[stubs] Use a single slot for context globals.

src/compiler/ast-graph-builder.cc

 // Copyright 2014 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/compiler/ast-graph-builder.h"
 
 #include "src/compiler.h"
 #include "src/compiler/ast-loop-assignment-analyzer.h"
 #include "src/compiler/control-builders.h"
 #include "src/compiler/js-type-feedback.h"
@@ skipping 3316 matching lines @@
   Node* the_hole = jsgraph()->TheHoleConstant();
   VariableMode mode = variable->mode();
   switch (variable->location()) {
     case VariableLocation::GLOBAL:
     case VariableLocation::UNALLOCATED: {
       // Global var, const, or let variable.
       Node* script_context = current_context();
       int slot_index = -1;
       if (variable->index() > 0) {
         DCHECK(variable->IsStaticGlobalObjectProperty());
-        // Each var occupies two slots in the context: for reads and writes.
         slot_index = variable->index();
         int depth = current_scope()->ContextChainLength(variable->scope());
         if (depth > 0) {
           const Operator* op = javascript()->LoadContext(
               depth - 1, Context::PREVIOUS_INDEX, true);
           script_context = NewNode(op, current_context());
         }
       }
       Node* global = BuildLoadGlobalObject();
       Handle<Name> name = variable->name();
@@ skipping 130 matching lines @@
   Node* the_hole = jsgraph()->TheHoleConstant();
   VariableMode mode = variable->mode();
   switch (variable->location()) {
     case VariableLocation::GLOBAL:
     case VariableLocation::UNALLOCATED: {
       // Global var, const, or let variable.
       Node* script_context = current_context();
       int slot_index = -1;
       if (variable->index() > 0) {
         DCHECK(variable->IsStaticGlobalObjectProperty());
-        // Each var occupies two slots in the context: for reads and writes.
         slot_index = variable->index();
         int depth = current_scope()->ContextChainLength(variable->scope());
         if (depth > 0) {
           const Operator* op = javascript()->LoadContext(
               depth - 1, Context::PREVIOUS_INDEX, true);
           script_context = NewNode(op, current_context());
         }
       }
       Node* global = BuildLoadGlobalObject();
       Handle<Name> name = variable->name();
@@ skipping 817 matching lines @@
     // Phi does not exist yet, introduce one.
     value = NewPhi(inputs, value, control);
     value->ReplaceInput(inputs - 1, other);
   }
   return value;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8