Optimize LocalVarDescriptor objects for functions that have no local variables

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"
 
@@ skipping 248 matching lines @@
       desc.info.end_pos = end_token_pos();
       ASSERT(desc.info.begin_pos <= desc.info.end_pos);
       desc.info.set_index(context_scope.ContextIndexAt(i));
       vars.Add(desc);
     }
   }
   // Now collect all variables from local scopes.
   int16_t scope_id = 0;
   CollectLocalVariables(&vars, &scope_id);
 
+  if (vars.length() == 0) {
+    return Object::empty_var_descriptors().raw();
+  }
   const LocalVarDescriptors& var_desc =
       LocalVarDescriptors::Handle(LocalVarDescriptors::New(vars.length()));
   for (int i = 0; i < vars.length(); i++) {
     var_desc.SetVar(i, *(vars[i].name), &vars[i].info);
   }
   return var_desc.raw();
 }
 
 
 // The parser creates internal variables that start with ":"
 static bool IsInternalIdentifier(const String& str) {
   ASSERT(str.Length() > 0);
   return str.CharAt(0) == ':';
 }
 
 
 // Add visible variables that are declared in this scope to vars, then
 // collect visible variables of children, followed by siblings.
 void LocalScope::CollectLocalVariables(GrowableArray<VarDesc>* vars,
                                        int16_t* scope_id) {
   (*scope_id)++;
   if (HasContextLevel() &&
       ((parent() == NULL) ||
       (!parent()->HasContextLevel()) ||
       (parent()->context_level() != context_level()))) {
     // This is the outermost scope with a context level or this scope's
     // context level differs from its parent's level.
     VarDesc desc;
-    desc.name = &String::Handle();  // No name.
+    desc.name = &Object::null_string();  // No name.
     desc.info.set_kind(RawLocalVarDescriptors::kContextLevel);
     desc.info.scope_id = *scope_id;
     desc.info.begin_pos = begin_token_pos();
     desc.info.end_pos = end_token_pos();
     desc.info.set_index(context_level());
     vars->Add(desc);
   }
   for (int i = 0; i < this->variables_.length(); i++) {
     LocalVariable* var = variables_[i];
     if ((var->owner() == this) && !var->is_invisible()) {
@@ skipping 361 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