On Tue, Sep 22, 2009 at 1:19 PM, Steven Schveighoffer <schvei...@yahoo.com> wrote: > On Tue, 22 Sep 2009 05:24:11 -0400, Daniel Keep > <daniel.keep.li...@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. ;)
