x +1 > x

[Spiros Bousbouras]

In the thread "Eventual undefined behaviour" Harald van Dijk
said:

The standard allows

int f(int x) {
return x + 1 > x;
}

to be optimised to

int f(int x) {
return 1;
}

How does this follow from the standard ? And what would
happen if x was unsigned int ?

 

[Thomas Lumley]

In the thread "Eventual undefined behaviour" Harald van Dijk
said:




How does this follow from the standard ? And what would
happen if x was unsigned int ?

If 'x' is not INT_MAX then f() must return 1, and if 'x is INT_MAX the
behaviour is undefined and so f() is permitted to return 1. Then by
the 'as if' rule, the function can compute the return value of 1 in
any way.

With unsigned int the optimization would not be permitted, since the
behaviour on overflow is defined and implies a return value of 0.


-thomas

 

[Richard Heathfield]

Thomas Lumley said:

With unsigned int the optimization would not be permitted, since the
behaviour on overflow is defined and implies a return value of 0.

Not quite a correct explanation. The C Standard does not define the
behaviour of unsigned ints on overflow. Rather, it specifies unsigned
integer arithmetic in such a way that overflow is impossible.

Your description of the outcome, however, is correct.