[Bug 65416] New: r300g does not eliminate unread varyings

[bugzilla-daemon@freedesktop.org]

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https://bugs.freedesktop.org/show_bug.cgi?id=65416

          Priority: medium
            Bug ID: 65416
          Assignee: dri-devel@lists.freedesktop.org
           Summary: r300g does not eliminate unread varyings
          Severity: normal
    Classification: Unclassified
                OS: All
          Reporter: stefandoesinger@gmx.at
          Hardware: Other
            Status: NEW
           Version: git
         Component: Drivers/Gallium/r300
           Product: Mesa

Created attachment 80344
  --> https://bugs.freedesktop.org/attachment.cgi?id=80344&action=edit
Example shaders

r300g tries to interpolate varyings written by the vertex shader even if the
fragment shader does not read them. The attached program code illustrates this
with two sample shaders.

The shaders in question were generated by Wine's fixed function pipeline
replacement, generated from a fixed function setup set up by 3DMark 2000.

The visible issues caused by this are broken fog because the driver runs out of
varyings and reduced performance due to the extra shader instructions.

An argument could be made that this is Wine's bug, and it should not generate
such inefficient shaders. Doing this would be a major inconvenience though
because the vertex and fragment shader are generated independently. As far as I
can see the proprietary drivers optimize this inefficiency away.

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