[Mesa-dev] [PATCH v3 024/104] nir: Support deref instructions in lower_system_values

[Jason Ekstrand]

On Mon, Apr 9, 2018 at 5:21 PM, Caio Marcelo de Oliveira Filho <
caio.oliveira at intel.com> wrote:

> Hi,
>
> > >> Question: nir_deref_instr_get_variable will walk the deref instr
> > >> chain, but does it even make sense if there's a array or struct in
> > >> this deref chain? Should this be asserted?
> > >>
> > >
> > > That's an interesting question.  Certainly, at this point in the patch
> > > series, we can't make that assumption.  This is because we haven't
> checked
> > > the mode yet.  However, once we can assume deref instructions, we can
> check
> > > the mode and then go on to find the var.  Maybe something like this
> > > (untested):
> > >
> > > https://gitlab.freedesktop.org/jekstrand/mesa/commit/
> > > 33aee39955eff842d6ea263da2f36e60287e62ee
> > >
> >
> > It turns out that there is one system value which is an array:
> > gl_SampleMask.  However, due to details, we only ever load element 0 so
> we
> > can ignore the array deref in that case.  Unfortunately, this means that
> we
> > can't do any better than what we have here. :-(
>
> I think we could still be strict while handling that case, by being
> explicit about it in the middle of the patch you shared:
>
>     nir_deref *deref = nir_src_as_deref(load_deref->src[0]);
>     if (deref->mode != nir_var_system_value) {
>        continue;
>     }
>
>     if (deref->deref_type != nir_deref_type_var) {
>        assert(deref->deref_type == nir_deref_type_array);
>        assert(nir_instr_get_variable(deref)->data.location ==
> SYSTEM_VALUE_SAMPLE_MASK);
>        /* Short explanation that we only load ever position zero, maybe
> even assert... */
>        deref = nir_deref_instr_parent(deref);
>     }
>
>     assert(deref->deref_type == nir_deref_type_var);
>     nir_variable *var = deref->var;
>
> Would something like that work?
>

I took another swing at it, and this one seems to make Jenkins happy:

https://gitlab.freedesktop.org/jekstrand/mesa/commit/ad3cc9f301da3519d4f76767a6d9e98e5a5c118e
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