More general dead phi elimination.

runtime/vm/intermediate_language.h

 // Copyright (c) 2013, 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_INTERMEDIATE_LANGUAGE_H_
 #define VM_INTERMEDIATE_LANGUAGE_H_
 
 #include "vm/allocation.h"
 #include "vm/ast.h"
 #include "vm/growable_array.h"
@@ skipping 1576 matching lines @@
   virtual EffectSet Dependencies() const { return EffectSet::None(); }
 
  private:
   // Classes that have access to predecessors_ when inlining.
   friend class BlockEntryInstr;
   friend class InlineExitCollector;
   friend class PolymorphicInliner;
 
   // Direct access to phis_ in order to resize it due to phi elimination.
   friend class ConstantPropagator;
+  friend class DeadCodeElimination;
 
   virtual void ClearPredecessors() { predecessors_.Clear(); }
   virtual void AddPredecessor(BlockEntryInstr* predecessor);
 
   GrowableArray<BlockEntryInstr*> predecessors_;
   ZoneGrowableArray<PhiInstr*>* phis_;
 
   DISALLOW_COPY_AND_ASSIGN(JoinEntryInstr);
 };
 
@@ skipping 384 matching lines @@
   BitVector* reaching_defs() const {
     return reaching_defs_;
   }
 
   void set_reaching_defs(BitVector* reaching_defs) {
     reaching_defs_ = reaching_defs;
   }
 
   virtual bool MayThrow() const { return false; }
 
+  // A phi is redundant if all input operands are the same.
+  bool IsRedundant() const;
+
  private:
   // Direct access to inputs_ in order to resize it due to unreachable
   // predecessors.
   friend class ConstantPropagator;
 
   void RawSetInputAt(intptr_t i, Value* value) { inputs_[i] = value; }
 
   JoinEntryInstr* block_;
   GrowableArray<Value*> inputs_;
   bool is_alive_;
@@ skipping 5946 matching lines @@
   ForwardInstructionIterator* current_iterator_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FlowGraphVisitor);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_INTERMEDIATE_LANGUAGE_H_