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Overflowing integers is not a bug. It's literally how an adder circuit operates on fixed precision. Ignoring them by default is worth criticizing but there are

Overflowing integers is not a bug. It's literally how an adder circuit operates on fixed precision. Ignoring them by default is worth criticizing but there are plenty of times where silent overflow is correct and desirable behavior.

tomp 4y ago
It's technically not a "bug" in Rust or C++, it's undefined behaviour, which is much worse than a bug! (and it's never correct behaviour)
- still_grokking 4y ago
  Little nitpick: In C/C++ it's "only" UB for signed ints. For unsigned ints it's arithmetic modulo 2^n (where n is the number of bits in the value representation of that particular size of integer). Such divergent definitions make the whole mess of course even bigger, that's not the point.
  - pratk 4y ago
    Also, the UB can be mitigated with -fwrapv (if desired) to handle overflowing integers with 2's complement
    - still_grokking 4y ago
      Stupid question, but how can a compiler flag "mitigate" some UB that is (to my knowledge) defined by the standard as UB? This would be possible only if the standard would define the behavior in question as implementation defined. But that's not the case afaik. Signed overflow is undefined behavior as it's afaik defined to be that. But maybe I'm just wrong here? But at least SO is confining my knowledge: https://stackoverflow.com/questions/16188263/is-signed-integer-overflow-still-undefined-behavior-in-c https://stackoverflow.com/questions/16188263/is-signed-integ...
      - pratk 4y ago
        -fwrapv forces the compiler to handle signed overflow with 2's complement. UB essentially means the compiler is free to do anything, and with the option it chooses the wrapping behavior. See https://stackoverflow.com/questions/47232954/what-does-fwrapv-do https://stackoverflow.com/questions/47232954/what-does-fwrap...
- steveklabnik 4y ago
  It’s not UB in Rust.
still_grokking 4y ago
> but there are plenty of times where silent overflow is correct and desirable behavior When or where? I would say it's always a bug in a program—except when (mis)using ints as bit-fields in very special circumstances. Simply, as more or less every computation in the real world is build around the concept of whole numbers, and not around the idea of overflowing two-complement bit-fields. The later is so exotic that there is afik almost no proper use for it (the only exception being as a very low level building block to enable computers do actually useful calculations).

tomp 4y ago

It's technically not a "bug" in Rust or C++, it's undefined behaviour, which is much worse than a bug! (and it's never correct behaviour)

still_grokking 4y ago
Little nitpick: In C/C++ it's "only" UB for signed ints. For unsigned ints it's arithmetic modulo 2^n (where n is the number of bits in the value representation of that particular size of integer). Such divergent definitions make the whole mess of course even bigger, that's not the point.
- pratk 4y ago
  Also, the UB can be mitigated with -fwrapv (if desired) to handle overflowing integers with 2's complement
  - still_grokking 4y ago
    Stupid question, but how can a compiler flag "mitigate" some UB that is (to my knowledge) defined by the standard as UB? This would be possible only if the standard would define the behavior in question as implementation defined. But that's not the case afaik. Signed overflow is undefined behavior as it's afaik defined to be that. But maybe I'm just wrong here? But at least SO is confining my knowledge: https://stackoverflow.com/questions/16188263/is-signed-integer-overflow-still-undefined-behavior-in-c https://stackoverflow.com/questions/16188263/is-signed-integ...
    - pratk 4y ago
      -fwrapv forces the compiler to handle signed overflow with 2's complement. UB essentially means the compiler is free to do anything, and with the option it chooses the wrapping behavior. See https://stackoverflow.com/questions/47232954/what-does-fwrapv-do https://stackoverflow.com/questions/47232954/what-does-fwrap...
steveklabnik 4y ago
It’s not UB in Rust.

still_grokking 4y ago

> but there are plenty of times where silent overflow is correct and desirable behavior When or where? I would say it's always a bug in a program—except when (mis)using ints as bit-fields in very special circumstances. Simply, as more or less every computation in the real world is build around the concept of whole numbers, and not around the idea of overflowing two-complement bit-fields. The later is so exotic that there is afik almost no proper use for it (the only exception being as a very low level building block to enable computers do actually useful calculations).