[RISCV] Provide a isOneSourceVECTOR_SHUFFLE function. NFC.

[HanKuanChen]

Summary

This function is used to reduce multiple check about whether a
vector_shuffle is from a same and contiguous source.

[craig.topper]

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

2734

I kind of think a function starting with is should return a bool. Maybe getSingleShuffleSource?

[craig.topper]

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

2768

Should isVnsrlShuffle return the SDValue returned from isOneSourceVECTOR_SHUFFLE? Looks like the caller makes an assumption about how to find the source.

[HanKuanChen]

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

2768

Should isVnsrlShuffle return the SDValue returned from isOneSourceVECTOR_SHUFFLE? Looks like the caller makes an assumption about how to find the source.

Does that mean isVnsrlShuffle should be renamed to something like getVnsrlShuffle?

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

static SDValue lowerScalarSplat(SDValue Passthru, SDValue Scalar, SDValue VL,
   if (isOneConstant(VL) && isNullConstant(Scalar))
     return DAG.getNode(RISCVISD::VMV_S_X_VL, DL, VT, Passthru,
                        DAG.getConstant(0, DL, XLenVT), VL);
 
   // Otherwise use the more complicated splatting algorithm.
   return splatSplitI64WithVL(DL, VT, Passthru, Scalar, VL, DAG);
 }
 
-// Mask can only be this form.
-// X X X X ... -1 -1 -1 ..., while X is not -1.
-// X X X X ... must be in ascending order.
-// for example,
-// [0,  2,  4,  6] -> vnsrl src, 0
-// [0,  4,  8, 12] -> vnsrl (vnsrl src, 0), 0
-// [3,  7, 11, 15] -> vnsrl (vnsrl src, EltSize * 2), EltSize
-// [2, 10, 18, 26] -> vnsrl (vnsrl (vnsrl src, 0), EltSize * 2), 0
-// In addition, N0 and N1 must from a same vector (or N1 is undef).
-static bool isVnsrlShuffle(SDValue N0, SDValue N1, ArrayRef<int> Mask, EVT VT,
-                           const RISCVSubtarget &Subtarget) {
-  if (!N1.isUndef()) {
-    // Both input must be extracts.
-    if (N0.getOpcode() != ISD::EXTRACT_SUBVECTOR ||
-        N1.getOpcode() != ISD::EXTRACT_SUBVECTOR)
-      return false;
+// If a vector_shuffle source is from a same and contiguous (and started with 0
+// index) vector, return the vector. Otherwise, return SDValue().
+static SDValue isOneSourceVECTOR_SHUFFLE(SDValue N0, SDValue N1,
+                                         ArrayRef<int> Mask) {
+  if (N1.isUndef())
+    return N0;
+
+    return SDValue();
 
-    // Extracting from the same source.
-    SDValue Src = N0.getOperand(0);
-    if (Src != N1.getOperand(0))
-      return false;
+    return SDValue();
 
-    // Make sure N0 and N1 are continuous.
-    if (N0.getConstantOperandVal(1) != 0 ||
-        N1.getConstantOperandVal(1) != Mask.size())
-      return false;
-  }
+    return SDValue();
+
+  return Src;
+
+// Mask can only be this form.
+// X X X X ... -1 -1 -1 ..., while X is not -1.
+// X X X X ... must be in ascending order.
+// for example,
+// [0,  2,  4,  6] -> vnsrl src, 0
+// [0,  4,  8, 12] -> vnsrl (vnsrl src, 0), 0
+// [3,  7, 11, 15] -> vnsrl (vnsrl src, EltSize * 2), EltSize
+// [2, 10, 18, 26] -> vnsrl (vnsrl (vnsrl src, 0), EltSize * 2), 0
+// In addition, N0 and N1 must from a same vector (or N1 is undef).
+static bool isVnsrlShuffle(SDValue N0, SDValue N1, ArrayRef<int> Mask, EVT VT,
+                           const RISCVSubtarget &Subtarget) {
+  if (!isOneSourceVECTOR_SHUFFLE(N0, N1, Mask))
+    return false;
   if (Mask.size() < 2)
     return false;
   // Find first -1 and check whether the mask behind is -1.
   auto FirstUndef = find(Mask, -1);
   if (std::any_of(FirstUndef, Mask.end(),
                   [](int MaskIdx) { return MaskIdx != -1; }))
     return false;
   ptrdiff_t ValidMaskEnd = std::distance(Mask.begin(), FirstUndef);

 // t49: v8i8 = extract_subvector t13, Constant:i64<0>
 // t109: v8i8 = extract_subvector t12, Constant:i64<8>
 // t108: v8i8 = vector_shuffle<1,2,3,4,5,6,7,8> t49, t106
 static SDValue lowerVECTOR_SHUFFLEAsVSlidedown(const SDLoc &DL, MVT VT,
                                                SDValue V1, SDValue V2,
                                                ArrayRef<int> Mask,
                                                const RISCVSubtarget &Subtarget,
                                                SelectionDAG &DAG) {
-  // Both input must be extracts.
-  if (V1.getOpcode() != ISD::EXTRACT_SUBVECTOR ||
-      V2.getOpcode() != ISD::EXTRACT_SUBVECTOR)
-    return SDValue();
-
-  // Extracting from the same source.
-  SDValue Src = V1.getOperand(0);
-  if (Src != V2.getOperand(0))
+  SDValue Src = isOneSourceVECTOR_SHUFFLE(V1, V2, Mask);
+  if (!Src)
     return SDValue();
-
-  // V1 must be started with 0.
-  // V1 and V2 are continuous.
-  if (V1.getConstantOperandVal(1) != 0 ||
-      VT.getVectorNumElements() != V2.getConstantOperandVal(1))
-    return SDValue();
-
   // First index must be known and non-zero. It will be used as the slidedown
   // amount.
   if (Mask[0] <= 0)
     return SDValue();
 
   // Mask is also continuous.
   for (unsigned i = 1; i != Mask.size(); ++i)
     if (Mask[i - 1] + 1 != Mask[i])