[llvm] [WIP][VectorCombine] Fold "shuffle (binop (shuffle, shuffle)), undef" --> "binop (shuffle), (shuffle)" (PR #114101)

[Simon Pilgrim via llvm-commits]

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@@ -1400,6 +1401,92 @@ bool VectorCombine::scalarizeLoadExtract(Instruction &I) {
   return true;
 }
 
+/// Try to fold "shuffle (binop (shuffle, shuffle)), undef" into "binop (shuffle), (shuffle)".
+bool VectorCombine::foldPermuteOfBinops(Instruction &I) {
+  BinaryOperator *BinOp;
+  ArrayRef<int> OuterMask;
+  if (!match(&I,
+             m_Shuffle(m_OneUse(m_BinOp(BinOp)), m_Undef(), m_Mask(OuterMask))))
+    return false;
+
+  // Don't introduce poison into div/rem.
+  if (llvm::is_contained(OuterMask, PoisonMaskElem) && BinOp->isIntDivRem())
+    return false;
+
+  Value *Op00, *Op01;
+  ArrayRef<int> Mask0;
+  if (!match(BinOp->getOperand(0),
+             m_OneUse(m_Shuffle(m_Value(Op00), m_Value(Op01), m_Mask(Mask0)))))
+    return false;
+
+  Value *Op10, *Op11;
+  ArrayRef<int> Mask1;
+  if (!match(BinOp->getOperand(1),
+             m_OneUse(m_Shuffle(m_Value(Op10), m_Value(Op11), m_Mask(Mask1)))))
+    return false;
+
+  Instruction::BinaryOps Opcode = BinOp->getOpcode();
+  auto *ShuffleDstTy = dyn_cast<FixedVectorType>(I.getType());
+  auto *BinOpTy = dyn_cast<FixedVectorType>(BinOp->getType());
+  auto *Op0Ty = dyn_cast<FixedVectorType>(Op00->getType());
+  auto *Op1Ty = dyn_cast<FixedVectorType>(Op10->getType());
+  if (!ShuffleDstTy || !BinOpTy || !Op0Ty || !Op1Ty)
+    return false;
+
+  unsigned NumSrcElts = BinOpTy->getNumElements();
+
+  // Don't accept shuffles that reference the second (undef/poison) operand.
+  if (any_of(OuterMask, [NumSrcElts](int M) { return M >= (int)NumSrcElts; }))
----------------
RKSimon wrote:

Yes, we can enable this for non-intdiv cases

https://github.com/llvm/llvm-project/pull/114101