If the optimizer can determine that "nothing happens" in the loop, it can optimize the loop away without attempting to determine whether or not the loop terminates.

It allows the optimizer to assume away the halting problem; all nontrivial loops are obligated to halt.

What's the point of this though? Why are you letting programmers write non-functional code? When does the loop exactly terminate? I'm guessing the standard discusses this but at this point idk if I care about memorizing more C trivia.

It's more useful in C++. Imagine a templated vector destructor:

    for(T* ptr = begin; ptr != end; ++ptr)
    {  destroy(ptr); }

For many types, 'destroy' will be empty (for example integers), so this just turns into:

    for(T* ptr = begin; ptr != end; ++ptr)
    { }

Which is an empty loop and can be optimised away -- assuming we can prove it isn't an infinite loop! Which can be quite hard sometimes (in this case, it needs that 'end' is some multiple of sizeof(T) bigger than begin).

Now in many cases you can prove these loops are finite, but in full generality it's quite hard, and it was considered easier to just let the compiler remove them.

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1528.htm discusses it in some detail.

> What's the point of this though? Why are you letting programmers write non-functional code?

He just told you. Because the only way to prevent it in general is to solve the halting problem.

> When does the loop exactly terminate?

In the general case this is provably impossible to determine.

(all you're seeing here is that the compiler authors felt no need to add special case logic to handle "trivial" cases of the halting problem. If the compiler sees any expression in a loop test, it assumes the loop will halt some of the time)

> He just told you. Because the only way to prevent it in general is to solve the halting problem.

I'm aware. I think that I might have misunderstood what the optimization really does.

The idea (I assume) is to let compilers optimize away loops without determining that they terminate. I.e. the rule is more aimed at loops which would terminate, but this lets compilers avoid proving that they do in fact terminate.

Grandparent presented this as a "surprising optimization" but I'd argue it's exactly what you'd expect when a compiler sees while(expression) -- he just happened to pick a trivial expression.

It is necessary in order for the compiler to do transformation optimizations which do impact production code. Newbies shouldn't be writing production code without guidance anyways IMO.

Newbies should probably turn off optimization entirely.

No argument there, or at the very least whole program optimizations