[InstCombine] Refactor fixed and scalable binop shuffle combine. NFCI

This extracts the logic that works out the "unshuffled" constant when pulling shuffle vectors out of binary ops, so the same combine can be generic over fixed and scalable vectors.

The plan is to reuse this helper to do the same canonicalization on intrinsics too.

Author: lukel97

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2094,6 +2094,50 @@ static bool shouldMergeGEPs(GEPOperator &GEP, GEPOperator &Src) {
   return true;
 }
 
+// Find constant NewC that has property:
+//   shuffle(NewC, ShMask) = C
+// Returns nullptr if such a constant does not exist e.g. ShMask=<0,0> C=<1,2>
+//
+// A 1-to-1 mapping is not required. Example:
+// ShMask = <1,1,2,2> and C = <5,5,6,6> --> NewC = <poison,5,6,poison>
+static Constant *unshuffleConstant(ArrayRef<int> ShMask, Constant *C,
+                                   VectorType *NewCTy) {
+  if (isa<ScalableVectorType>(NewCTy)) {
+    Constant *Splat = C->getSplatValue();
+    if (!Splat)
+      return nullptr;
+    return ConstantVector::getSplat(
+        cast<VectorType>(C->getType())->getElementCount(), Splat);
+  }
+
+  if (cast<FixedVectorType>(NewCTy)->getNumElements() >
+      cast<FixedVectorType>(C->getType())->getNumElements())
+    return nullptr;
+
+  unsigned NewCNumElts = cast<FixedVectorType>(NewCTy)->getNumElements();
+  PoisonValue *PoisonScalar = PoisonValue::get(C->getType()->getScalarType());
+  SmallVector<Constant *, 16> NewVecC(NewCNumElts, PoisonScalar);
+  unsigned NumElts = cast<FixedVectorType>(C->getType())->getNumElements();
+  for (unsigned I = 0; I < NumElts; ++I) {
+    Constant *CElt = C->getAggregateElement(I);
+    if (ShMask[I] >= 0) {
+      assert(ShMask[I] < (int)NumElts && "Not expecting narrowing shuffle");
+      Constant *NewCElt = NewVecC[ShMask[I]];
+      // Bail out if:
+      // 1. The constant vector contains a constant expression.
+      // 2. The shuffle needs an element of the constant vector that can't
+      //    be mapped to a new constant vector.
+      // 3. This is a widening shuffle that copies elements of V1 into the
+      //    extended elements (extending with poison is allowed).
+      if (!CElt || (!isa<PoisonValue>(NewCElt) && NewCElt != CElt) ||
+          I >= NewCNumElts)
+        return nullptr;
+      NewVecC[ShMask[I]] = CElt;
+    }
+  }
+  return ConstantVector::get(NewVecC);
+}
+
 Instruction *InstCombinerImpl::foldVectorBinop(BinaryOperator &Inst) {
   if (!isa<VectorType>(Inst.getType()))
     return nullptr;
@@ -2213,50 +2257,15 @@ Instruction *InstCombinerImpl::foldVectorBinop(BinaryOperator &Inst) {
   // other binops, so they can be folded. It may also enable demanded elements
   // transforms.
   Constant *C;
-  auto *InstVTy = dyn_cast<FixedVectorType>(Inst.getType());
-  if (InstVTy &&
-      match(&Inst, m_c_BinOp(m_OneUse(m_Shuffle(m_Value(V1), m_Poison(),
+  if (match(&Inst, m_c_BinOp(m_OneUse(m_Shuffle(m_Value(V1), m_Poison(),
                                                 m_Mask(Mask))),
-                             m_ImmConstant(C))) &&
-      cast<FixedVectorType>(V1->getType())->getNumElements() <=
-          InstVTy->getNumElements()) {
-    assert(InstVTy->getScalarType() == V1->getType()->getScalarType() &&
+                             m_ImmConstant(C)))) {
+    assert(Inst.getType()->getScalarType() == V1->getType()->getScalarType() &&
            "Shuffle should not change scalar type");
 
