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

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

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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@lists.freedesktop.org
          Reporter: jlegg@feralinteractive.com
        QA Contact: dri-devel@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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[Bug 95130] Derivatives of gl_Color wrong when helper pixels used

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

--- Comment #1 from James Legg <jlegg@feralinteractive.com> ---
Created attachment 123240
  --> https://bugs.freedesktop.org/attachment.cgi?id=123240&action=edit
Apitrace trace of test program

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

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

--- Comment #2 from James Legg <jlegg@feralinteractive.com> ---
Created attachment 123241
  --> https://bugs.freedesktop.org/attachment.cgi?id=123241&action=edit
Appearance when test is failing

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

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

--- Comment #3 from Nicolai Hähnle <nhaehnle@gmail.com> ---
Hi James, thank you for the detailed report. I can reproduce this and am going
to investigate.

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

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

--- Comment #4 from Nicolai Hähnle <nhaehnle@gmail.com> ---
As a temporary workaround, the error should disappear when you run with the
environment variable setting R600_DEBUG=mono.

The cause is that with non-monolithic shaders, input interpolation happens in
the prolog. However, when we compile the prolog we do not see the derivative
computations and therefore fail to turn the helper pixels on.

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

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

Nicolai Hähnle <nhaehnle@gmail.com> changed:

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

--- Comment #5 from Nicolai Hähnle <nhaehnle@gmail.com> ---
This is now fixed in Mesa Git b42bc90b6add0d0f81d915d49712761d32329afa with
LLVM r272063.

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