Ok, but in the context of the language at hand? Presumably the IR has distinct semantics from the language that generates the IR. Does UB just strictly resolve to LLVM UB? That's very reasonable!
The ferroscene language spec was recently donated to the rust foundation.
[1] https://doc.rust-lang.org/reference/behavior-considered-unde... [2] https://spec.ferrocene.dev/undefined-behavior.html
Consider a hypothetical non-LLVM full reimplementation of the compiler. If it optimizes and there are invalid assumptions then there is likely UB. LLVM isn't involved in that case though.
It's the distinguishing from bugs that concerns me.
No, not at all. UB can still produce correct and expected results for the entire input domain.
Just like segfault or logic bug, it’s a class of bugs. Why is special though is that in most bugs you just hit an invalid state. In UB you can end up executing code that never existed or not executing code that does exist. Or any number of other things can happen because the compiler applies an optimization assuming a runtime state you promised it would never occur but did.
It’s slightly different from being a strict subset because UB is actually exploited to perform optimizations - UB is not allowed so the compiler is able to emit more efficient code is taught to exploit that and the language allows for it (eg the niche optimization the blog describes)