Pure dynamic casts?

[Jarrett Billingsley]

On Tue, Sep 22, 2009 at 1:19 PM, Steven Schveighoffer
<schveiguy at yahoo.com> wrote:
> On Tue, 22 Sep 2009 05:24:11 -0400, Daniel Keep
> <daniel.keep.lists at gmail.com> wrote:
>
>>
>>
>> Jason House wrote:
>>>
>>> Dynamic casts are pure. They don't use global state, and have the same
>>> output for the same reference as input. Interestingly, dynamic cast results
>>> are independent of intervening mutable calls... So there's even greater
>>> opportunity for optimization.
>>
>> What if the GC just happens to re-use that address for a different object?
>
> That's only if memoization is used.
>
> I think what Jason said is correct -- you can view dynamic cast as taking 2
> arguments, one is the reference which is simply echoed as the return value,
> and one is the classinfo, which is an immutable argument that causes a
> decision to be made.
>
> In fact, you could use memoization on a dynamic cast subfunction that
> memoizes on the target type and the classinfo of the source, regardless of
> the reference value, and returns an offset to add to the return value.
>
> It's pretty easy for the compiler to prove that o has not be reassigned, so
> even without memoization, the compiler can logically assume that the result
> from the first dynamic cast can be reused.  I think this is the major
> optimization for pure functions anyways, not memoization.

Or you could - I dunno - cache the result of the dynamic cast in a
local, since doing multiple dynamic casts is terrible style anyway.

Just saying. ;)