Is it allowed for a compiler to optimize away a local volatile variable?

Question

Is the compiler allowed to optimize this (according to the C++17 standard):

int fn() {
    volatile int x = 0;
    return x;
}

to this?

Answer 1

This loop can be optimised away by the as-if rule because it has no observable behaviour:

for (unsigned i = 0; i < n; ++i) { bool looped = true; }

This one cannot:

for (unsigned i = 0; i < n; ++i) { volatile bool looped = true; }

The second loop does something on every iteration, which means the loop takes O(n) time. I have no idea what the constant is, but I can measure it and then I have a way of busy looping for a (more or less) known amount of time.

I can do that because the standard says that access to volatiles must happen, in order. If a compiler were to decide that in this case the standard didn't apply, I think I would have the right to file a bug report.

If the compiler chooses to put looped into a register, I suppose I have no good argument against that. But it still must set the value of that register to 1 for every loop iteration.