[Mesa-dev] [PATCH 1/2] glsl: Don't remove XFB-only varyings.

[Kenneth Graunke]

On Sunday, April 3, 2016 9:34:05 PM PDT Timothy Arceri wrote:
> On Sun, 2016-04-03 at 01:37 -0700, Kenneth Graunke wrote:
> > Consider the case of linking a program with both a vertex and
> > fragment
> > shader.  The VS may compute output varyings that are intended for
> > transform feedback, and not read by the fragment shader.
> > 
> > In this case, var->data.is_unmatched_generic_inout will be true,
> > but we still cannot eliminate the varyings.  We need to also check
> > !var->data.is_xfb_only.
> > 
> > Fixes failures in ES31-CTS.gpu_shader5.fma_precision_*, which happen
> > to use transform feedback in a way we apparently hadn't seen before.
> 
> Hmm ... are you sure there is not something else going wrong here
> somewhere? is_xfb_only is set in the same code block that should change
> is_unmatched_generic_inout to false.
> 
>       if (matched_candidate->toplevel_var->data.is_unmatched_generic_inout) 
{
>          matched_candidate->toplevel_var->data.is_xfb_only = 1;
>          matches.record(matched_candidate->toplevel_var, NULL);
>       }
> 
> matches.record() should set is_unmatched_generic_inout to false and
> setup everything else needed for xfb. Seems like there is another bug
> here somewhere.

The shader actually contains:

    layout(location = 0) out flat int outTextureSize;

So it triggers the very first condition of matches.record():

   if ((producer_var && (!producer_var->data.is_unmatched_generic_inout ||
       producer_var->data.explicit_location)) ||
       (consumer_var && (!consumer_var->data.is_unmatched_generic_inout ||
       consumer_var->data.explicit_location))) {
      /* Either a location already exists for this variable (since it is part
       * of fixed functionality), or it has already been recorded as part of a
       * previous match.
       */
      return;
   }

and so it returns early.  No match is recorded, and it never sets
producer_var->data.is_unmatched_generic_inout = 0.

Do you think we should do some of it anyway?

--Ken
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