[llvm] [RFC][BPF] Report Unreachable Behavior from IR (PR #126858)

[via llvm-commits]

eddyz87 wrote:

I was curious if there are other cases besides switch instruction, where `unreachable` could survive up to codegen. Unfortunately, `unreachable` is generated in a lot of places with complex logic, e.g. loop fusion or SCCP. It looks like it is impossible to know the answer for sure.

Given this, I tend to agree that a runtime trap would be a simpler solution. E.g. something along the lines libbpf does [here](https://github.com/libbpf/libbpf/blob/42a6ef63161a8dc4288172b27f3870e50b3606f7/src/libbpf.c#L6062), where it inserts a call to unknown helper function. In kernel `verifier.c:get_helper_proto()` is only called from `check_helper_call()` on main verification pass. Meaning that if `unreachable` is a dead code, verifier would delete it, otherwise it would report an error.

Verifier can be changed to report something about undefined behaviour trap to simplify debugging. BPF backend can be changed to avoid clobbering registers because of this special trap call.

Just my 5 cents.

https://github.com/llvm/llvm-project/pull/126858