4 ms·

Overflow from signed arithmetic always results in UB. But an unsigned-to-signed conversion outside the range of the target type just results in implementation-d

Overflow from signed arithmetic always results in UB. But an unsigned-to-signed conversion outside the range of the target type just results in implementation-defined behavior; see C17 §6.3.1.3 ("Signed and unsigned integers"). And most people these days (including me) take the simple approach of not supporting any implementation that doesn't just perform two's-complement wrapping for all conversions.

jcranmer 3y ago
> And most people these days (including me) take the simple approach of not supporting any implementation that doesn't just perform two's-complement wrapping for all conversions. C2x in fact requires 2's complement.
- LegionMammal978 3y ago
  Not quite; C23 requires a two's-complement representation, but it doesn't require two's-complement wrapping for unsigned-to-signed conversions. §6.3.1.3 hasn't been changed, so it's still left to the implementation to guarantee that. The only easily-visible effects of requiring a two's-complement representation are that X_MIN == -X_MAX - 1 for all signed integer types, and in general that the object representations of x and -x - 1 differ by only 1 bit, if the integer type has no padding bits.
  - j16sdiz 3y ago
    Is there any modern day architecture don't use twos complements?
    - olliej 3y ago
      GPUs notionally - I can't speak for truly "modern" GPUs but I recall older GPUs having language level "char"/"byte" implemented as a float (presumably with some minimal support to get expected semantics like clamping?)
  - mananaysiempre 3y ago
    Ah, so the “two’s complement from outer space” option (with X_MIN == -X_MAX and a trap representation in place of -XMAX-1) is also finally gone? I did not notice that, thanks. C18 6.2.6.2p3 had: > [It] is implementation-defined [...] whether the value with sign bit 1 and all value bits zero [...] is a trap representation or a normal value [for two’s complement].

jcranmer 3y ago

> And most people these days (including me) take the simple approach of not supporting any implementation that doesn't just perform two's-complement wrapping for all conversions. C2x in fact requires 2's complement.

LegionMammal978 3y ago
Not quite; C23 requires a two's-complement representation, but it doesn't require two's-complement wrapping for unsigned-to-signed conversions. §6.3.1.3 hasn't been changed, so it's still left to the implementation to guarantee that. The only easily-visible effects of requiring a two's-complement representation are that X_MIN == -X_MAX - 1 for all signed integer types, and in general that the object representations of x and -x - 1 differ by only 1 bit, if the integer type has no padding bits.
- j16sdiz 3y ago
  Is there any modern day architecture don't use twos complements?
  - olliej 3y ago
    GPUs notionally - I can't speak for truly "modern" GPUs but I recall older GPUs having language level "char"/"byte" implemented as a float (presumably with some minimal support to get expected semantics like clamping?)
- mananaysiempre 3y ago
  Ah, so the “two’s complement from outer space” option (with X_MIN == -X_MAX and a trap representation in place of -XMAX-1) is also finally gone? I did not notice that, thanks. C18 6.2.6.2p3 had: > [It] is implementation-defined [...] whether the value with sign bit 1 and all value bits zero [...] is a trap representation or a normal value [for two’s complement].