6 ms·

> Integer division is a slow operation, and under the rules of C, it has no side effects. Then C isn't following this rule - crashing is a pretty major side ef

> Integer division is a slow operation, and under the rules of C, it has no side effects. Then C isn't following this rule - crashing is a pretty major side effect.

Dylan16807 4y ago
Side effects are a type of defined behavior. Crashing is not a "side effect" in C terms.
t0mas88 4y ago
Only non-zero integer division is specified as having no side effects. Division by zero is in the C standard as "undefined behavior" meaning the compiler can decide what to do with it, crashing would be nice but it doesn't have to. It could also give you a wrong answer if it wanted to. Edit: And just to illustrate, I tried in clang++ and it gave me "5 / 0 = 0" so some compilers in some cases indeed make use of their freedom to give you a wrong answer.
zerocrates 4y ago
The basic deal is that in the presence of undefined behavior, there are no rules about what the program should do. So if you as a compiler writer see: we can do this optimization and cause no problems _except_ if there's division by zero, which is UB, then you can just do it anyway without checking.
vikingerik 4y ago
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.
- a1369209993 4y ago
  > I've been corrected 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).
  - nayuki 4y ago
    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.
    - a1369209993 4y ago
      > I wrote: > > 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.
      - DougBTX 4y ago
        > 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.

> Integer division is a slow operation, and under the rules of C, it has no side effects. Then C isn't following this rule - crashing is a pretty major side effect.

Dylan16807 4y ago

Side effects are a type of defined behavior. Crashing is not a "side effect" in C terms.

t0mas88 4y ago

Only non-zero integer division is specified as having no side effects. Division by zero is in the C standard as "undefined behavior" meaning the compiler can decide what to do with it, crashing would be nice but it doesn't have to. It could also give you a wrong answer if it wanted to. Edit: And just to illustrate, I tried in clang++ and it gave me "5 / 0 = 0" so some compilers in some cases indeed make use of their freedom to give you a wrong answer.

zerocrates 4y ago

The basic deal is that in the presence of undefined behavior, there are no rules about what the program should do. So if you as a compiler writer see: we can do this optimization and cause no problems _except_ if there's division by zero, which is UB, then you can just do it anyway without checking.

vikingerik 4y ago

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.

a1369209993 4y ago
> I've been corrected 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).
- nayuki 4y ago
  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.
  - a1369209993 4y ago
    > I wrote: > > 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.
    - DougBTX 4y ago
      > 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.