A64: Minor code generation improvement: only load the object map if needed.

src/a64/stub-cache-a64.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 870 matching lines @@
              (current->property_dictionary()->FindEntry(*name) ==
               NameDictionary::kNotFound));
 
       GenerateDictionaryNegativeLookup(masm(), miss, reg, name,
                                        scratch1, scratch2);
 
       __ Ldr(scratch1, FieldMemOperand(reg, HeapObject::kMapOffset));
       reg = holder_reg;  // From now on the object will be in holder_reg.
       __ Ldr(reg, FieldMemOperand(scratch1, Map::kPrototypeOffset));
     } else {
-      Register map_reg = scratch1;
-      // TODO(jbramley): Skip this load when we don't need the map.
-      __ Ldr(map_reg, FieldMemOperand(reg, HeapObject::kMapOffset));
+      bool need_map = (depth != 1 || check == CHECK_ALL_MAPS) ||
+                      heap()->InNewSpace(*prototype);
+      Register map_reg = NoReg;
+      if (need_map) {
+        map_reg = scratch1;
+        __ Ldr(map_reg, FieldMemOperand(reg, HeapObject::kMapOffset));
+      }
 
       if (depth != 1 || check == CHECK_ALL_MAPS) {
         __ CheckMap(map_reg, current_map, miss, DONT_DO_SMI_CHECK);
       }
 
       // 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.
       if (current_map->IsJSGlobalProxyMap()) {
         __ CheckAccessGlobalProxy(reg, scratch2, miss);
@@ skipping 631 matching lines @@
 
   // Miss case, call the runtime.
   __ Bind(&miss);
   TailCallBuiltin(masm, Builtins::kKeyedLoadIC_Miss);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_TARGET_ARCH_A64