[llvm] [RISCV] Recognize VLA shift pairs from shuffle masks (PR #127710)

[Craig Topper via llvm-commits]

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@@ -5651,6 +5694,74 @@ static SDValue lowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG,
     return getWideningInterleave(EvenV, OddV, DL, DAG, Subtarget);
   }
 
+  // Recognize a pattern which can handled via a pair of vslideup/vslidedown
+  // instructions (in any combination) with masking on the second instruction.
+  // Avoid matching bit rotates as slide pairs.  This is a performance
+  // heuristic, not a functional check.
+  // TODO: Generalize this slightly to allow single instruction cases, and
+  // prune the logic above which is mostly covered by this already.
+  std::pair<int, int> SrcInfo[2];
+  unsigned RotateAmt;
+  MVT RotateVT;
+  if (isMaskedSlidePair(Mask, SrcInfo) &&
+      !isLegalBitRotate(Mask, VT, Subtarget, RotateVT, RotateAmt)) {
+    SDValue Sources[2];
+    auto getSourceFor = [&](const std::pair<int, int> &Info) {
+      int SrcIdx = Info.first;
+      assert(SrcIdx == 0 || SrcIdx == 1);
+      SDValue &Src = Sources[SrcIdx];
+      if (!Src) {
+        SDValue SrcV = SrcIdx == 0 ? V1 : V2;
+        Src = convertToScalableVector(ContainerVT, SrcV, DAG, Subtarget);
+      }
+      return Src;
+    };
+    auto getSlide = [&](const std::pair<int, int> &Src, SDValue Mask,
+                        SDValue Passthru) {
+      SDValue SrcV = getSourceFor(Src);
+      int SlideAmt = -Src.second;
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topperc wrote:

Should we calculate `Diff` as `(int)i - (M % NumElts)` to avoid the negate here?

https://github.com/llvm/llvm-project/pull/127710