[stubs] Use a single slot for context globals.

src/full-codegen/x64/full-codegen-x64.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/v8.h"
 
 #if V8_TARGET_ARCH_X64
 
 #include "src/code-factory.h"
 #include "src/code-stubs.h"
@@ skipping 2596 matching lines @@
     // Global var, const, or let.
     __ Move(StoreDescriptor::NameRegister(), var->name());
     __ movp(StoreDescriptor::ReceiverRegister(), GlobalObjectOperand());
     if (FLAG_vector_stores) EmitLoadStoreICSlot(slot);
     CallStoreIC();
 
   } else if (var->IsGlobalSlot()) {
     // Global var, const, or let.
     DCHECK(var->index() > 0);
     DCHECK(var->IsStaticGlobalObjectProperty());
-    // Each var occupies two slots in the context: for reads and writes.
-    int const slot = var->index() + 1;
+    int const slot = var->index();
     int const depth = scope()->ContextChainLength(var->scope());
     if (depth <= StoreGlobalViaContextStub::kMaximumDepth) {
       __ Set(StoreGlobalViaContextDescriptor::SlotRegister(), slot);
       DCHECK(StoreGlobalViaContextDescriptor::ValueRegister().is(rax));
       StoreGlobalViaContextStub stub(isolate(), depth, language_mode());
       __ CallStub(&stub);
     } else {
       __ Push(Smi::FromInt(slot));
       __ Push(rax);
       __ CallRuntime(is_strict(language_mode())
@@ skipping 2732 matching lines @@
             Assembler::target_address_at(call_target_address,
                                          unoptimized_code));
   return OSR_AFTER_STACK_CHECK;
 }
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_TARGET_ARCH_X64