[PATCH] D75145: [PassManager] adjust VectorCombine placement

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D75145#1909542 <https://reviews.llvm.org/D75145#1909542>, @lebedev.ri wrote:

> In D75145#1909503 <https://reviews.llvm.org/D75145#1909503>, @dmgreen wrote:
>
> > > just curious since we got regressions downstream after this patch... haven't looked deeper at that
> >
> > Same here. It looks like running cse before instcombine is altering a fair amount, at least in a way that our Low Overhead loop pass does not like. I'm not sure if there are other problems or if it's just that.
> >
> > Looking at it, the way the iteration count is calculated is done differently now. This code:
> >  https://godbolt.org/z/2gBwF2
> >  Has changed the way that the vector preheader calculated the loop iteration values. This is after (top) and before (bottom):
> >  https://godbolt.org/z/tocy_x
> >  Notice the differences in `%n.mod.vf = and i32 %blockSize, 7` vs `%n.vec = and i32 %blockSize, -8`. The SCEV of the BETC for the vector body is then unknown in the new case. I think that's what's causing the low overhead loop pass to go wrong, probably the unrolling too.
> >
> > Any thoughts?
>
>
> I don't know if that helps the problem overall, but i see yet another seemingly-bogus one-use check restriction there.
>  https://godbolt.org/z/z8oyUC
>  I'll see if i can post a patch..


Hm, no, doesn't help //much//  https://godbolt.org/z/G24anE
Though from SCEV side i'd say that was overall helpful, less `<<Unknown>>`s.


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