DAGCombiner: Fold pointless truncate, bitcast, buildvector series

(2xi32) (truncate ((2xi64) bitcast (buildvector i32 a, i32 x, i32 b, i32 y)))
can be folded into a (2xi32) (buildvector i32 a, i32 b).

Such a DAG would cause uneccessary vdup instructions followed by vmovn
instructions.

We generate this code on ARM NEON for a setcc olt, 2xf64, 2xf64. For example, in
the vectorized version of the code below.

double A[N];
double B[N];

void test_double_compare_to_double() {
  int i;
  for(i=0;i<N;i++)
    A[i] = (double)(A[i] < B[i]);
}

radar://13191881

Fixes bug 15283.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175670 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 32 insertions, 0 deletions

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 9d40ff799d..2777d7c2eb 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5361,6 +5361,38 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
     }
   }
 
+  // Fold a series of buildvector, bitcast, and truncate if possible.
+  // For example fold
+  //   (2xi32 trunc (bitcast ((4xi32)buildvector x, x, y, y) 2xi64)) to
+  //   (2xi32 (buildvector x, y)).
+  if (Level == AfterLegalizeVectorOps && VT.isVector() &&
+      N0.getOpcode() == ISD::BITCAST && N0.hasOneUse() &&
+      N0.getOperand(0).getOpcode() == ISD::BUILD_VECTOR &&
+      N0.getOperand(0).hasOneUse()) {
+
+    SDValue BuildVect = N0.getOperand(0);
+    EVT BuildVectEltTy = BuildVect.getValueType().getVectorElementType();
+    EVT TruncVecEltTy = VT.getVectorElementType();
+
+    // Check that the element types match.
+    if (BuildVectEltTy == TruncVecEltTy) {
+      // Now we only need to compute the offset of the truncated elements.
+      unsigned BuildVecNumElts =  BuildVect.getNumOperands();
+      unsigned TruncVecNumElts = VT.getVectorNumElements();
+      unsigned TruncEltOffset = BuildVecNumElts / TruncVecNumElts;
+
+      assert((BuildVecNumElts % TruncVecNumElts) == 0 &&
+             "Invalid number of elements");
+
+      SmallVector<SDValue, 8> Opnds;
+      for (unsigned i = 0, e = BuildVecNumElts; i != e; i += TruncEltOffset)
+        Opnds.push_back(BuildVect.getOperand(i));
+
+      return DAG.getNode(ISD::BUILD_VECTOR, N->getDebugLoc(), VT, &Opnds[0],
+                         Opnds.size());
+    }
+  }
+
   // See if we can simplify the input to this truncate through knowledge that
   // only the low bits are being used.
   // For example "trunc (or (shl x, 8), y)" // -> trunc y