For C/C++ nerds out there, Felix von Letiner wrote a very interesting paper on Source Code Optimization. This will only apply for very specific cases (think string algorithms, hash code generation, numerical loops), but the conclusion is: write more readable code, it will run in less time. It's tough to outsmart the newer GCC versions:
People often write less readable code because they think it will produce faster code. Unfortunately, in most cases, the code will not be faster. Warning: advanced topic, contains assembly language code.