https://bugs.freedesktop.org/show_bug.cgi?id=104143

            Bug ID: 104143
           Summary: r600/sb: clobbers gl_Position -> gl_FragCoord
           Product: Mesa
           Version: git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/Gallium/r600
          Assignee: dri-devel@lists.freedesktop.org
          Reporter: gw.fossdev@gmail.com
        QA Contact: dri-devel@lists.freedesktop.org

Created attachment 135998
  --> https://bugs.freedesktop.org/attachment.cgi?id=135998&action=edit
piglit drawing output with sb enabled

With a variation of the piglit 

    spec/glsl-1.10/execution/variable-indexing/vs-output-array-vec2-index-wr

r600/sb clobbers the gl_Position/gl_FragCoord.

The variation consists in replacing two arrays of vec2 by one array of vec4 and
swizzling the elements to achieve the same result (by effectively interleaving
the two arrays). 

A few observations: 

* As can be seen from the colour coding screen shots, the array content is  
correctly passed from the vertex shader to the fragment shader. 

* The error only occurs for the uniform index value 0. 

* When passing an additional parameter that contains a copy of gl_Position,
then   this parameter seems to have the correct value (i.e. the vertex shader
correctly evaluates gl_Position).

* The byte code doesn't doesn't give any obvious indication why things go wrong
with the optimized shader. 

My mesa is at fa8c1b92b7. 

best, 
Gert

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