To my downvoters, since I can no longer edit: I've been corrected that the rule is integer division has no side effects except for dividing by zero. This was not the rule my parent poster stated.
No you haven't. The incorrect statement was a verbatim quote from nayuki's post, which you were responding to. Please refrain from apologising for other people gaslighting you (edit: particularly, but not exclusively, since it sets a bad precedent for everyone else).
At the CPU level, division by zero can behave in a number of ways. It can trap and raise an exception. It can silently return 0 or leave a register unchanged. It might hang and crash the whole system. The C language standard acknowledges that different CPUs may behave differently, and chose to categorize division-by-zero under "undefined behavior", not "implementation-defined behavior" or "must trap".
I wrote:
> Integer division is a slow operation, and under the rules of C, it has no side effects.
This statement is correct because if the divisor is not zero, then division truly has no side effects and can be reordered anywhere, otherwise if the divisor is zero, the C standard says it's undefined behavior so this case is irrelevant and can be disregarded, so we can assume that division always has no side effects. It doesn't matter if the underlying CPU has a side effect for div-zero or not; the C standard permits the compiler to completely ignore this case.
> > Integer division is a slow operation, and under the rules of C, it has no side effects.
Yes, you did, and while that's a reasonable approximation in some contexts, it is false in the general case, since division by zero has a side effect in the form of invoking undefined behaviour. (Arguably that means it has every possible side effect, but that's more of a philosophical issue. In practice it has various specific side effects like crashing, which are specific realizations of its theoretical side effect of invoking undefined behaviour.)
vikingerik's statement was correct:
> [If "Integer division [...] has no side effects",] Then C isn't following this rule - crashing is a pretty major side effect.
> it is false in the general case, since division by zero has a side effect in the form of invoking undefined behaviour.
They were careful to say “under the rules of C,” the rules define the behaviour of C. On the other hand, undefined behaviour is outside the rules, so I think they’re correct in what they’re saying.
The problem for me is that the compiler is not obliged to check that the code is following the rules. It puts so much extra weight on the shoulders of the programmer, though I appreciate that using only rules which can be checked by the compiler is hard too, especially back when C was standardised.
> They were careful to say "under the rules of C,"
Yes, and under the rules of C, division by zero has a side effect, namely invoking undefined behaviour.
> The problem for me is that the compiler is not obliged to check that the code is following the rules.
That part's actually fine (annoying, but ultimately a reasonable consequence of the "rules the compiler can check" issue); the real(ly bad and insidious) problem is that when the compiler does check that the code is following the rules, it's allowed to do it in deliberately backward way that uses any case of not following the rules as a excuse to break unrelated code.
Undefined behavior is not a side effect to be "invoked" by the rules of C. If UB happens, it means your program isn't valid. UB is not a side effect or any effect at all, it is the void left behind when the system of rules disappears.
