[Interpreter] Localize the CanonicalHandleScope to parsing and renumbering.

src/ast/ast-numbering.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/ast/ast-numbering.h"
 
 #include "src/ast/ast.h"
 #include "src/ast/scopes.h"
 
 namespace v8 {
@@ skipping 559 matching lines @@
     DisableCrankshaft(kRestParameter);
   }
 
   if (IsGeneratorFunction(node->kind()) || IsAsyncFunction(node->kind())) {
     // TODO(neis): We may want to allow Turbofan optimization here if
     // --turbo-from-bytecode is set and we know that Ignition is used.
     // Unfortunately we can't express that here.
     DisableOptimization(kGenerator);
   }
 
+  // Create a canonical handle scope if compiling ignition bytecode. This is
+  // required by the constant array builder to de-duplicate objects without
+  // dereferencing handles.
+  std::unique_ptr<CanonicalHandleScope> canonical;
+  if (FLAG_ignition) canonical.reset(new CanonicalHandleScope(isolate_));

[marja, 2016/09/12 07:26:41]

Why do we need to create the canonical handle scope here? What internalizes
below?

+
   VisitDeclarations(scope->declarations());
   VisitStatements(node->body());
 
   node->set_ast_properties(&properties_);
   node->set_dont_optimize_reason(dont_optimize_reason());
   node->set_yield_count(yield_count_);
   return !HasStackOverflow();
 }
 
 
 bool AstNumbering::Renumber(Isolate* isolate, Zone* zone,
                             FunctionLiteral* function) {
   AstNumberingVisitor visitor(isolate, zone);
   return visitor.Renumber(function);
 }
 }  // namespace internal
 }  // namespace v8