[stubs] Use a single slot for context globals.

src/hydrogen.cc

 // Copyright 2013 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/hydrogen.h"
 
 #include <sstream>
 
 #include "src/v8.h"
 
@@ skipping 5546 matching lines @@
             instr = New<HLoadNamedField>(cell_constant, nullptr, access,
                                          field_maps, HType::HeapObject());
           }
           instr->ClearDependsOnFlag(kInobjectFields);
           instr->SetDependsOnFlag(kGlobalVars);
           return ast_context()->ReturnInstruction(instr, expr->id());
         }
       } else if (variable->IsGlobalSlot()) {
         DCHECK(variable->index() > 0);
         DCHECK(variable->IsStaticGlobalObjectProperty());
-        // Each var occupies two slots in the context: for reads and writes.
         int slot_index = variable->index();
         int depth = scope()->ContextChainLength(variable->scope());
 
         HLoadGlobalViaContext* instr =
             New<HLoadGlobalViaContext>(depth, slot_index);
         return ast_context()->ReturnInstruction(instr, expr->id());
 
       } else {
         HValue* global_object = Add<HLoadNamedField>(
             context(), nullptr,
@@ skipping 1211 matching lines @@
     }
     HInstruction* instr = Add<HStoreNamedField>(cell_constant, access, value);
     instr->ClearChangesFlag(kInobjectFields);
     instr->SetChangesFlag(kGlobalVars);
     if (instr->HasObservableSideEffects()) {
       Add<HSimulate>(ast_id, REMOVABLE_SIMULATE);
     }
   } else if (var->IsGlobalSlot()) {
     DCHECK(var->index() > 0);
     DCHECK(var->IsStaticGlobalObjectProperty());
-    // Each var occupies two slots in the context: for reads and writes.
-    int slot_index = var->index() + 1;
+    int slot_index = var->index();
     int depth = scope()->ContextChainLength(var->scope());
 
     HStoreGlobalViaContext* instr = Add<HStoreGlobalViaContext>(
         value, depth, slot_index, function_language_mode());
     USE(instr);
     DCHECK(instr->HasObservableSideEffects());
     Add<HSimulate>(ast_id, REMOVABLE_SIMULATE);
 
   } else {
     HValue* global_object = Add<HLoadNamedField>(
@@ skipping 6524 matching lines @@
     isolate()->GetHTracer()->TraceHydrogen(name(), graph_);
   }
 
 #ifdef DEBUG
   graph_->Verify(false);  // No full verify.
 #endif
 }
 
 }  // namespace internal
 }  // namespace v8