[Mesa-dev] SpvOpSelect w/ float operands

[Connor Abbott]

On Tue, Nov 17, 2020 at 9:56 PM Brian Paul <brianp at vmware.com> wrote:
>
> On 11/17/2020 11:49 AM, Ian Romanick wrote:
> > On 11/17/20 9:25 AM, Brian Paul wrote:
> >>
> >> Using the Intel Vulkan driver, we've found some cases where SpvOpSelect
> >> is returning -0.0 (negative zeros) instead of normal 0.0 depending on
> >> the arguments.
> >
> > Do you have a specific test case that fails?
>
> Yeah, but as with the NMin/NMax issue it's not a simple test case.  It
> comes from a Windows WHCK test suite.
>
>
> > It seems like on some platforms there was an errata about the version of
> > the SEL instruction that is used for min or max that can return the
> > wrong signed zero in some cases.
> >
> > It's also possible that some optimizations are causing problems.  I
> > don't remember exactly how it works in SPIR-V, but does marking those
> > SPIR-V instructions as precise (that's what it was in GLSL) make a
> > difference?
>
> AFAIK, there's only a SPIR-V decoration for tagging things for relaxed
> precision.

The SPIR-V equivalent of "precise" is NoContract. But for emulating DX
stuff you're supposed to use KHR_shader_float_controls which was
specifically designed for emulating DX floating-point requirements. In
NIR that just marks everything as "exact" if you force correct
NaN/signed zero/Inf handling.

>
> -Brian
>
> >
> >> I'm wondering if "SpvOpSelect x, a, b" for floats is being implemented
> >> with something like "a*x + b*(1-x)" ?  That might explain where the
> >> negative zeros are coming from.
> >>
> >> Our work-around is to implement selection with bitwise operations: (a &
> >> x) | (b & ~x)
> >>
> >> It seems to me that SpvOpSelect shouldn't interpret the bits and just
> >> return an exact copy of the argument.
> >>
> >> -Brian
> >> _______________________________________________
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