[C++] Optimizing dynamic_cast to a final C++ class

[Florian Weimer <fweimer at redhat dot com>]

I've looked at optimizing dynamic_cast to final C++ class.  In theory, 
it should be possible to load the vtable pointer, compare it to the 
expected value, and use the original pointer if it matches, or NULL 
otherwise (for pointers, references are similar but need the conditional 
throw).

But I remember a discussion on the cxxabi list that vtables are not 
deduplicated if they are contained in multiple libraries, so a simple 
pointer comparison won't do.  Is there anything else that could be used 
as a reliable marker?  If not, do you think it would be feasible to add 
a fast-path for final target classes to the __dynamic_cast implementation?

--
Florian Weimer / Red Hat Product Security Team