ARM NEON: Handle v16i8 and v8i16 reverse shuffles

Lower reverse shuffles to a vrev64 and a vext instruction instead of the default
legalization of storing and loading to the stack. This is important because we
generate reverse shuffles in the loop vectorizer when we reverse store to an
array.

  uint8_t Arr[N];
  for (i = 0; i < N; ++i)
    Arr[N - i - 1] = ...

radar://13171760

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174929 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 37 insertions, 1 deletions

diff --git a/lib/Target/ARM/ARMISelLowering.cpp b/lib/Target/ARM/ARMISelLowering.cpp
index 4c9d2da2ed..9d7a37929a 100644
--- a/lib/Target/ARM/ARMISelLowering.cpp
+++ b/lib/Target/ARM/ARMISelLowering.cpp
@@ -4294,6 +4294,21 @@ static bool isVZIP_v_undef_Mask(ArrayRef<int> M, EVT VT, unsigned &WhichResult){
   return true;
 }
 
+/// \return true if this is a reverse operation on an vector.
+static bool isReverseMask(ArrayRef<int> M, EVT VT) {
+  unsigned NumElts = VT.getVectorNumElements();
+  // Make sure the mask has the right size.
+  if (NumElts != M.size())
+      return false;
+
+  // Look for <15, ..., 3, -1, 1, 0>.
+  for (unsigned i = 0; i != NumElts; ++i)
+    if (M[i] >= 0 && M[i] != (int) (NumElts - 1 - i))
+      return false;
+
+  return true;
+}
+
 // If N is an integer constant that can be moved into a register in one
 // instruction, return an SDValue of such a constant (will become a MOV
 // instruction).  Otherwise return null.
@@ -4689,7 +4704,8 @@ ARMTargetLowering::isShuffleMaskLegal(const SmallVectorImpl<int> &M,
           isVZIPMask(M, VT, WhichResult) ||
           isVTRN_v_undef_Mask(M, VT, WhichResult) ||
           isVUZP_v_undef_Mask(M, VT, WhichResult) ||
-          isVZIP_v_undef_Mask(M, VT, WhichResult));
+          isVZIP_v_undef_Mask(M, VT, WhichResult) ||
+          ((VT == MVT::v8i16 || VT == MVT::v16i8) && isReverseMask(M, VT)));
 }
 
 /// GeneratePerfectShuffle - Given an entry in the perfect-shuffle table, emit
@@ -4793,6 +4809,23 @@ static SDValue LowerVECTOR_SHUFFLEv8i8(SDValue Op,
                                  &VTBLMask[0], 8));
 }
 
+static SDValue LowerReverse_VECTOR_SHUFFLEv16i8_v8i16(SDValue Op,
+                                                      SelectionDAG &DAG) {
+  DebugLoc DL = Op.getDebugLoc();
+  SDValue OpLHS = Op.getOperand(0);
+  EVT VT = OpLHS.getValueType();
+
+  assert((VT == MVT::v8i16 || VT == MVT::v16i8) &&
+         "Expect an v8i16/v16i8 type");
+  OpLHS = DAG.getNode(ARMISD::VREV64, DL, VT, OpLHS);
+  // For a v16i8 type: After the VREV, we have got <8, ...15, 8, ..., 0>. Now,
+  // extract the first 8 bytes into the top double word and the last 8 bytes
+  // into the bottom double word. The v8i16 case is similar.
+  unsigned ExtractNum = (VT == MVT::v16i8) ? 8 : 4;
+  return DAG.getNode(ARMISD::VEXT, DL, VT, OpLHS, OpLHS,
+                     DAG.getConstant(ExtractNum, MVT::i32));
+}
+
 static SDValue LowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG) {
   SDValue V1 = Op.getOperand(0);
   SDValue V2 = Op.getOperand(1);
@@ -4930,6 +4963,9 @@ static SDValue LowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG) {
     return DAG.getNode(ISD::BITCAST, dl, VT, Val);
   }
 
+  if ((VT == MVT::v8i16 || VT == MVT::v16i8) && isReverseMask(ShuffleMask, VT))
+    return LowerReverse_VECTOR_SHUFFLEv16i8_v8i16(Op, DAG);
+
   if (VT == MVT::v8i8) {
     SDValue NewOp = LowerVECTOR_SHUFFLEv8i8(Op, ShuffleMask, DAG);
     if (NewOp.getNode())