[PATCH] D79003: [DAG] Add SimplifyDemandedVectorElts binop SimplifyMultipleUseDemandedBits handling

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp:2646-2649
     KnownUndef = getKnownUndefForVectorBinop(Op, TLO.DAG, UndefLHS, UndefRHS);
+
+    // Attempt to avoid multi-use ops if we don't need anything from them.
+    if (!DemandedElts.isAllOnesValue()) {
----------------
RKSimon wrote:
> lebedev.ri wrote:
> > Just wondering, should we not demand elts that are `KnownUndef`?
> That's a good idea - decent tests might be tricky but should be doable. A slight thing to watch out for is KnownUndef/KnownZero bits are only guaranteed to be correct for demanded elts.
> 
> Would it be OK just to add this as a TODO comment for now? It might need some investigation (and improvements to KnownUndef/KnownZero emission which isn't great yet as we don't use it enough).
> That's a good idea - decent tests might be tricky but should be doable.

Aha. That //might// even be more generic than this if-block.

> Would it be OK just to add this as a TODO comment for now?

Yes, absolutely.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D79003/new/

https://reviews.llvm.org/D79003