[Mesa-dev] NaN behavior in GLSL (was Re: [PATCH] glsl: always do sqrt(abs()) and inversesqrt(abs()))

[Jason Ekstrand]

On Fri, Jan 13, 2017 at 4:05 AM, Marek Olšák <maraeo at gmail.com> wrote:

> On Fri, Jan 13, 2017 at 3:37 AM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> > On Thu, Jan 12, 2017 at 9:13 PM, Jason Ekstrand <jason at jlekstrand.net>
> wrote:
> >> Unless, of course, it's controlled by the same hardware bit... Clearly,
> we
> >> can can give you abs on rsq without denorm flushing (easy shader hacks)
> but
> >> not the other way around.
> >
> > OK, so somehow I missed that earlier. However there's an interesting
> > section in the PRM:
> >
> > https://01.org/sites/default/files/documentation/intel-gfx-
> prm-osrc-skl-vol07-3d_media_gpgpu.pdf
> >
> > on PDF page 854, "Dismissed Legacy Behaviors" which has a list of
> > suggested IEEE 754 deviations for DX9. One of them is indeed that 0 *
> > x = 0, but another is that input NaNs be propagated with certain
> > exceptions. Also they suggest that RCP(0)/RSQ(0) = fmax. Interesting.
> >
> > So at this point, the zero_wins thing is pretty much blown. i965
> > appears to have an all-or-nothing approach, and additionally that
> > approach doesn't match up exactly to what NVIDIA does (or at least I'm
> > not aware of a clamp-everything mode).
> >
> > This will take some thought to figure out how something can be
> > specified so that a single spec works for both i965 and nv/amd. OTOH
> > we could have two different specs that just expose different things -
> > e.g. i965 could expose a MESA_shader_float_alt_mode or whatever which
> > is spec'd to do the things that the PRM says, and nv/amd have the
> > MESA_shader_float_zero_wins ext which does what we were talking about
> > earlier.
> >
> > I'm open to other suggestions too.
>
> There is also the "small" problem that it would take a non-trivial
> effort for us on the LLVM side. You guys can flip a switch. We can't.
>

Don't you have to expend that effort for ARB programs anyway?  I thought
they weren't supposed to generate NaN either.
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