C - how would compiler optimization affect a for loop with no body?

Question

I have some legacy code where a timewasting loop has been included to allow time for an eeprom read to complete (bad practice):

for(i = 0; i < 50; i++);

However, peculiar things happen when compiler optimizations are switched on for speed. It is not necessarily connected with that statement, but I would like to know if the compiler might just optimize the time delay away

Answer 1

It depends on the type of i. If it is just a plain integer type that isn't used apart from inside the loop, there are no side effects and the compiler is free to optimize away the whole thing.

If you declare i as volatile however, the compiler is forced to generate code that increments the variable and reads it, at each lap of the loop.

This is one of many reasons why you should not use "burn-away" loops like these in embedded systems. You also occupy 100% CPU and consume 100% current. And you create a tight coupling between your system clock and the loop, which isn't necessarily linear.

The professional solution is always to use an on-chip hardware timer instead of "burn-away" loops.