[DAGCombiner] Recognize another rotation idiom

This is the new idiom:

  x<<(y&31) | x>>((0-y)&31)

which is recognized as:

  x ROTL (y&31)

The change refines matchRotateSub.  In
Neg & (OpSize - 1) == (OpSize - Pos) & (OpSize - 1), if Pos is
Pos' & (OpSize - 1) we can just use Pos' instead of Pos.

llvm-svn: 203315

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 97885d4f3f8..f9388185b43 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3466,6 +3466,14 @@ static bool matchRotateSub(SDValue Pos, SDValue Neg, unsigned OpSize) {
     return 0;
   SDValue NegOp1 = Neg.getOperand(1);
 
+  // On the RHS of [A], if Pos is Pos' & (OpSize - 1), just replace Pos with
+  // Pos'.  The truncation is redundant for the purpose of the equality.
+  if (MaskLoBits &&
+      Pos.getOpcode() == ISD::AND &&
+      Pos.getOperand(1).getOpcode() == ISD::Constant &&
+      cast<ConstantSDNode>(Pos.getOperand(1))->getAPIntValue() == OpSize - 1)
+    Pos = Pos.getOperand(0);
+
   // The condition we need is now:
   //
   //     (NegC - NegOp1) & Mask == (OpSize - Pos) & Mask