[Mesa-dev] [PATCH 1/2] nir: add a compiler option for disabling float comparison simplifications

[Jason Ekstrand]

On Fri, Nov 30, 2018 at 4:34 PM Ian Romanick <idr at freedesktop.org> wrote:

> On 11/30/2018 01:29 PM, Jason Ekstrand wrote:
> > On Fri, Nov 30, 2018 at 3:18 PM Ian Romanick <idr at freedesktop.org
> > <mailto:idr at freedesktop.org>> wrote:
> >
> >     On 11/29/2018 07:47 AM, Connor Abbott wrote:
> >     > On Thu, Nov 29, 2018 at 4:22 PM Jason Ekstrand
> >     <jason at jlekstrand.net <mailto:jason at jlekstrand.net>> wrote:
> >     >>
> >     >> Can you provide some context for this?  Those rules are already
> >     flagged "inexact" (that's what the ~ means) so they won't apply to
> >     anything that's "precise" or "invariant".
> >     >
> >     > I think the concern is that this isn't allowed in SPIR-V, even
> without
> >     > exact or invariant. We even go out of our way to do the correct
> thing
> >     > in the frontend by inserting an "&& a == a" or "|| a != a", but
> then
> >
> >     If you're that paranoid about it, why not just mark the operations
> are
> >     precise?  That's literally why it exists.
> >
> >     > opt_algebraic removes it with another rule and then this rule can
> flip
> >     > it from ordered to unordered. The spec says that operations don't
> have
> >     > to produce NaN, but it doesn't say anything on comparisons other
> than
> >     > the generic "everything must follow IEEE rules" and an entry in the
> >     > table that says "produces correct results." Then again, I can't
> find
> >     > anything in GLSL allowing these transforms either, so maybe we just
> >     > need to get rid of them.
> >
> >     What I hear you saying is, "The behavior isn't defined."  Unless you
> can
> >     point to a CTS test or an application that has incorrect behavior,
> I'm
> >     going to oppose removing this pretty strongly.  *Every* GLSL compiler
> >     does this.
> >
> >
> > The test case came from VKD3D which does D3D12 on Vulkan.  Someone
> > (Samuel, maybe?) was going to ask around and see if we can figure out
> > what D3D12's rules are.  It's possible that it requires IEEE or
> > something close.  If that's the case, as I said to Samuel on IRC, we're
> > probably looking at an extension.  I don't think we want a flag like
> > this that's set per-API.
>
> Why isn't it sufficient to mark the operation as precise?  Was that on
> IRC, and I missed it?
>
> It is also possible that we could improve the handling of 'if
> (!condition)' in the backend to make these transformations less
> important.  I have some patches for that somewhere.  They didn't really
> help with these transformations in place, and I never measured the
> result without these transformations.
>

I think we can and that would be better than this flag.

--Jason
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/mesa-dev/attachments/20181130/3b0699ce/attachment.html>