[llvm] [VectorCombine] Handle shuffle of selects (PR #128032)

[Simon Pilgrim via llvm-commits]

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@@ -1899,6 +1900,74 @@ bool VectorCombine::foldShuffleOfBinops(Instruction &I) {
   return true;
 }
 
+/// Try to convert,
+/// (shuffle(select(c1,t1,f1)), (select(c2,t2,f2)), m) into
+/// (select (shuffle c1,c2,m), (shuffle t1,t2,m), (shuffle f1,f2,m))
+bool VectorCombine::foldShuffleOfSelects(Instruction &I) {
+  ArrayRef<int> Mask;
+  Value *C1, *T1, *F1, *C2, *T2, *F2;
+  if (!match(&I, m_Shuffle(
+                     m_OneUse(m_Select(m_Value(C1), m_Value(T1), m_Value(F1))),
+                     m_OneUse(m_Select(m_Value(C2), m_Value(T2), m_Value(F2))),
+                     m_Mask(Mask))))
+    return false;
+
+  auto *DstVecTy = dyn_cast<FixedVectorType>(I.getType());
+  auto *C1VecTy = dyn_cast<FixedVectorType>(C1->getType());
+  auto *C2VecTy = dyn_cast<FixedVectorType>(C2->getType());
+  if (!C1VecTy || !C2VecTy)
+    return false;
+
+  // SelectInsts must have the same FMF.
+  auto *Select0 = cast<Instruction>(I.getOperand(0));
+  if (auto *SI0FOp = dyn_cast<FPMathOperator>(Select0))
+    if (auto *SI1FOp = dyn_cast<FPMathOperator>((I.getOperand(1))))
+      if (SI0FOp->getFastMathFlags() != SI1FOp->getFastMathFlags())
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RKSimon wrote:

Should we assert that both/neither of the selects are FPMathOperator ?

https://github.com/llvm/llvm-project/pull/128032