Add an assigned variables analysis....

src/data-flow.h

Index: src/data-flow.h
===================================================================
--- src/data-flow.h	(revision 4066)
+++ src/data-flow.h	(working copy)
@@ -40,7 +40,6 @@
  public:
   explicit BitVector(int length)
       : length_(length), bits_(Vector<uint32_t>::New(1 + length / 32)) {
-    ASSERT(length > 0);
     for (int i = 0; i < bits_.length(); i++) {
       bits_[i] = 0;
     }
@@ -497,6 +496,41 @@
 };
 
 
+// Computes the set of assigned variables and annotates variables proxies
+// that are trivial sub-expressions and for-loops where the loop variable
+// is guaranteed to be a smi.
+class AssignedVariablesAnalyzer : public AstVisitor {
+ public:
+  explicit AssignedVariablesAnalyzer(FunctionLiteral* fun);
+
+  void Analyze();
+
+ private:
+  Variable* FindSmiLoopVariable(ForStatement* stmt);
+
+  int BitIndex(Variable* var);
+
+  void RecordAssignedVar(Variable* var);
+
+  void MarkIfTrivial(Expression* expr);
+
+  // Visits an expression saving the accumulator before, clearing
+  // it before visting and restoring it after visiting.
+  void ProcessExpression(Expression* expr);
+
+  // AST node visit functions.
+#define DECLARE_VISIT(type) virtual void Visit##type(type* node);
+  AST_NODE_LIST(DECLARE_VISIT)
+#undef DECLARE_VISIT
+
+  FunctionLiteral* fun_;
+
+  // Accumulator for assigned variables set.
+  BitVector av_;
+
+  DISALLOW_COPY_AND_ASSIGN(AssignedVariablesAnalyzer);
+};
+
 } }  // namespace v8::internal