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

[via llvm-commits]

yonghong-song wrote:

I also discussed with @WenleiHe a little bit about this. Another solution is to extend 'unreachable' insn in LLVM. For example, we could have 'unreachable' like below
    unreachable <reason>
The reason could include
  - due to switch statement where unreachable is truely unreachable
  - due to generating llvm.trap where any code after that is truely unreachable
  - maybe unreachable for naked function
  - unreachable due to code is optimized by taking advantage of undef behavior and it is actually reachable.
  
  With <reason> carried and passed from various optimization, downstream can reason about 'unreachable' insn. This will make BPF backend pass easier to only trigger error for <reason> due to undef bahavior.
  
WDYT? Is this a reasonable approach?

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