RFR: 8138659: Speed up InstanceKlass subclass discrimination

[Coleen Phillimore]

On 10/7/15 8:43 PM, Kim Barrett wrote:
> On Oct 7, 2015, at 7:35 PM, Coleen Phillimore <coleen.phillimore at oracle.com> wrote:
>> Kim,
>>
>> This looks *very* nice.   Thank you for doing this:
>>
>> +    return static_cast<InstanceMirrorKlass*>(k);
>>
>>
>> For bonus points can you change this to a static_cast<>  ???
>>
>>   // Casting from Klass*
>>   static InstanceKlass* cast(Klass* k) {
>>     assert(k == NULL || k->is_klass(), "must be");
>>     assert(k == NULL || k->oop_is_instance(), "cast to InstanceKlass");
>>     return (InstanceKlass*) k;
>>   }
> Yikes!  I somehow forgot that InstanceKlass::cast accepted NULL.  But I wonder if that case really
> arises?  Certainly a large percentage of uses of it will (nearly) instantly crash if k is NULL, because
> we immediately call some member function on the result of the cast.

Oh, yeah, this looks bad!  File another bug and someone will look for 
why we allow null here.

Coleen
>
> [And the is_klass is questionable too, since for it to even potentially return false someone must
> have performed a bad cast, so we’re already in the realm of undefined behavior and that call
> could do anything.]
>