Is this a guaranteed signed/unsigned conversion?

Consider:

#include <stdlib.h>

/* An elementary optimizer is expected to remove most code */
/* Return the absolute value */
unsigned long ulabs(long j)
{
if (0 == LONG_MIN + LONG_MAX) {
if (j < 0) return -j;
else return j;
}
else if (LONG_MIN == j) return 1U + j + ULONG_MAX;
else if (j < 0) return -j;
else return j;
} /* ulabs */

Is this guaranteed to convert all long values to their absolute
unsigned long equivalent, assuming that the desired result is
representable as an unsigned long. Does it work over 2's, 1's
complement and sign-magnitude? Is there a better method? labs()
is not guaranteed.

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