[PATCH] D22898: AMDGPU: Fix ffloor for SI

[Marek Olšák via llvm-commits]

mareko added a subscriber: mareko.
mareko added a comment.

In https://reviews.llvm.org/D22898#501345, @arsenm wrote:

> In https://reviews.llvm.org/D22898#501301, @arsenm wrote:
>
> > In https://reviews.llvm.org/D22898#499043, @nhaehnle wrote:
> >
> > > Is the MIN needed for correctness at all? Looking at the workaround docs, I see the explanation that "[FRACT] is outputting 1.0 for very small negative inputs). Sounds to me like v_fract is correctly in the range [0, 1.0), except for those very small negative inputs, where it returns 1.0 (which happens to be correct for the ffloor lowering).
> >
> >
> > I guess so? I don't know the details of the bug but this passes conformance now
>
>
> Thinking about it more this makes sense. 1.0 will skip the fract at exactly 1.0. up to 0.99... fract is used


How does it make any sense? fract should return values in [0, 1). SI has a bug that it returns 1 incorrectly in one case. Doing min(x, 1) will have no effect on the result of buggy fract. That min() is a no-op operation.


https://reviews.llvm.org/D22898