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!

You can find a general overview for the language at hand in "The rust reference"[1]. For a more formal document, you can have a look in to the ferroscene language specification list of undefined behaviour[2] section. From there you can jump to different section, and see legality rules, and undefined behavior sections for each.

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

No. UB is a term of art here.

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.

> If it optimizes and there are invalid assumptions then there is likely UB.

It's the distinguishing from bugs that concerns me.

It is a bug - you’ve violated the contract between the language and the compiler.

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)

Anything a compiler does with code which is UB is not a bug in the compiler. That's pretty much the definition of UB.

I don't follow. Isn't UB a subset of bugs or alternatively a follow on consequence that causes observable behavior to further deviate?

> Isn't UB a subset of bugs

No, not at all. UB can still produce correct and expected results for the entire input domain.

If I have a bug that only triggers between 9 and 10 am EST on Mondays that is still a bug, no? Now extend that to "rand(1.0) < 0.01". Now extend that to a check using __TIME__ that goes off at compile time instead of runtime (some binaries are buggy, some aren't). Now extend that to UB.

"can" is extremely different than "will"!