[Bug 103743] Nier:Automata - "if" in fragment shader incorrectly evaluated, causes artifacts

[bugzilla-daemon@freedesktop.org]

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https://bugs.freedesktop.org/show_bug.cgi?id=103743

            Bug ID: 103743
           Summary: Nier:Automata - "if" in fragment shader incorrectly
                    evaluated, causes artifacts
           Product: Mesa
           Version: git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/Gallium/radeonsi
          Assignee: dri-devel@lists.freedesktop.org
          Reporter: philip.rebohle@tu-dortmund.de
        QA Contact: dri-devel@lists.freedesktop.org

Created attachment 135457
  --> https://bugs.freedesktop.org/attachment.cgi?id=135457&action=edit
Fragment shader with workaround

When compiling Mesa against LLVM >= 5.0, Nier:Automata shows lighting artifacts
that are caused by a misbehaving 'if' in a fragment shader.

Apitrace that shows the issue (2.5G, bz2-compressed):
https://mega.nz/#!JaBD0IoT!yBekRb7ZmvsSY60mo0E7N6uwaYfp_KfRDlnQWL0-QOo

I attached a copy of the fragment shader that causes the problem, but also
includes a workaround. The file name for MESA_SHADER_READ_PATH is
FS_feba74122c9590d1522b92bbec52d662ecd99012.glsl.

The offending code is located at line 135:

    R1.x = uintBitsToFloat(floatBitsToUint(R0).y >= 0x4u ? 0xffffffffu : 0u);
    if (bool(floatBitsToUint(R1).x))

The artifacts go away when changing the original 'if' condition to the
following:

    if (floatBitsToUint(R0).y >= 0x4u)

This happens with both Mesa 17.2 and latest Mesa-git when compiling against
LLVM 5.0 or later. This does *not* happen with LLVM 4.0.1. My GPU is an RX 480.

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