Is negating INT_MIN undefined behaviour?

Question

Let\'s say I have a variable i that comes from external sources:

int i = get_i();

Assuming i is INT_MIN

Answer 1

Platforms may choose to define the behavior, but the C Standard does not require that they guarantee anything about it. While historically microcomputer compilers have relatively consistently behaved as though -INT_MIN would yield INT_MIN or in some cases a number that behaves like a value one larger than INT_MAX, it has become more fashionable to instead have it retroactively change the value of whatever was being negated. Thus, given:

int wowzers(int x)
{
  if (x != INT_MIN) printf("Not int min!");
  return -x;
}

a hyper-modern compiler may use the expression -x to determine that x can't have been equal to INT_MIN when the previous comparison was performed, and may thus perform the printf unconditionally.

Incidentally, gcc 8.2 will use the UB-ness of negating INT_MIN to "optimize" the following

int qq,rr;
void test(unsigned short q)
{
    for (int i=0; i<=q; i++)
    {
        qq=-2147483647-i;
        rr=qq;
        rr=-rr;
    }
}

into code that unconditionally stores -2147483647 to qq and 2147483647 to rr. Removing the rr=-rr line will make the code store -2147483647 or -2147483648 into both qq and rr, depending upon whether q is zero.