Optimize conditional branches that have same true/false targets.

runtime/vm/flow_graph_optimizer.h

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_FLOW_GRAPH_OPTIMIZER_H_
 #define VM_FLOW_GRAPH_OPTIMIZER_H_
 
 #include "vm/intermediate_language.h"
 #include "vm/flow_graph.h"
 
@@ skipping 228 matching lines @@
  public:
   ConstantPropagator(FlowGraph* graph,
                      const GrowableArray<BlockEntryInstr*>& ignored);
 
   static void Optimize(FlowGraph* graph);
 
   // Only visit branches to optimize away unreachable blocks discovered
   // by range analysis.
   static void OptimizeBranches(FlowGraph* graph);
 
+  // Eliminate branches that have the same true- and false-target: For example,
+  // this occurs after expressions like
+  //
+  // if (a == null) || (b == null) {

[Kevin Millikin (Google), 2013/09/27 11:03:43]

You need more parentheses in the comment :)

[Florian Schneider, 2013/09/30 12:19:23]

Less is more :)

Done.

+  //   ...
+  // }
+  //
+  // where b is known to be null.
+  static void RemoveRedundantBranches(FlowGraph* graph);
+
   // Used to initialize the abstract value of definitions.
   static RawObject* Unknown() { return Object::unknown_constant().raw(); }
 
  private:
   void Analyze();
   void VisitBranches();
   void Transform();
+  void EliminateRedundantBranches();
 
   void SetReachable(BlockEntryInstr* block);
   void SetValue(Definition* definition, const Object& value);
 
   // Assign the join (least upper bound) of a pair of abstract values to the
   // first one.
   void Join(Object* left, const Object& right);
 
   bool IsUnknown(const Object& value) {
     return value.raw() == unknown_.raw();
@@ skipping 99 matching lines @@
 // Optimize spill stores inside try-blocks by identifying values that always
 // contain a single known constant at catch block entry.
 class TryCatchAnalyzer : public AllStatic {
  public:
   static void Optimize(FlowGraph* flow_graph);
 };
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_OPTIMIZER_H_