Don't CheckSmi when type is Smi.

src/hydrogen.cc

 // Copyright 2012 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 1111 matching lines @@
     HValue* load_dependency,
     ElementsKind elements_kind,
     bool is_store,
     KeyedAccessStoreMode store_mode) {
   Zone* zone = this->zone();
   if (is_store) {
     ASSERT(val != NULL);
     switch (elements_kind) {
       case FAST_SMI_ELEMENTS:
       case FAST_HOLEY_SMI_ELEMENTS:
-        // Smi-only arrays need a smi check.
-        AddInstruction(new(zone) HCheckSmi(val));
+        if (!val->type().IsSmi()) {
+          // Smi-only arrays need a smi check.
+          AddInstruction(new(zone) HCheckSmi(val));
+        }
         // Fall through.
       case FAST_ELEMENTS:
       case FAST_HOLEY_ELEMENTS:
       case FAST_DOUBLE_ELEMENTS:
       case FAST_HOLEY_DOUBLE_ELEMENTS:
         return new(zone) HStoreKeyed(elements, checked_key, val, elements_kind);
       default:
         UNREACHABLE();
         return NULL;
     }
@@ skipping 5783 matching lines @@
 
     elements = AddLoadElements(literal);
 
     HValue* key = AddInstruction(
         new(zone()) HConstant(Handle<Object>(Smi::FromInt(i), isolate()),
                               Representation::Integer32()));
 
     switch (boilerplate_elements_kind) {
       case FAST_SMI_ELEMENTS:
       case FAST_HOLEY_SMI_ELEMENTS:
-        // Smi-only arrays need a smi check.
-        AddInstruction(new(zone()) HCheckSmi(value));
-        // Fall through.
+        if (!value->type().IsSmi()) {
+          // Smi-only arrays need a smi check.
+          AddInstruction(new(zone()) HCheckSmi(value));
+          // Fall through.
+        }
       case FAST_ELEMENTS:
       case FAST_HOLEY_ELEMENTS:
       case FAST_DOUBLE_ELEMENTS:
       case FAST_HOLEY_DOUBLE_ELEMENTS:
         AddInstruction(new(zone()) HStoreKeyed(
             elements,
             key,
             value,
             boilerplate_elements_kind));
         break;
@@ skipping 5416 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal