[llvm] [InstCombine] Do not perform binop-of-shuffle when mask is poison (PR #82185)

[Nikita Popov via llvm-commits]

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@@ -1870,14 +1870,16 @@ Value *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V,
         Value *ShufOp = MatchShufAsOp0 ? X : Y;
         Value *OtherOp = MatchShufAsOp0 ? Y : X;
         for (User *U : OtherOp->users()) {
-          auto Shuf = m_Shuffle(m_Specific(ShufOp), m_Value(), m_ZeroMask());
+          ArrayRef<int> Mask;
+          auto Shuf = m_Shuffle(m_Specific(ShufOp), m_Value(), m_Mask(Mask));
           if (BO->isCommutative()
                   ? match(U, m_c_BinOp(Opcode, Shuf, m_Specific(OtherOp)))
                   : MatchShufAsOp0
                         ? match(U, m_BinOp(Opcode, Shuf, m_Specific(OtherOp)))
                         : match(U, m_BinOp(Opcode, m_Specific(OtherOp), Shuf)))
-            if (DT.dominates(U, I))
-              return U;
+            if (match(Mask, m_ZeroMask()) && Mask[0] != PoisonMaskElem)
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nikic wrote:

There is a `DemandedElts == 1` check above, which guarantees only lane 0 is demanded.

https://github.com/llvm/llvm-project/pull/82185