Add a flag controlling context sharing.

runtime/vm/scopes.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/scopes.h"
 
 #include "vm/object.h"
 #include "vm/stack_frame.h"
 #include "vm/symbols.h"
 
 namespace dart {
 
+DEFINE_FLAG(bool, share_enclosing_context, true,
+            "Allocate captured variables in the existing context of an "
+            "enclosing scope (up to innermost loop) and spare the allocation "
+            "of a local context.");
+
 int SourceLabel::FunctionLevel() const {
   ASSERT(owner() != NULL);
   return owner()->function_level();
 }
 
 
 LocalScope::LocalScope(LocalScope* parent, int function_level, int loop_level)
     : parent_(parent),
       child_(NULL),
       sibling_(NULL),
@@ skipping 118 matching lines @@
   ASSERT(variable->is_captured());
   ASSERT(variable->owner() == this);
   // The context level in the owner scope of a captured variable indicates at
   // code generation time how far to walk up the context chain in order to
   // access the variable from the current context level.
   if ((*context_owner) == NULL) {
     ASSERT(num_context_variables_ == 0);
     // This scope becomes the current context owner.
     set_context_level(1);
     *context_owner = this;
+  } else if (!FLAG_share_enclosing_context && ((*context_owner) != this)) {
+    // The captured variable is in a child scope of the context owner and we do
+    // not share contexts.
+    // This scope will allocate and chain a new context.
+    ASSERT(num_context_variables_ == 0);
+    // This scope becomes the current context owner.
+    set_context_level((*context_owner)->context_level() + 1);
+    *context_owner = this;
   } else if ((*context_owner)->loop_level() < loop_level()) {
+    ASSERT(FLAG_share_enclosing_context);
     // The captured variable is at a deeper loop level than the current context.
     // This scope will allocate and chain a new context.
     ASSERT(num_context_variables_ == 0);
     // This scope becomes the current context owner.
     set_context_level((*context_owner)->context_level() + 1);
     *context_owner = this;
   } else {
     // Allocate the captured variable in the current context.
     if (!HasContextLevel()) {
       ASSERT(variable->owner() != *context_owner);
@@ skipping 491 matching lines @@
     return fixed_parameter_count - (index() - kParamEndSlotFromFp);
   } else {
     // Shift negative indexes so that the lowest one is 0 (they are still
     // non-positive).
     return fixed_parameter_count - (index() - kFirstLocalSlotFromFp);
   }
 }
 
 
 }  // namespace dart