[DAGCombiner] Match (add X, X) as (shl X, 1) when detecting rotate.

Summary: The combiner transforms (shl X, 1) into (add X, X).

Reviewers: craig.topper, efriedma, RKSimon, lebedev.ri

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66882

llvm-svn: 370578

1 files changed, 20 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 7c73212c294..b37985ed6c6 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -6023,6 +6023,9 @@ static bool matchRotateHalf(SelectionDAG &DAG, SDValue Op, SDValue &Shift,
 /// Otherwise, returns an expansion of \p ExtractFrom based on the following
 /// patterns:
 ///
+///   (or (add v v) (shrl v bitwidth-1)):
+///     expands (add v v) -> (shl v 1)
+///
 ///   (or (mul v c0) (shrl (mul v c1) c2)):
 ///     expands (mul v c0) -> (shl (mul v c1) c3)
 ///
@@ -6045,6 +6048,23 @@ static SDValue extractShiftForRotate(SelectionDAG &DAG, SDValue OppShift,
       "Existing shift must be valid as a rotate half");
 
   ExtractFrom = stripConstantMask(DAG, ExtractFrom, Mask);
+
+  // Value and Type of the shift.
+  SDValue OppShiftLHS = OppShift.getOperand(0);
+  EVT ShiftedVT = OppShiftLHS.getValueType();
+
+  // Amount of the existing shift.
+  ConstantSDNode *OppShiftCst = isConstOrConstSplat(OppShift.getOperand(1));
+
+  // (add v v) -> (shl v 1)
+  if (OppShift.getOpcode() == ISD::SRL && OppShiftCst &&
+      ExtractFrom.getOpcode() == ISD::ADD &&
+      ExtractFrom.getOperand(0) == ExtractFrom.getOperand(1) &&
+      ExtractFrom.getOperand(0) == OppShiftLHS &&
+      OppShiftCst->getAPIntValue() == ShiftedVT.getScalarSizeInBits() - 1)
+    return DAG.getNode(ISD::SHL, DL, ShiftedVT, OppShiftLHS,
+                       DAG.getShiftAmountConstant(1, ShiftedVT, DL));
+
   // Preconditions:
   //    (or (op0 v c0) (shiftl/r (op0 v c1) c2))
   //
@@ -6068,15 +6088,11 @@ static SDValue extractShiftForRotate(SelectionDAG &DAG, SDValue OppShift,
 
   // op0 must be the same opcode on both sides, have the same LHS argument,
   // and produce the same value type.
-  SDValue OppShiftLHS = OppShift.getOperand(0);
-  EVT ShiftedVT = OppShiftLHS.getValueType();
   if (OppShiftLHS.getOpcode() != ExtractFrom.getOpcode() ||
       OppShiftLHS.getOperand(0) != ExtractFrom.getOperand(0) ||
       ShiftedVT != ExtractFrom.getValueType())
     return SDValue();
 
-  // Amount of the existing shift.
-  ConstantSDNode *OppShiftCst = isConstOrConstSplat(OppShift.getOperand(1));
   // Constant mul/udiv/shift amount from the RHS of the shift's LHS op.
   ConstantSDNode *OppLHSCst = isConstOrConstSplat(OppShiftLHS.getOperand(1));
   // Constant mul/udiv/shift amount from the RHS of the ExtractFrom op.