X86: Make shuffle -> shift conversion more aggressive about undefs.

Shuffles that only move an element into position 0 of the vector are common in
the output of the loop vectorizer and often generate suboptimal code when SSSE3
is not available. Lower them to vector shifts if possible.

We still prefer palignr over psrldq because it has higher throughput on
sandybridge.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182102 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 28 insertions, 18 deletions

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 45fe69aa29..8a7faf60e6 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -4756,19 +4756,27 @@ static SDValue getShuffleScalarElt(SDNode *N, unsigned Index, SelectionDAG &DAG,
 /// getNumOfConsecutiveZeros - Return the number of elements of a vector
 /// shuffle operation which come from a consecutively from a zero. The
 /// search can start in two different directions, from left or right.
-static
-unsigned getNumOfConsecutiveZeros(ShuffleVectorSDNode *SVOp, unsigned NumElems,
-                                  bool ZerosFromLeft, SelectionDAG &DAG) {
-  unsigned i;
-  for (i = 0; i != NumElems; ++i) {
-    unsigned Index = ZerosFromLeft ? i : NumElems-i-1;
+/// We count undefs as zeros until PreferredNum is reached.
+static unsigned getNumOfConsecutiveZeros(ShuffleVectorSDNode *SVOp,
+                                         unsigned NumElems, bool ZerosFromLeft,
+                                         SelectionDAG &DAG,
+                                         unsigned PreferredNum = -1U) {
+  unsigned NumZeros = 0;
+  for (unsigned i = 0; i != NumElems; ++i) {
+    unsigned Index = ZerosFromLeft ? i : NumElems - i - 1;
     SDValue Elt = getShuffleScalarElt(SVOp, Index, DAG, 0);
-    if (!(Elt.getNode() &&
-         (Elt.getOpcode() == ISD::UNDEF || X86::isZeroNode(Elt))))
+    if (!Elt.getNode())
+      break;
+
+    if (X86::isZeroNode(Elt))
+      ++NumZeros;
+    else if (Elt.getOpcode() == ISD::UNDEF) // Undef as zero up to PreferredNum.
+      NumZeros = std::min(NumZeros + 1, PreferredNum);
+    else
       break;
   }
 
-  return i;
+  return NumZeros;
 }
 
 /// isShuffleMaskConsecutive - Check if the shuffle mask indicies [MaskI, MaskE)
@@ -4806,8 +4814,9 @@ bool isShuffleMaskConsecutive(ShuffleVectorSDNode *SVOp,
 static bool isVectorShiftRight(ShuffleVectorSDNode *SVOp, SelectionDAG &DAG,
                                bool &isLeft, SDValue &ShVal, unsigned &ShAmt) {
   unsigned NumElems = SVOp->getValueType(0).getVectorNumElements();
-  unsigned NumZeros = getNumOfConsecutiveZeros(SVOp, NumElems,
-              false /* check zeros from right */, DAG);
+  unsigned NumZeros = getNumOfConsecutiveZeros(
+      SVOp, NumElems, false /* check zeros from right */, DAG,
+      SVOp->getMaskElt(0));
   unsigned OpSrc;
 
   if (!NumZeros)
@@ -4839,8 +4848,9 @@ static bool isVectorShiftRight(ShuffleVectorSDNode *SVOp, SelectionDAG &DAG,
 static bool isVectorShiftLeft(ShuffleVectorSDNode *SVOp, SelectionDAG &DAG,
                               bool &isLeft, SDValue &ShVal, unsigned &ShAmt) {
   unsigned NumElems = SVOp->getValueType(0).getVectorNumElements();
-  unsigned NumZeros = getNumOfConsecutiveZeros(SVOp, NumElems,
-              true /* check zeros from left */, DAG);
+  unsigned NumZeros = getNumOfConsecutiveZeros(
+      SVOp, NumElems, true /* check zeros from left */, DAG,
+      NumElems - SVOp->getMaskElt(NumElems - 1) - 1);
   unsigned OpSrc;
 
   if (!NumZeros)
@@ -6871,6 +6881,11 @@ X86TargetLowering::LowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG) const {
                                 TargetMask, DAG);
   }
 
+  if (isPALIGNRMask(M, VT, Subtarget))
+    return getTargetShuffleNode(X86ISD::PALIGNR, dl, VT, V1, V2,
+                                getShufflePALIGNRImmediate(SVOp),
+                                DAG);
+
   // Check if this can be converted into a logical shift.
   bool isLeft = false;
   unsigned ShAmt = 0;
@@ -6988,11 +7003,6 @@ X86TargetLowering::LowerVECTOR_SHUFFLE(SDValue Op, SelectionDAG &DAG) const {
   // inlined here right now to enable us to directly emit target specific
   // nodes, and remove one by one until they don't return Op anymore.
 
-  if (isPALIGNRMask(M, VT, Subtarget))
-    return getTargetShuffleNode(X86ISD::PALIGNR, dl, VT, V1, V2,
-                                getShufflePALIGNRImmediate(SVOp),
-                                DAG);
-
   if (ShuffleVectorSDNode::isSplatMask(&M[0], VT) &&
       SVOp->getSplatIndex() == 0 && V2IsUndef) {
     if (VT == MVT::v2f64 || VT == MVT::v2i64)