[cfe-dev] [llvm-dev] Handling of FP denormal values

[Cameron McInally via cfe-dev]

On Mon, Sep 16, 2019 at 9:43 PM Matt Arsenault via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

>
>
> On Sep 16, 2019, at 19:57, Kaylor, Andrew via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>
> Do we need an ftz fast-math flag?
>
>
> This would be useful for matching a handful of AMDGPU instructions (a fmad
> that only always flushes being the most important). We have a dedicated
> intrinsic to allow flushing in this case when denormals are enabled
>

+1

For FTZ/DAZ, we're currently getting cases like this incorrect:

  %add = fadd nnan ninf nsz float %a, 0.000000e+00

That cannot be safely optimized to 'a' with FTZ/DAZ enabled. Although,
there's admittedly a small chance of problems, since a following FP
operation would normalize it, but here be dragons.

Are there any other facets to this problem that I've overlooked?
>
>
> For AMDGPU we need to split -denormal-fp-math into per-FP type flags (and
> the corresponding IR attribute). The denorm mode register has separate
> fields for f32, and f64+f16. The default for each of these is different
> depending on the subtarget/language combination. Mostly we want f64+f16 to
> always be on, and only change the f32 mode. The current naming implies
> changing all of the modes.
>
> The different sign of 0 modes as exist now aren’t available. There are
> however separate flags for enabling flushing on input and output. This
> isn’t particular important, and currently we just set both bits at the same
> time but it might be something to think about if this is being expanded.
>

At the command-line level, I don't see a lot of value in separating the two
flags. At the Function/Loop/Block/Instruction level, separating the two
would be more useful though. E.g. normalizing input/output; or sacrificing
accuracy to speed up a hot loop.
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