[generators] Add some explanation on forcing context allocation.

src/parsing/parser.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/parsing/parser.h"
 
 #include "src/api.h"
 #include "src/ast/ast.h"
 #include "src/ast/ast-expression-rewriter.h"
 #include "src/ast/ast-expression-visitor.h"
@@ skipping 4085 matching lines @@
   {
     AstNodeFactory function_factory(ast_value_factory());
     FunctionState function_state(&function_state_, &scope_, scope, kind,
                                  &function_factory);
     scope_->SetScopeName(function_name);
     ExpressionClassifier formals_classifier(this, &duplicate_finder);
 
     if (is_generator) {
       // For generators, allocating variables in contexts is currently a win
       // because it minimizes the work needed to suspend and resume an
-      // activation.
+      // activation.  The machine code produced for generators (by full-codegen)
+      // relies on this forced context allocation, but not in an essential way.

[Michael Starzinger, 2016/03/17 12:48:00]

nit: The "[...], but not in an essential way" part is misleading. Can we just
point to the place where it is relied upon by saying something along the lines
of "[...], because arguments on the stack to the generator are not restored when
resuming a generator" instead. WDYT?

       scope_->ForceContextAllocation();
 
       // Calling a generator returns a generator object.  That object is stored
       // in a temporary variable, a definition that is used by "yield"
       // expressions. This also marks the FunctionState as a generator.
       Variable* temp = scope_->NewTemporary(
           ast_value_factory()->dot_generator_object_string());
       function_state.set_generator_object_variable(temp);
     }
 
@@ skipping 2665 matching lines @@
                         try_block, target);
     final_loop = target;
   }
 
   return final_loop;
 }
 
 
 }  // namespace internal
 }  // namespace v8