[VectorUtils] Add llvm::scaleShuffleMaskElts wrapper for narrowShuffleMaskElts/widenShuffleMaskElts, NFC. (llvm#96646)

Using the target number of vector elements, scaleShuffleMaskElts will try to use narrowShuffleMaskElts/widenShuffleMaskElts to scale the shuffle mask accordingly.

Working on llvm#58895 I didn't want to create yet another case where we have to handle both re-scaling cases.

Author: RKSimon

llvm/include/llvm/Analysis/VectorUtils.h

@@ -224,6 +224,13 @@ void narrowShuffleMaskElts(int Scale, ArrayRef<int> Mask,
 bool widenShuffleMaskElts(int Scale, ArrayRef<int> Mask,
                           SmallVectorImpl<int> &ScaledMask);
 
+/// Attempt to narrow/widen the \p Mask shuffle mask to the \p NumDstElts target
+/// width. Internally this will call narrowShuffleMaskElts/widenShuffleMaskElts.
+/// This will assert unless NumDstElts is a multiple of Mask.size (or vice-versa).
+/// Returns false on failure, and ScaledMask will be in an undefined state.
+bool scaleShuffleMaskElts(unsigned NumDstElts, ArrayRef<int> Mask,
+                          SmallVectorImpl<int> &ScaledMask);
+
 /// Repetitively apply `widenShuffleMaskElts()` for as long as it succeeds,
 /// to get the shuffle mask with widest possible elements.
 void getShuffleMaskWithWidestElts(ArrayRef<int> Mask,

llvm/lib/Analysis/VectorUtils.cpp

@@ -418,6 +418,31 @@ bool llvm::widenShuffleMaskElts(int Scale, ArrayRef<int> Mask,
   return true;
 }
 
+bool llvm::scaleShuffleMaskElts(unsigned NumDstElts, ArrayRef<int> Mask,
+                                SmallVectorImpl<int> &ScaledMask) {
+  unsigned NumSrcElts = Mask.size();
+  assert(NumSrcElts > 0 && NumDstElts > 0 && "Unexpected scaling factor");
+
+  // Fast-path: if no scaling, then it is just a copy.
+  if (NumSrcElts == NumDstElts) {
+    ScaledMask.assign(Mask.begin(), Mask.end());
+    return true;
+  }
+
+  // Ensure we can find a whole scale factor.
+  assert(((NumSrcElts % NumDstElts) == 0 || (NumDstElts % NumSrcElts) == 0) &&
+         "Unexpected scaling factor");
+
+  if (NumSrcElts > NumDstElts) {
+    int Scale = NumSrcElts / NumDstElts;
+    return widenShuffleMaskElts(Scale, Mask, ScaledMask);
+  }
+
+  int Scale = NumDstElts / NumSrcElts;
+  narrowShuffleMaskElts(Scale, Mask, ScaledMask);
+  return true;
+}
+
 void llvm::getShuffleMaskWithWidestElts(ArrayRef<int> Mask,
                                         SmallVectorImpl<int> &ScaledMask) {
   std::array<SmallVector<int, 16>, 2> TmpMasks;

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -2837,15 +2837,7 @@ Instruction *InstCombinerImpl::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
     auto *XType = cast<FixedVectorType>(X->getType());
     unsigned XNumElts = XType->getNumElements();
     SmallVector<int, 16> ScaledMask;
-    if (XNumElts >= VWidth) {
-      assert(XNumElts % VWidth == 0 && "Unexpected vector bitcast");
-      narrowShuffleMaskElts(XNumElts / VWidth, Mask, ScaledMask);
-    } else {
-      assert(VWidth % XNumElts == 0 && "Unexpected vector bitcast");
-      if (!widenShuffleMaskElts(VWidth / XNumElts, Mask, ScaledMask))
-        ScaledMask.clear();
-    }
-    if (!ScaledMask.empty()) {
+    if (scaleShuffleMaskElts(XNumElts, Mask, ScaledMask)) {
       // If the shuffled source vector simplifies, cast that value to this
       // shuffle's type.
       if (auto *V = simplifyShuffleVectorInst(X, UndefValue::get(XType),