UB doesn't mean there will be nasal demons. It means there can be nasal demons, if the implementation says so. It means the language standard does not define a behavior. POSIX can still define the behavior. The implementation can still define the behavior.
Plenty of things are UB just because major implementations do things wildly differently. For example:
realloc(p, 0)
> UB doesn't mean there will be nasal demons. It means there can be nasal demons, if the implementation says so.
Rather "if the implementation doesn't say otherwise".
Generally speaking compiler writers are not mustache-twirling villains stroking a white cat thinking of the most dastardly miscompilation they could implement as punishment. Rather they implement optimisation passes hewing as close as they can to the spec's requirements. Which means if you're out of the spec's guarantees you get whatever emergent behaviour occurs when the optimisation passes run rampant.
This is both factually incorrect and philosophically unsound.
Every asm or IR instruction is emitted by the compiler. It isn't a "doesn't say otherwise" kind of thing. Whatever the motivations are, the compiler and its authors are responsible for everything that results.
"if you're out of the spec's guarantees you get whatever emergent behaviour occurs" is simply and patently not factual. There isn't a single compiler in existence for which this is true. Every compiler makes additional guarantees beyond the ISO standard, sometimes due to local dialect, sometimes due to other standards like POSIX, sometimes controlled by configuration or switches (e.g., -fwrapv).
Yeah that’s just really bad language design. Which, again, literally no modern languages do because it’s just terrible horrible awful no good very bad design.
It's describing rather than prescribing, which yeah isn't really design. Most modern languages don't even (plan to) have multiple implementations, much less a standard.
All of that implementation freedom is also available if the behavior is erroneous instead. Having it defined as UB just gets you nasal demons, which incidentally this rule leads to on modern compilers. For example:
https://godbolt.org/z/ncaKGnoTb