Eliminate temporary locals for some expressions

runtime/vm/flow_graph_builder.h

Index: runtime/vm/flow_graph_builder.h
===================================================================
--- runtime/vm/flow_graph_builder.h	(revision 23243)
+++ runtime/vm/flow_graph_builder.h	(working copy)
@@ -140,6 +140,9 @@
   bool IsInlining() const { return (exit_collector_ != NULL); }
   InlineExitCollector* exit_collector() const { return exit_collector_; }
 
+  intptr_t args_pushed() const { return args_pushed_; }
+  void add_args_pushed(intptr_t n) { args_pushed_ += n; }
+
  private:
   intptr_t parameter_count() const {
     return num_copied_params_ + num_non_copied_params_;
@@ -162,6 +165,9 @@
   intptr_t try_index_;
   GraphEntryInstr* graph_entry_;
 
+  // Outgoing argument stack height.
+  intptr_t args_pushed_;
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(FlowGraphBuilder);
 };
 
@@ -318,6 +324,10 @@
     temp_index_ -= n;
   }
 
+  // Returns a local variable index for a temporary local that is
+  // on top of the current expression stack.
+  intptr_t GetCurrentTempLocalIndex() const;
+
   Value* BuildObjectAllocation(ConstructorCallNode* node);
   void BuildConstructorCall(ConstructorCallNode* node,
                             PushArgumentInstr* alloc_value);
@@ -346,21 +356,23 @@
 
   Value* BuildNullValue();
 
+  // Returns true if the run-time type check can be eliminated.
+  bool CanSkipTypeCheck(intptr_t token_pos,
+                        Value* value,
+                        const AbstractType& dst_type,
+                        const String& dst_name);
+
+  void BuildLetTempExpressions(LetNode* node);
+
  private:
+  friend class TempLocalScope;  // For ReturnDefinition.
+
   // Specify a definition of the final result.  Adds the definition to
   // the graph, but normally overridden in subclasses.
   virtual void ReturnDefinition(Definition* definition) {
     Do(definition);
   }
 
- protected:
-  // Returns true if the run-time type check can be eliminated.
-  bool CanSkipTypeCheck(intptr_t token_pos,
-                        Value* value,
-                        const AbstractType& dst_type,
-                        const String& dst_name);
-
- private:
   // Shared global state.
   FlowGraphBuilder* owner_;
 
@@ -401,6 +413,7 @@
   virtual void VisitStoreStaticFieldNode(StoreStaticFieldNode* node);
   virtual void VisitTypeNode(TypeNode* node);
   virtual void VisitCommaNode(CommaNode* node);
+  virtual void VisitLetNode(LetNode* node);
 
   Value* value() const { return value_; }