[Bug 95130] Derivatives of gl_Color wrong when helper pixels used

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=95130

            Bug ID: 95130
           Summary: Derivatives of gl_Color wrong when helper pixels used
           Product: Mesa
           Version: git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/Gallium/radeonsi
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: jlegg at feralinteractive.com
        QA Contact: dri-devel at lists.freedesktop.org

Created attachment 123239
  --> https://bugs.freedesktop.org/attachment.cgi?id=123239&action=edit
Test program

Along the edges of triangles and around where fragments are rejected due to
depth testing, the GLSL functions dFdx, dFdy, and fwidth return incorrect
values when the argument is dependant on gl_Color or gl_SecondaryColor.

I've attached a program reproducing the issue. It takes derivatives of a
constant, which should be 0. It is expected to produce a black window. However,
the edges of the triangle it draws are visible because of this bug.

The error does not occur when environment variable LIBGL_ALWAYS_SOFTWARE is set
to 1.

I'm using Mesa git 0831eb9 with an AMD R9 270 on x86_64 Fedora 22.

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