Re: [PATCH v6 9/9] drm/amd/display: Mark dc_fixpt_from_fraction() noinline

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Dec 20, 2024 at 11:31:00AM +0100, Peter Zijlstra wrote:
> Also, curse the DRM Makefiles, you can't do:
> 
>   make drivers/gpu/drm/amd/display/dc/basics/fixpt31_32.s

Small tip: You can get the path of the target by building
drivers/gpu/drm/amd/amdgpu/ and finding it in the output. In this case,
it'd be

  $ make drivers/gpu/drm/amd/amdgpu/../display/dc/basics/fixpt31_32.s

Not excusing that it does not work as it should but sometimes you have
to work with what you can *shrug*

> > $ clang --version | head -1
> > clang version 20.0.0git (https://github.com/llvm/llvm-project.git
> > 8daf4f16fa08b5d876e98108721dd1743a360326)
> 
> So I didn't have a recent build at hand.. so I've not validated the
> below.
...
> If you put them size-by-side, you'll see it's more or less the same
> code-gen (trivial differences), but now it just stops code-gen, where
> previously it would continue.
> 
> So this really is a compiler problem, this needs no annotation, it's
> straight up broken.
> 
> Now, the thing is, these ASSERT()s are checking for divide-by-zero, I
> suspect clang figured that out and invokes UB on us and just stops
> code-gen.

Yeah, I think your analysis is spot on, as this was introduced by a
change in clang from a few months ago according to my bisect:

https://github.com/llvm/llvm-project/commit/37932643abab699e8bb1def08b7eb4eae7ff1448

Since the ASSERT does not do anything to prevent the divide by zero (it
just flags it with WARN_ON) and the rest of the code doesn't either, I
assume that the codegen stops as soon as it encounters the unreachable
that change created from the path where divide by zero would occur via

  dc_fixpt_recip() ->
    dc_fixpt_from_fraction() ->
      complete_integer_division_u64() ->
        div64_u64_rem()

Shouldn't callers of division functions harden them against dividing by
zero?

> Nathan, Nick, don't we have a compiler flag that forces __builtin_trap()
> whenever clang pulls something like this? I think UBSAN does this, but
> we really shouldn't pull in the whole of that for sanity.

Right, I think that LLVM has a hidden flag for this:

  -mllvm -trap-unreachable

That makes this particular warning disappear.

It isn't the greatest because '-mllvm' flags need to be passed along to
the linker for LTO but that's easy enough to deal with. I know we have
talked about enabling that flag in the past but I cannot remember why we
decided against it (maybe code size concerns and other optimization
restrictions)? It looks like GCC has a similar flag,
-funreachable-traps.

Cheers,
Nathan