1 files changed, 48 insertions, 0 deletions
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 188a808f569..03181a625b8 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -16506,6 +16506,49 @@ static SDValue lower512BitShuffle(const SDLoc &DL, ArrayRef<int> Mask,
   }
 }
 
+static SDValue lower1BitShuffleAsKSHIFTR(const SDLoc &DL, ArrayRef<int> Mask,
+                                         MVT VT, SDValue V1, SDValue V2,
+                                         const X86Subtarget &Subtarget,
+                                         SelectionDAG &DAG) {
+  // Shuffle should be unary.
+  if (!V2.isUndef())
+    return SDValue();
+
+  int ShiftAmt = -1;
+  int NumElts = Mask.size();
+  for (int i = 0; i != NumElts; ++i) {
+    int M = Mask[i];
+    assert((M == SM_SentinelUndef || (0 <= M && M < NumElts)) &&
+           "Unexpected mask index.");
+    if (M < 0)
+      continue;
+
+    // The first non-undef element determines our shift amount.
+    if (ShiftAmt < 0) {
+      ShiftAmt = M - i;
+      // Need to be shifting right.
+      if (ShiftAmt <= 0)
+        return SDValue();
+    }
+    // All non-undef elements must shift by the same amount.
+    if (ShiftAmt != M - i)
+      return SDValue();
+  }
+  assert(ShiftAmt >= 0 && "All undef?");
+
+  // Great we found a shift right.
+  MVT WideVT = VT;
+  if ((!Subtarget.hasDQI() && NumElts == 8) || NumElts < 8)
+    WideVT = Subtarget.hasDQI() ? MVT::v8i1 : MVT::v16i1;
+  SDValue Res = DAG.getNode(ISD::INSERT_SUBVECTOR, DL, WideVT,
+                            DAG.getUNDEF(WideVT), V1,
+                            DAG.getIntPtrConstant(0, DL));
+  Res = DAG.getNode(X86ISD::KSHIFTR, DL, WideVT, Res,
+                    DAG.getConstant(ShiftAmt, DL, MVT::i8));
+  return DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, VT, Res,
+                     DAG.getIntPtrConstant(0, DL));
+}
+
 // Determine if this shuffle can be implemented with a KSHIFT instruction.
 // Returns the shift amount if possible or -1 if not. This is a simplified
 // version of matchShuffleAsShift.
@@ -16586,6 +16629,11 @@ static SDValue lower1BitShuffle(const SDLoc &DL, ArrayRef<int> Mask,
                        Extract, DAG.getIntPtrConstant(0, DL));
   }
 
+  // Try a simple shift right with undef elements. Later we'll try with zeros.
+  if (SDValue Shift = lower1BitShuffleAsKSHIFTR(DL, Mask, VT, V1, V2, Subtarget,
+                                                DAG))
+    return Shift;
+
   // Try to match KSHIFTs.
   unsigned Offset = 0;
   for (SDValue V : { V1, V2 }) {