[llvm] [RISCV] Attempt to widen SEW before generic shuffle lowering (PR #122311)

[Luke Lau via llvm-commits]

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@@ -5261,6 +5261,42 @@ static SDValue lowerDisjointIndicesShuffle(ShuffleVectorSDNode *SVN,
   return DAG.getVectorShuffle(VT, DL, Select, DAG.getUNDEF(VT), NewMask);
 }
 
+/// Try to widen element type to get a new mask value for a better permutation
+/// sequence.  This doesn't try to inspect the widened mask for profitability;
+/// we speculate the widened form is equal or better.  This has the effect of
+/// reducing mask constant sizes - allowing cheaper materialization sequences
+/// - and index sequence sizes - reducing register pressure and materialization
+/// cost, at the cost of (possibly) an extra VTYPE toggle.
+static SDValue tryWidenMaskForShuffle(SDValue Op, SelectionDAG &DAG) {
+  SDLoc DL(Op);
+  EVT VT = Op.getValueType();
+  EVT ScalarVT = VT.getVectorElementType();
+  unsigned ElementSize = ScalarVT.getFixedSizeInBits();
+  SDValue V0 = Op.getOperand(0);
+  SDValue V1 = Op.getOperand(1);
+  ArrayRef<int> Mask = cast<ShuffleVectorSDNode>(Op)->getMask();
+
+  // Avoid wasted work leading to isTypeLegal check failing below
+  if (ElementSize > 32)
+    return SDValue();
+
+  SmallVector<int, 8> NewMask;
+  if (widenShuffleMaskElts(Mask, NewMask)) {
+    MVT NewEltVT = VT.isFloatingPoint()
+                       ? MVT::getFloatingPointVT(ElementSize * 2)
+                       : MVT::getIntegerVT(ElementSize * 2);
+    MVT NewVT = MVT::getVectorVT(NewEltVT, VT.getVectorNumElements() / 2);
+    if (DAG.getTargetLoweringInfo().isTypeLegal(NewVT)) {
+      V0 = DAG.getBitcast(NewVT, V0);
+      V1 = DAG.getBitcast(NewVT, V1);
+      return DAG.getBitcast(VT,
+                            DAG.getVectorShuffle(NewVT, DL, V0, V1, NewMask));
+    }
+  }
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lukel97 wrote:

Nit, structure these checks as early returns?

https://github.com/llvm/llvm-project/pull/122311