[llvm] [RISCV] Match single source deinterleave shuffles for vnsrl (PR #114878)

[Philip Reames via llvm-commits]

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@@ -4441,48 +4441,58 @@ static SDValue lowerScalarInsert(SDValue Scalar, SDValue VL, MVT VT,
 }
 
 // Is this a shuffle extracts either the even or odd elements of a vector?
-// That is, specifically, either (a) or (b) below.
-// t34: v8i8 = extract_subvector t11, Constant:i64<0>
-// t33: v8i8 = extract_subvector t11, Constant:i64<8>
-// a) t35: v8i8 = vector_shuffle<0,2,4,6,8,10,12,14> t34, t33
-// b) t35: v8i8 = vector_shuffle<1,3,5,7,9,11,13,15> t34, t33
-// Returns {Src Vector, Even Elements} on success
-static bool isDeinterleaveShuffle(MVT VT, MVT ContainerVT, SDValue V1,
-                                  SDValue V2, ArrayRef<int> Mask,
-                                  const RISCVSubtarget &Subtarget) {
+// That is, specifically, either (a) or (b) in the options below.
+// Single operand shuffle is easy:
+//   a) t35: v8i8 = vector_shuffle<0,2,4,6,u,u,u,u> t34, undef
+//   b) t35: v8i8 = vector_shuffle<1,3,5,7,u,u,u,u> t34, undef
+// Double operand shuffle:
+//   t34: v8i8 = extract_subvector t11, Constant:i64<0>
+//   t33: v8i8 = extract_subvector t11, Constant:i64<8>
+//   a) t35: v8i8 = vector_shuffle<0,2,4,6,8,10,12,14> t34, t33
+//   b) t35: v8i8 = vector_shuffle<1,3,5,7,9,11,13,15> t34, t33
+static SDValue isDeinterleaveShuffle(MVT VT, MVT ContainerVT, SDValue V1,
+                                     SDValue V2, ArrayRef<int> Mask,
+                                     const RISCVSubtarget &Subtarget) {
   // Need to be able to widen the vector.
   if (VT.getScalarSizeInBits() >= Subtarget.getELen())
-    return false;
+    return SDValue();
+
+  // First index must be the first even or odd element from V1.
+  if (Mask[0] != 0 && Mask[0] != 1)
+    return SDValue();
+
+  // The others must increase by 2 each time.
+  for (unsigned i = 1; i != Mask.size(); ++i)
+    if (Mask[i] != -1 && Mask[i] != Mask[0] + (int)i * 2)
+      return SDValue();
+
+  if (1 == count_if(Mask, [](int Idx) { return Idx != -1; }))
+    return SDValue();
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preames wrote:

Slides actually.  If we can use a vslide, I think that is mildly preferable to vnsrl.  Not a big deal either way mind you.

https://github.com/llvm/llvm-project/pull/114878