[PATCH] D22898: AMDGPU: Fix ffloor for SI

[Matt Arsenault via llvm-commits]

arsenm added a comment.

In https://reviews.llvm.org/D22898#518537, @mareko wrote:

> Yeah I know about the CI bug, but it's not important for OpenGL.
>
> 0x3ff0000000000000 is 1.0.
>  0x3fefffffffffffff isn't 1.0. It's the largest number smaller than 1.0, which is 0.9999999999999........... it can also be written as: bitcast(1.0, i32) - 1
>  If you print 0x3fefffffffffffff with 6 decimal places, it will be rounded to 1.0 for the purpose of printing. I guess that's where the confusion comes from.
>
> I don't know why 1.0 passes OpenCL conformance and bitcast(1.0, i32) - 1 doesn't. I suggest you check what the closed compiler does in this case.


Closed OpenCL on the AMDIL path uses a library expansion for floor, and doesn't try to use any of these instructions

In https://reviews.llvm.org/D22898#518537, @mareko wrote:

> Yeah I know about the CI bug, but it's not important for OpenGL.
>
> 0x3ff0000000000000 is 1.0.
>  0x3fefffffffffffff isn't 1.0. It's the largest number smaller than 1.0, which is 0.9999999999999........... it can also be written as: bitcast(1.0, i32) - 1
>  If you print 0x3fefffffffffffff with 6 decimal places, it will be rounded to 1.0 for the purpose of printing. I guess that's where the confusion comes from.
>
> I don't know why 1.0 passes OpenCL conformance and bitcast(1.0, i32) - 1 doesn't. I suggest you check what the closed compiler does in this case.


It implements floor with a library expansion with bitops and doesn't attempt to use fract (though I don't think this is intentional). The custom floor lowering that I found was dead deleted in this patch also passes conformance


https://reviews.llvm.org/D22898