[ARM] Look through concat when lowering in-place shuffles (VZIP, ..)

Currently, we canonicalize shuffles that produce a result larger than
their operands with:
  shuffle(concat(v1, undef), concat(v2, undef))
->
  shuffle(concat(v1, v2), undef)

because we can access quad vectors (see PerformVECTOR_SHUFFLECombine).

This is useful in the general case, but there are special cases where
native shuffles produce larger results: the two-result ops.

We can look through the concat when lowering them:
  shuffle(concat(v1, v2), undef)
->
  concat(VZIP(v1, v2):0, :1)

This lets us generate the native shuffles instead of scalarizing to
dozens of VMOVs.

Differential Revision: http://reviews.llvm.org/D10424

llvm-svn: 240118

1 files changed, 38 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index e00e338bd28..ac4233cf92e 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -5715,6 +5715,44 @@ static SDValue LowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG) {
           .getValue(WhichResult);
     }
 
+    // Also check for these shuffles through CONCAT_VECTORS: we canonicalize
+    // shuffles that produce a result larger than their operands with:
+    //   shuffle(concat(v1, undef), concat(v2, undef))
+    // ->
+    //   shuffle(concat(v1, v2), undef)
+    // because we can access quad vectors (see PerformVECTOR_SHUFFLECombine).
+    //
+    // This is useful in the general case, but there are special cases where
+    // native shuffles produce larger results: the two-result ops.
+    //
+    // Look through the concat when lowering them:
+    //   shuffle(concat(v1, v2), undef)
+    // ->
+    //   concat(VZIP(v1, v2):0, :1)
+    //
+    if (V1->getOpcode() == ISD::CONCAT_VECTORS &&
+        V2->getOpcode() == ISD::UNDEF) {
+      SDValue SubV1 = V1->getOperand(0);
+      SDValue SubV2 = V1->getOperand(1);
+      EVT SubVT = SubV1.getValueType();
+
+      // We expect these to have been canonicalized to -1.
+      assert(std::all_of(ShuffleMask.begin(), ShuffleMask.end(), [&](int i) {
+        return i < (int)VT.getVectorNumElements();
+      }) && "Unexpected shuffle index into UNDEF operand!");
+
+      if (unsigned ShuffleOpc = isNEONTwoResultShuffleMask(
+              ShuffleMask, SubVT, WhichResult, isV_UNDEF)) {
+        if (isV_UNDEF)
+          SubV2 = SubV1;
+        assert((WhichResult == 0) &&
+               "In-place shuffle of concat can only have one result!");
+        SDValue Res = DAG.getNode(ShuffleOpc, dl, DAG.getVTList(SubVT, SubVT),
+                                  SubV1, SubV2);
+        return DAG.getNode(ISD::CONCAT_VECTORS, dl, VT, Res.getValue(0),
+                           Res.getValue(1));
+      }
+    }
   }
 
   // If the shuffle is not directly supported and it has 4 elements, use