Re: Undefined Behavior Optimizations in C

gah4 <gah4@u.washington.edu> writes:
[snip]
> For many years, C allowed for sign-magnitude and ones' complement
> representation.


It still does. The upcoming 2023 ISO C standard mandates 2's-complement
for signed integer types, but it hasn't been published yet.


[...]


> Now, C has unsigned int which you can use when you need specific
> overflow behavior (except on some Unisys machines).


If a Unisys C implementation doesn't behave as the standard requires
with respect to unsigned overflow, then it's a non-conforming
implementation. (Non-conforming implementations can still be useful.)


[...]


> As C allows for both UB and system-dependent behavior, and it is
> hard for people to remember every case of each one, it is unreasonable
> to me, to assume fixed point overflow is UB.


C has:
- Undefined behavior (the standard imposes no requirements);
- Unspecified behavior (the standard provides 2 or more possibilities
    and implementations can choose arbitrarily; an example is the order of
    evaluation of function arguments); and
- Implementation-defined behavior (unspecified behavior where the
    implementation must document its choice).


Signed integer overflow has undefined behavior -- not because it is or
isn't reasonable, but because the standard says so.


Even in C23, the mandate for 2's-complement representation doesn't
imply a requirement for 2's-complement behavior on overflow.


[...]


--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for XCOM Labs
void Void(void) { Void(); } /* The recursive call of the void */

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