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

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

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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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[Bug 65416] r300g does not eliminate unread varyings

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

--- Comment #1 from Stefan Dösinger <stefandoesinger@gmx.at> ---
Fwiw, I don't know if r600g is affected. The HW has enough varyings to run the
unoptimized shaders, and the applications affected by this performance wise are
CPU limited on my r600g system.

Wine has some checks to prevent writing texture coordinates in the vertex
shader when there is no input to generate them. Unfortunately 3DMark 2000 and
Unreal Tournament 2004 use a strange vertex processing setup that breaks those
checks(D3DTSS_TEXCOORDINDEX of all texture stages is 0, thus they're reading
the first texture coordinate input, which does exist).

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[Bug 65416] r300g does not eliminate unread varyings

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

--- Comment #2 from Alex Deucher <agd5f@yahoo.com> ---
Couldn't this be done in a device independent manner in mesa when linking
shaders?  Drop outputs if there is no matching input in the subsequent shader
stage?

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[Bug 65416] r300g does not eliminate unread varyings

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

--- Comment #3 from Marek Olšák <maraeo@gmail.com> ---
I think the GLSL compiler only eliminates unused user-defined varyings, unused
legacy varyings are not eliminated. I'm taking this task.

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[Bug 65416] r300g does not eliminate unread varyings

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

--- Comment #4 from Stefan Dösinger <stefandoesinger@gmx.at> ---
Cool, thanks!

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[Bug 65416] r300g does not eliminate unread varyings

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

--- Comment #5 from Marek Olšák <maraeo@gmail.com> ---
I have implemented it, but there is a problem. If I enable the optimization,
EXT_separate_shader_objects must be disabled. Is it okay with you?

This is a valid sequence with EXT_sso:
glUseProgram(prog_with_vs_and_fs);
glUseShaderProgramEXT(prog_with_fs);

prog_with_vs_and_fs cannot be optimized, because the program object can be
bound partially (in this case only the vertex shader from the program object is
bound).

ARB_separate_shader_objects doesn't have this issue.

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[Bug 65416] r300g does not eliminate unread varyings

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

--- Comment #6 from Stefan Dösinger <stefandoesinger@gmx.at> ---
I guess that's ok for wined3d's purposes. We don't use either extension right
now, and if we ever use one of them we'll probably go with the ARB one anyway.

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[Bug 65416] r300g does not eliminate unread varyings

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

Marek Olšák <maraeo@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #7 from Marek Olšák <maraeo@gmail.com> ---
Fixed by a commit series which starts here:
http://cgit.freedesktop.org/mesa/mesa/commit/?id=74edd56927801e8c646c7d5cddba397c2f54b4ef

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