[X86] Teach the DAGCombiner how to fold a OR of two shufflevector nodes.

This patch teaches the DAGCombiner how to fold a binary OR between two
shufflevector into a single shuffle vector when possible.

The rules are:
  1. fold (or (shuf A, V_0, MA), (shuf B, V_0, MB)) -> (shuf A, B, Mask1)
  2. fold (or (shuf A, V_0, MA), (shuf B, V_0, MB)) -> (shuf B, A, Mask2)

The DAGCombiner can take advantage of the fact that OR is commutative and
compute two possible shuffle masks (Mask1 and Mask2) for the resulting
shuffle node.

Before folding a dag according to either rule 1 or 2, DAGCombiner verifies
that the resulting shuffle mask is legal for the target.
DAGCombiner would firstly try to fold according to 1.; If not possible
then it will try to fold according to 2.
If both Mask1 and Mask2 are illegal then we conservatively don't fold
the OR instruction.

llvm-svn: 203156

1 files changed, 54 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 8650154c1a9..aa35f6d3e77 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3200,6 +3200,60 @@ SDValue DAGCombiner::visitOR(SDNode *N) {
       return N0;
     if (ISD::isBuildVectorAllOnes(N1.getNode()))
       return N1;
+
+    // fold (or (shuf A, V_0, MA), (shuf B, V_0, MB)) -> (shuf A, B, Mask1)
+    // fold (or (shuf A, V_0, MA), (shuf B, V_0, MB)) -> (shuf B, A, Mask2)
+    // Do this only if the resulting shuffle is legal.
+    if (isa<ShuffleVectorSDNode>(N0) &&
+        isa<ShuffleVectorSDNode>(N1) &&
+        N0->getOperand(1) == N1->getOperand(1) &&
+        ISD::isBuildVectorAllZeros(N0.getOperand(1).getNode())) {
+      bool CanFold = true;
+      unsigned NumElts = VT.getVectorNumElements();
+      const ShuffleVectorSDNode *SV0 = cast<ShuffleVectorSDNode>(N0);
+      const ShuffleVectorSDNode *SV1 = cast<ShuffleVectorSDNode>(N1);
+      // We construct two shuffle masks:
+      // - Mask1 is a shuffle mask for a shuffle with N0 as the first operand
+      // and N1 as the second operand.
+      // - Mask2 is a shuffle mask for a shuffle with N1 as the first operand
+      // and N0 as the second operand.
+      // We do this because OR is commutable and therefore there might be
+      // two ways to fold this node into a shuffle.
+      SmallVector<int,4> Mask1;
+      SmallVector<int,4> Mask2;
+      
+      for (unsigned i = 0; i != NumElts && CanFold; ++i) {
+        int M0 = SV0->getMaskElt(i);
+        int M1 = SV1->getMaskElt(i);
+   
+        // Both shuffle indexes are undef. Propagate Undef.
+        if (M0 < 0 && M1 < 0) {
+          Mask1.push_back(M0);
+          Mask2.push_back(M0);
+          continue;
+        }
+
+        if (M0 < 0 || M1 < 0 ||
+            (M0 < (int)NumElts && M1 < (int)NumElts) ||
+            (M0 >= (int)NumElts && M1 >= (int)NumElts)) {
+          CanFold = false;
+          break;
+        }
+        
+        Mask1.push_back(M0 < (int)NumElts ? M0 : M1 + NumElts);
+        Mask2.push_back(M1 < (int)NumElts ? M1 : M0 + NumElts);
+      }
+
+      if (CanFold) {
+        // Fold this sequence only if the resulting shuffle is 'legal'.
+        if (TLI.isShuffleMaskLegal(Mask1, VT))
+          return DAG.getVectorShuffle(VT, SDLoc(N), N0->getOperand(0),
+                                      N1->getOperand(0), &Mask1[0]);
+        if (TLI.isShuffleMaskLegal(Mask2, VT))
+          return DAG.getVectorShuffle(VT, SDLoc(N), N1->getOperand(0),
+                                      N0->getOperand(0), &Mask2[0]);
+      }
+    }
   }
 
   // fold (or x, undef) -> -1