[PATCH] D156497: [BPF] Emit UNDEF rather than constant 0

[Tamir Duberstein via Phabricator via llvm-commits]

tamird added a comment.

In D156497#4562602 <https://reviews.llvm.org/D156497#4562602>, @yonghong-song wrote:

> So if I understand correctly, BPF backend can handle clang frontend generated IR properly and issue proper warning. But with rust frontend, the IR is a little bit different and BPF backend will behave differently and allow to generate illegal code. Is this right? The bpf-unsupported-emit.c is exactly the case to demonstrate that rust frontend may generate different IR and the BPF backend will generate incorrect code. Is my understanding correct?

No, that's not correct. As you can see, all the IR in the tests I added was generated from C. There's nothing about the IR generated from Rust that is to blame.

The problem is that it's very easy to accidentally *not* exit from the diagnostic handler, as the test demonstrate -- and in case you don't, you will get "success" with broken code.

> Regarding to diag from constant 0 to undef, I am not 100% confident that this will solve all the problems. What if now r0 is undef but later continued compilation generates an assignment to r0? So we should solve the problem to ensure for buggy code the compilation fail rather succeed.

Yeah, that's a fair point. Do you have ideas on how to achieve that?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D156497/new/

https://reviews.llvm.org/D156497