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 18843)
+++ runtime/vm/flow_graph_optimizer.h	(working copy)
@@ -29,6 +29,10 @@
   // Use propagated class ids to optimize, replace or eliminate instructions.
   void ApplyClassIds();
 
+  // Optimize (a << b) & c pattern: if c is a positive Smi or zero, then the
+  // shift can be a truncating Smi shift-left and result is always Smi.
+  void TryOptimizeLeftShiftWithBitAndPattern();
+
   void Canonicalize();
 
   void EliminateDeadPhis();
@@ -97,6 +101,12 @@
                      Environment* deopt_environment,
                      Instruction* insert_before);
 
+  // Insert a Smi check if needed.
+  void AddCheckSmi(Definition* to_check,
+                   intptr_t deopt_id,
+                   Environment* deopt_environment,
+                   Instruction* insert_before);
+
   // Add a class check for a call's first argument immediately before the
   // call, using the call's IC data to determine the check, and the call's
   // deopt ID and deoptimization environment if the check fails.
@@ -138,6 +148,10 @@
   void HandleEqualityCompare(EqualityCompareInstr* comp,
                              T current_instruction);
 
+  void OptimizeLeftShiftBitAndSmiOp(Definition* bit_and_instr,
+                                    Definition* left_instr,
+                                    Definition* right_instr);
+
   FlowGraph* flow_graph_;
 
   DISALLOW_COPY_AND_ASSIGN(FlowGraphOptimizer);