-    // Find constant NewC that has property:
-    //   shuffle(NewC, ShMask) = C
-    // If such constant does not exist (example: ShMask=<0,0> and C=<1,2>)
-    // reorder is not possible. A 1-to-1 mapping is not required. Example:
-    // ShMask = <1,1,2,2> and C = <5,5,6,6> --> NewC = <undef,5,6,undef>
     bool ConstOp1 = isa<Constant>(RHS);
-    ArrayRef<int> ShMask = Mask;
-    unsigned SrcVecNumElts =
-        cast<FixedVectorType>(V1->getType())->getNumElements();
-    PoisonValue *PoisonScalar = PoisonValue::get(C->getType()->getScalarType());
-    SmallVector<Constant *, 16> NewVecC(SrcVecNumElts, PoisonScalar);
-    bool MayChange = true;
-    unsigned NumElts = InstVTy->getNumElements();
-    for (unsigned I = 0; I < NumElts; ++I) {
-      Constant *CElt = C->getAggregateElement(I);
-      if (ShMask[I] >= 0) {
-        assert(ShMask[I] < (int)NumElts && "Not expecting narrowing shuffle");
-        Constant *NewCElt = NewVecC[ShMask[I]];
-        // Bail out if:
-        // 1. The constant vector contains a constant expression.
-        // 2. The shuffle needs an element of the constant vector that can't
-        //    be mapped to a new constant vector.
-        // 3. This is a widening shuffle that copies elements of V1 into the
-        //    extended elements (extending with poison is allowed).
-        if (!CElt || (!isa<PoisonValue>(NewCElt) && NewCElt != CElt) ||
-            I >= SrcVecNumElts) {
-          MayChange = false;
-          break;
-        }
-        NewVecC[ShMask[I]] = CElt;
-      }
-    }
-    if (MayChange) {
-      Constant *NewC = ConstantVector::get(NewVecC);
+    if (Constant *NewC =
+            unshuffleConstant(Mask, C, cast<VectorType>(V1->getType()))) {
       // It may not be safe to execute a binop on a vector with poison elements
       // because the entire instruction can be folded to undef or create poison
       // that did not exist in the original code.
@@ -2272,27 +2281,6 @@ Instruction *InstCombinerImpl::foldVectorBinop(BinaryOperator &Inst) {
     }
   }
 
-  // Similar to the combine above, but handles the case for scalable vectors
-  // where both shuffle(V1, 0) and C are splats.
-  //
-  // Op(shuffle(V1, 0), (splat C)) -> shuffle(Op(V1, (splat C)), 0)
-  if (isa<ScalableVectorType>(Inst.getType()) &&
-      match(&Inst, m_c_BinOp(m_OneUse(m_Shuffle(m_Value(V1), m_Poison(),
-                                                m_ZeroMask())),
-                             m_ImmConstant(C)))) {
-    if (Constant *Splat = C->getSplatValue()) {
-      bool ConstOp1 = isa<Constant>(RHS);
-      VectorType *V1Ty = cast<VectorType>(V1->getType());
-      Constant *NewC = ConstantVector::getSplat(V1Ty->getElementCount(), Splat);
-
-      Value *NewLHS = ConstOp1 ? V1 : NewC;
-      Value *NewRHS = ConstOp1 ? NewC : V1;
-      VectorType *VTy = cast<VectorType>(Inst.getType());
-      SmallVector<int> Mask(VTy->getElementCount().getKnownMinValue(), 0);
-      return createBinOpShuffle(NewLHS, NewRHS, Mask);
-    }
-  }
-
   // Try to reassociate to sink a splat shuffle after a binary operation.
   if (Inst.isAssociative() && Inst.isCommutative()) {
     // Canonicalize shuffle operand as LHS.