[llvm] [RISCV] Match deinterleave(4,8) shuffles to SHL/TRUNC when legal (PR #118509)

[Craig Topper via llvm-commits]

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@@ -5332,11 +5301,31 @@ static SDValue lowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG,
   if (ShuffleVectorInst::isReverseMask(Mask, NumElts) && V2.isUndef())
     return DAG.getNode(ISD::VECTOR_REVERSE, DL, VT, V1);
 
-  // If this is a deinterleave and we can widen the vector, then we can use
-  // vnsrl to deinterleave.
-  if (SDValue Src =
-          isDeinterleaveShuffle(VT, ContainerVT, V1, V2, Mask, Subtarget))
-    return getDeinterleaveViaVNSRL(DL, VT, Src, Mask[0] == 0, DAG);
+  // If this is a deinterleave(2,4,8) and we can widen the vector, then we can
+  // use shift and truncate to perform the shuffle.
+  // TODO: For Factor=6, we can perform the first step of the deinterleave via
+  // shift-and-trunc reducing total cost for everything except an mf8 result.
+  // TODO: For Factor=4,8, we can do the same when the ratio isn't high enough
+  // to do the entire operation.
+  if (VT.getScalarSizeInBits() < Subtarget.getELen()) {
+    const unsigned MaxFactor = Subtarget.getELen() / VT.getScalarSizeInBits();
+    assert(MaxFactor == 2 || MaxFactor == 4 || MaxFactor == 8);
+    for (unsigned Factor = 2; Factor <= MaxFactor; Factor <<= 1) {
+      unsigned Index = 0;
+      if (ShuffleVectorInst::isDeInterleaveMaskOfFactor(Mask, Factor, Index) &&
+          1 < count_if(Mask, [](int Idx) { return Idx != -1; })) {
+        if (SDValue Src = getSingleShuffleSrc(VT, ContainerVT, V1, V2)) {
+          if (Src.getValueType() == VT) {
+            EVT WideVT = VT.getDoubleNumVectorElementsVT();
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topperc wrote:

Not directly related to this patch, but I think for large minimum VLEN this might create an illegal type. There's an attempt to protect this in `getSingleShuffleSrc` by checking `RISCVTargetLowering::getLMUL(ContainerVT) != RISCVII::VLMUL::LMUL_8`. For sufficiently large minimum VLEN, LMUL==4 can be the largest legal type and LMUL=8 would be an illegal type because we don't have a large enough MVT to represent it.

https://github.com/llvm/llvm-project/pull/118509