Re: Hotspot. Dynamic compilers "better" than static one?

[John Volan <johnv@ac3i.dseg.ti.com>]

Norman H. Cohen wrote:
> 
> (Translation into Java:  Suppose there is a class T with method p and
> final subclasses T1, T2, T3, ....  If we know that most of the calls on
> p are going to be for objects of class T1, the call
> 
>    x.p();
> 
> (where x is of type T) can, in effect, be transformed into
> 
>    if ( x instanceof T1 ) {
>       ((T1)x).p();
>    } else {
>       x.p();
>    }
> 
> where the check for the cast to T1 can be optimized away and the call to
> ((T1)x).p can be compiled as direct method call to T1's version of p.)

Doesn't that presume that T1 is a "final" class (i.e., a leaf on the
inheritance tree, with no further subclasses allowed)?  Or at least that
T1.p() is a "final" method (i.e., locked out from all further overriding
in any subclass of T1)?  If T1 isn't final, then even the expression
(T1)x has to be treated as a potentially polymorphic. Given the JVM's
dynamic class-loading mechanism, you can never know when a subclass of
T1 might suddenly be pulled into the JVM.  If T1.p() isn't final, then
the call ((T1)x).p() is still potentially dispatching, so how can the
optimization described above be done legally?

Note that the Java expression

   ( x instanceof T1 )

is NOT equivalent to the Ada expression

   ( A in T1 )        -- i.e., A'Tag = T1'Tag

it's equivalent to

   ( A in T1'Class )  -- i.e., A'Tag = 'Tag of T1 or any subclass of T1

If I'm missing something here, I'd be grateful for a clarification.

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