On 18/01/2009, at 1:51 PM, Erik Buck wrote:
Both Objective-C message sending and C++ virtual function calls
commonly prevent in-lining because the _compiler_ can not determine
which code will actually be called. If you use Objective-C message
sending or C++ virtual member functions, you forego most low level
optimizations at the call sites.
Nicely stated. Presumably, one is using a dynamic method because you
are having to defer to runtime exactly what code will be executed. If
you did not have dynamic method dispatch, you would have to explicitly
write the decision mechanism in the code yourself (conditionals or
case statements). That is, the complexity does not go away – it may be
that the complexity of the method dispatch has to be explicitly coded.
A recent article on computer architecture states this "complexity
preservation principle" rather clearly:
http://www.embedded.com/columns/technicalinsights/29111964?_requestid=1581
Now in Objective-C, the compiler (or linker) can't do static analysis
to remove unnecessary method dispatches because your system is
effectively open (dynamic) to have various add ins (categories, etc)
that can happen at runtime. If a system is closed (as with Eiffel), a
compiler can be much more ruthless in its optimization attempts, but
of course you lose the ability to dynamically add functionality. With C
++, the complexity is to a very large extent left inelegantly up to
the coder. Many programmers seem to feel safer that way.
Ian_______________________________________________
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