Comment by bloak
3 years ago
That's right, I think. If you replace the "fflush()" (which should have an argument by the way) with "f()" and declare "void f(void);" then the test and the call appear in the binary. But if you declare "__attribute__((pure)) void f(void);" then the test and the call disappear.
I played around a bit in godbolt: https://godbolt.org/z/vWPWcGM1P
It seems this is correct, but there are very quick cases where the compiler does not consider a program 'pure'. Even a simple call to 'puts' already is enough to be compiled. Probably because it has side-effects in setting a value for ferror(file) to return.
I wonder if we can find an example of a function that is externally observable to a user, but that is guaranteed to finish. Then specifically i wonder if the compiler can proof that the undefined behavior is guaranteed to happen so it elides the branch, proving 'real' timetravel. That is observable.
> I wonder if we can find an example of a function that is externally observable to a user, but that is guaranteed to finish.
I don't think the standard has such a thing, but if it did, the closest thing would probably be a write to a volatile variable. You'd have to make sure the compiler sees the variable as having a side-effect in the first place (so it would probably need external linkage).