Re: Undefined Behavior Optimizations in C

On Wednesday, January 18, 2023 at 8:35:40 AM UTC-8, Spiros Bousbouras wrote:
...
> I have encountered several times the claim that compilers assume that UB does
> not happen and I don't understand it. Lets consider 2 examples :
>
> x + 1 > x
>
> in C where x is a signed integer. Compilers will often treat this as
> always true with the following reasoning :
>
> - if x does not have the maximum value which fits in its type then the
> meaning of the C expressions is the same as their mathematical meaning
> so the expression evaluates to true.
>
> - if x has the maximum value which fits in its type then x + 1 is not
> defined so any translation (including treating the whole expression as
> true) is valid.
>
> There's no assumption that UB (undefined behaviour) will not happen, both
> possibilities are accounted for.


I believe in a case like this a modern C/C++ compiler reasons that x must be
less than the maximum representable value and it generates code according
to this, possibly removing dead code that depends on x being the maximum
representable value. If the compiler's assumption that x is less than the
maximum is wrong, it's perfectly fine, it's UB, any "broken" code generated
is allowed.


Alex

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