Restore performance of accesses to JSGlobalProxy after r22802

src/x64/stub-cache-x64.cc

Index: src/x64/stub-cache-x64.cc
diff --git a/src/x64/stub-cache-x64.cc b/src/x64/stub-cache-x64.cc
index 69781ebea1551a7e8c18906b3fe8e755ffd1833e..7ef59cd8202705afa528a2d1aa1fbabaf35b3f26 100644
--- a/src/x64/stub-cache-x64.cc
+++ b/src/x64/stub-cache-x64.cc
@@ -651,8 +651,8 @@ Register PropertyHandlerCompiler::CheckPrototypes(
 
     prototype = handle(JSObject::cast(current_map->prototype()));
     if (current_map->is_dictionary_map() &&
-        !current_map->IsJSGlobalObjectMap() &&
-        !current_map->IsJSGlobalProxyMap()) {
+        !current_map->IsJSGlobalObjectMap()) {
+      DCHECK(!current_map->IsJSGlobalProxyMap());  // Proxy maps are fast.
       if (!name->IsUniqueName()) {
         DCHECK(name->IsString());
         name = factory()->InternalizeString(Handle<String>::cast(name));
@@ -685,6 +685,9 @@ Register PropertyHandlerCompiler::CheckPrototypes(
       // Check access rights to the global object.  This has to happen after
       // the map check so that we know that the object is actually a global
       // object.
+      // This allows us to install generated handlers for accesses to the
+      // global proxy (as opposed to using slow ICs). See corresponding code
+      // in LookupForRead().
       if (current_map->IsJSGlobalProxyMap()) {
         __ CheckAccessGlobalProxy(reg, scratch2, miss);
       } else if (current_map->IsJSGlobalObjectMap()) {