Re: clang and drm issue: objtool warnings from clang build

[Nathan Chancellor <nathan@kernel.org>]

On Sat, Apr 26, 2025 at 01:56:59PM -0700, Linus Torvalds wrote:
> IOW, there's a *big* difference between "the programmer told me this
> is unreachable, so I won't generate code past this point" and "I have
> decided this is undefined behavior, so now I won't generate code past
> this point".
> 
> So what I'm asking for is absolutely not "trap on unreachable". That's
> wrong and just plain stupid.
> 
> I'm asking for "trap on UD instead of *assuming* it's unreachable".
> 
> Because clearly that code *can* be reached, it's just doing something undefined.
> 
> See? Big big difference.

Ah yes, that is a big yet subtle difference that I had not considered,
my bad for missing that. I was only thinking about the implicitly
inserted __builtin_unreachable() from potential UB, not the explicitly
added ones from the developers.

I suspect that it would not be easy to split that distinction in LLVM
but since I am not a compiler person, I will see if this has come up
before and talk to people otherwise. I know there has been work in LLVM
to try and stop undefined behavior from destroying control flow with
things like the freeze instruction but I am not sure that would help us
in this situation. Pardon my ignorance though, isn't something like this
basically just '-fsanitize=undefined -fsanitize-trap=all'?

Cheers,
Nathan