RFR: 8138659: Speed up InstanceKlass subclass discrimination

[Kim Barrett]

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.

[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.]