8/8 When I compile C++ with dynamic dispatch, will the actual generated
code (assembly) contain a lot of if-else check for each method
call? How about Java? Is the compiler smart enough to do flow
analysis and do type checking to narrow down types, thus speeding
up the method call?
\_ This sounds like one of the weenies 6-8 years ago who
always loathed the use of "virtual" in C++, afraid of what
the cost of a couple of table lookups would do to their
overhead.
\_ It uses a function dispatch table. Very efficient.
\_ If you are smart enough to worry about whether your compiler
does flow analysis, you should be smart enough to research your
vague assumptions. Check out _The C++ Object Model_, Stroustrup
\_ Yes, Java and C++ use runtime dispatching, which adds around 1-5
cycles of overhead per function call (for commonly executed
routines, where the function table is in cache) and 10+ cycles
(for infrequently executed ones, or where you're just executing
lots of functions or somehow else thrashing cache). If you're
invoking lots of small methods (which is how these
languages are typically used), that can be a huge loss of
performance. THere has been recent work on, e.g. dynamic
compilation to specialize away the function table lookup. Can't
recall the authors off the top of my head. -nick
\_ Sun HotSpot (and presumbly IBM/Tower equivalents) easily inline
and compile virtual calls at runtime. At least with Sun's, you are
back to very small methods because the compilation of a method is
triggered simply after several thousands iterations are counted -
if you inline everything into one flat main() or run(), it'll never
get compiled but you'll pay dearly for all the profiling. You won't
hear this from Sun. -muchandr |
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