[lmkg]

> If it only exists in some hardware, how should the standard deal with that?

Generally seems to me the C standard makes things like that UB. Signed integer overflow, for example. Implemented as wrapping two's-complement on modern architectures, defined as such in many modern languages, but UB in C due to ongoing support for niche architectures.

The issues around pointer provenance are inherent to the C abstract machine. It's a much more immediate show-stopper on architectures that don't have a flat address space, and the C abstract machine doesn't assume a flat address space because it supports architecture where that's not true. My understanding is that reflects some oddball historical architectures that aren't relevant anymore, nowadays that includes CHERI.

[uecker]

Historically, the reason was was often niche architectures. But sometimes certain behavior dies out and we can make semantics more strict. For example, two's complement is now a requirement for C. Still, we did not make signed overflow defined. The reasons are optimization and - maybe surprising for some - safety. UB can be used to insert the compile-time checks we need to make things safe, but often we can not currently require everyone to do this. At the same time, making things defined may make things worse. For example, finding wraparound bugs in unsigned arithmetic - though well-defined - is a difficult and serious problem. For signed overflow, you use a compiler flag and this is not exploitable anymore (could still be a DoS).