Stefan Behnel, 16.04.2010 09:38:
A compiler for a statically compiled language can see that the above loop yields a constant result, so it can calculate the result in advance (or at least reduce the loop overhead for the calculation) instead of generating code for the loop as it stands.
That reminds me of an optimisation anecdote I experienced during a talk I gave on Cython. I was showing how a couple of simple type annotations could be used to make Cython run arithmetic code a couple of hundred times faster than it runs in CPython. A tiny little change at the end brought the factor from about 200x to an increadible 18000x. That was still one order of magniture larger than the highest speedup I had seen from Cython up to then, so I was perplex at the time and had to take a deeper look.
It turned out that this was exactly the point where Cython had enough type annotations available to make the Python code turn completely into C code, so the C compiler noticed that the whole benchmark code didn't actually yield any result, optimised away the whole thing and just wrote out an empty module. A tremendous speedup, obviously...
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