Recognize pattern (a << b) & c with c being a positive Smi and allow left shift to truncate the res…

runtime/vm/flow_graph_optimizer.h

Index: runtime/vm/flow_graph_optimizer.h
===================================================================
--- runtime/vm/flow_graph_optimizer.h	(revision 18774)
+++ runtime/vm/flow_graph_optimizer.h	(working copy)
@@ -27,6 +27,10 @@
   // Use propagated class ids to optimize, replace or eliminate instructions.
   void ApplyClassIds();
 
+  // Optimize (a << b) & c pattern: if c is a positive smi, then the
+  // shift can be a truncating Smi shift and result is always Smi.
+  void TryOptimizeLeftShiftWithBitAndPattern();
+
   void Canonicalize();
 
   void EliminateDeadPhis();
@@ -132,6 +136,10 @@
   void HandleEqualityCompare(EqualityCompareInstr* comp,
                              T current_instruction);
 
+  void OptimizeLeftShiftBitAndSmiOp(Definition* bit_and_instr,
+                                    Value* left,
+                                    Value* right);
+
   FlowGraph* flow_graph_;
 
   DISALLOW_COPY_AND_ASSIGN(FlowGraphOptimizer);