Introduce 'trivial' expressions, use them for this property assignments.

src/ast.h

-// Copyright 2006-2008 the V8 project authors. All rights reserved.
+// Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
 //       with the distribution.
@@ skipping 180 matching lines @@
   // Symbols that cannot be parsed as array indices are considered property
   // names.  We do not treat symbols that can be array indexes as property
   // names because [] for string objects is handled only by keyed ICs.
   virtual bool IsPropertyName() { return false; }
 
   // True if the expression does not have (evaluated) subexpressions.
   // Function literals are leaves because their subexpressions are not
   // evaluated.
   virtual bool IsLeaf() { return false; }
 
+  // True if the expression has no side effects and is safe to
+  // evaluate out of order.
+  virtual bool IsTrivial() { return false; }
+
   // Mark the expression as being compiled as an expression
   // statement. This is used to transform postfix increments to
   // (faster) prefix increments.
   virtual void MarkAsStatement() { /* do nothing */ }
 
   // Static type information for this expression.
   StaticType* type() { return &type_; }
 
   int num() { return num_; }
 
@@ skipping 519 matching lines @@
 
   virtual bool IsPropertyName() {
     if (handle_->IsSymbol()) {
       uint32_t ignored;
       return !String::cast(*handle_)->AsArrayIndex(&ignored);
     }
     return false;
   }
 
   virtual bool IsLeaf() { return true; }
+  virtual bool IsTrivial() { return true; }
 
   // Identity testers.
   bool IsNull() const { return handle_.is_identical_to(Factory::null_value()); }
   bool IsTrue() const { return handle_.is_identical_to(Factory::true_value()); }
   bool IsFalse() const {
     return handle_.is_identical_to(Factory::false_value());
   }
 
   Handle<Object> handle() const { return handle_; }
 
@@ skipping 168 matching lines @@
 
   virtual bool IsValidLeftHandSide() {
     return var_ == NULL ? true : var_->IsValidLeftHandSide();
   }
 
   virtual bool IsLeaf() {
     ASSERT(var_ != NULL);  // Variable must be resolved.
     return var()->is_global() || var()->rewrite()->IsLeaf();
   }
 
+  // Reading from a mutable variable is a side effect, but 'this' is
+  // immutable.
+  virtual bool IsTrivial() { return is_this(); }
+
   bool IsVariable(Handle<String> n) {
     return !is_this() && name().is_identical_to(n);
   }
 
   bool IsArguments() {
     Variable* variable = AsVariable();
     return (variable == NULL) ? false : variable->is_arguments();
   }
 
   Handle<String> name() const  { return name_; }
@@ skipping 901 matching lines @@
 #undef DEF_VISIT
 
  private:
   bool stack_overflow_;
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_AST_H_