Actually, you don't need to execute it at all if there are no side effects. You might as well turn the machine off and save all the electricity.

Heating your computer is a side effect.

For full documentation on this effect, please see the Haskell Heat-Monad package.

In the book Permutation City, people are just able to upload a copy of their brains into a computer. But the simulations are so big that they run slowly. So a billionaire not only uploads a copy of his consciousness, he also runs a massive optimization routine on it! After a few years of chugging along, the optimizer spits out an empty file and reports: "This program has no output."