[PATCH] D126532: [SVE] Add a DAG combiner fold to visitADD for vscale with truncate

[Philip Reames via Phabricator via llvm-commits]

reames added a comment.

I don't claim to fully understand this, so my comment here might be off base.

I suspect your fold can be generalized as: fold a+truncate(vscale(c1))+truncate(vscale(c2)) to a+truncate(vscale(c1)+vscale(c2))

The vscale(c1)+vscale(c2) to vscale(C1 <https://reviews.llvm.org/C1> + C2) is handled separately above already.

If this is true, that your transform reduces to proving that it's legal to common the truncate.  However, as a far as I known trunc(x) + trunc(y) is always equal to trunc(x+y).  So why do we need this transform at all?  Shouldn't this be covered by generic trunc folds and the existing rule?

Anyways, I'm clearly missing something here.  Any idea what?


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  https://reviews.llvm.org/D126532/new/

https://reviews.llvm.org/D126532