoerdMeijer wrote:

> Quick question on the perf numbers. Do you know what the problem is with MCF? Would be good if we can avoid this regression, then the numbers are even better....

No idea why MCF regresses. My plan was to look at MCF regressions and why `-O2 -enable-gvnhoist` gives better numbers than `-O3` + this patch.

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