[llvm] [RISCV] Optimize gather/scatter to unit-stride memop + shuffle (PR #66279)

[Craig Topper via llvm-commits]

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@@ -13510,6 +13510,40 @@ static bool legalizeScatterGatherIndexType(SDLoc DL, SDValue &Index,
   return true;
 }
 
+/// Match the index vector of a scatter or gather node as the shuffle mask
+/// which performs the rearrangement if possible.  Will only match if
+/// all lanes are touched, and thus replacing the scatter or gather with
+/// a unit strided access and shuffle is legal.
+static bool matchIndexAsShuffle(EVT VT, SDValue Index, SDValue Mask,
+                                SmallVector<int> &ShuffleMask) {
+  if (!ISD::isConstantSplatVectorAllOnes(Mask.getNode()))
+    return false;
+  if (!ISD::isBuildVectorOfConstantSDNodes(Index.getNode()))
+    return false;
+
+  const unsigned ElementSize = VT.getScalarStoreSize();
+  const unsigned NumElems = VT.getVectorNumElements();
+
+  // Create the shuffle mask and check all bits active
+  assert(ShuffleMask.empty());
+  BitVector ActiveLanes(NumElems);
+  for (const auto Idx : enumerate(Index->ops())) {
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topperc wrote:

Why are we using the ops iterator but then discarding the value() part? Can we just use `for (unsigned i = 0; i < Index->getNumOperands(); ++i)`?

https://github.com/llvm/llvm-project/pull/66279