Comment by Retr0id

6 hours ago

There are lots of better ways of doing this, but knowing why this one is bad/wrong requires the mental model described upthread.

(But also, what you describe would be incorrect, since two <MAX values can add to a value that is >MAX, and overflow)

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Retr0id

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skydhash  6 hours ago

> But also, what you describe would be incorrect, since two <MAX values can add to a value that is >MAX, and overflow

I was maybe unclear. I meant, if you know a sum can introduce overflow (because you have a check right after), why not check the inputs before doing the sum, instead of checking the sum?

  • mbrock  6 hours ago

    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.