VM: Eliminate a redundant store of the context at function entry.

runtime/vm/flow_graph_builder.cc

 // 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.
 
 #include "vm/flow_graph_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/class_finalizer.h"
@@ skipping 3442 matching lines @@
     Value* allocated_context =
         Bind(new AllocateContextInstr(node->token_pos(),
                                       num_context_variables));
     { LocalVariable* tmp_var = EnterTempLocalScope(allocated_context);
       // If this node_sequence is the body of the function being compiled, and
       // if this function allocates context variables, but none of its enclosing
       // functions do, the context on entry is not linked as parent of the
       // allocated context but saved on entry and restored on exit as to prevent
       // memory leaks.
       // In this case, the parser pre-allocates a variable to save the context.
+      Value* parent_context = NULL;
       if (MustSaveRestoreContext(node)) {
         BuildSaveContext(
             *owner()->parsed_function()->saved_entry_context_var());
-        Value* null_context = Bind(new ConstantInstr(Object::ZoneHandle()));
-        AddInstruction(new StoreContextInstr(null_context));
+        parent_context = Bind(new ConstantInstr(Object::ZoneHandle()));
+      } else {
+        parent_context = Bind(new CurrentContextInstr());
       }
-      Value* current_context = Bind(new CurrentContextInstr());
       Value* tmp_val = Bind(new LoadLocalInstr(*tmp_var));
       Do(new StoreVMFieldInstr(tmp_val,
                                Context::parent_offset(),
-                               current_context,
+                               parent_context,
                                Type::ZoneHandle()));
       AddInstruction(
           new StoreContextInstr(Bind(ExitTempLocalScope(tmp_var))));
     }
 
     // If this node_sequence is the body of the function being compiled, copy
     // the captured parameters from the frame into the context.
     if (node == owner()->parsed_function()->node_sequence()) {
       ASSERT(scope->context_level() == 1);
       const Function& function = owner()->parsed_function()->function();
@@ skipping 457 matching lines @@
                                   LanguageError::kError,
                                   Heap::kNew,
                                   "FlowGraphBuilder Bailout: %s %s",
                                   String::Handle(function.name()).ToCString(),
                                   reason));
   Isolate::Current()->long_jump_base()->Jump(1, error);
 }
 
 
 }  // namespace dart