Comment by mbrock

You can do something like

       (y > 0 && x > INT_MAX - y) 
    || (y < 0 && x < INT_MIN - y)

and hope the optimizer turns it back into just checking the result. Or you use -fwrapv to concretize the ISO ambiguity and specify the natural two's complement semantics, checking overflow with the classic Hacker's Delight formula;

    ((x ^ s) & (y ^ s)) < 0

But the best way is to use the intrinsic __builtin_add_overflow or, depending on compiler support, its C23 standardization via <stdckdint.h> and ckd_add etc.