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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

On a conventional target, in general, code with undefined behavior is fine, as long as it doesn't actually execute at runtime.  People write source code based on this, and transforms duplicate code based on this.  And this makes diagnostics in the backend impractical.

I understand that BPF operates under different constraints than conventional targets... in particular, BPF-C is a subset of C.  It only allows programs that pass the BPF verifier.   So maybe you don't run into exactly the same issues with late diagnostics.  But, I'm not sure what the implications are here for transforms.  For example, can JumpThreading introduce an "unreachable" that triggers the error here?  If so, do we need to restrict JumpThreading on BPF?

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