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

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

[bugzilla-daemon]

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

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

--- Comment #1 from Philip Rebohle <philip.rebohle@tu-dortmund.de> ---
Created attachment 135458
  --> https://bugs.freedesktop.org/attachment.cgi?id=135458&action=edit
Frame 2400 with artifacts

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

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

--- Comment #2 from Philip Rebohle <philip.rebohle@tu-dortmund.de> ---
Created attachment 135459
  --> https://bugs.freedesktop.org/attachment.cgi?id=135459&action=edit
Frame 2400 without artifacts

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

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

--- Comment #3 from Philip Rebohle <philip.rebohle@tu-dortmund.de> ---
PS: While bug 102218 is not directly related to this one, that's another case
where two shaders that *should* be functionally equivalent yield different
results. In the second screenshot, workarounds for both issues are applied in
order to obtain correct rendering.

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

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

--- Comment #4 from Nicolai Hähnle <nhaehnle@gmail.com> ---
Thanks for the very lucid bug report. I can reproduce this and am
investigating.

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

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

--- Comment #5 from Philip Rebohle <philip.rebohle@tu-dortmund.de> ---
I found another pair of equivalent expressions that produce different results.

Working as expected:
    if ((floatBitsToUint(R0).y >= 0x4u ? 0xffffffffu : 0u) == 0xffffffffu)

Showing artifacts:
    if ((floatBitsToUint(R0).y >= 0x4u ? 0xffffffffu : 0u) != 0u)

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

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

--- Comment #6 from Nicolai Hähnle <nhaehnle@gmail.com> ---
Thanks. To give you an update, this is a super subtle control flow handling bug
in LLVM -- and the difference between the original shader and your modification
at the LLVM input is merely that the sense of one branch is (correctly)
inverted, leading LLVM down a subtly different path in the end.

A sledge-hammer fix is quite simple, but I'm still thinking about how to get a
better fix which doesn't pessimize a bunch of cases.

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

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

--- Comment #7 from Nicolai Hähnle <nhaehnle@gmail.com> ---
Created attachment 135749
  --> https://bugs.freedesktop.org/attachment.cgi?id=135749&action=edit
patch for related issue

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

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

--- Comment #8 from Nicolai Hähnle <nhaehnle@gmail.com> ---
Created attachment 135750
  --> https://bugs.freedesktop.org/attachment.cgi?id=135750&action=edit
patch to fix this issue

Two patches for LLVM attached that fix the issue for me.

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

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

--- Comment #9 from Philip Rebohle <philip.rebohle@tu-dortmund.de> ---
Thanks for looking into this. While these two patches certainly improve the
situation significantly, they don't fix the issue entirely.

The remaining artifacts are hardly noticable in the original apitrace, so I
created another one (2.9G, bz2-compressed):
https://mega.nz/#!FHxSgZjL!0MzRQhI6W6Lk0beTv9LrPxDWjNTQveDuXVq3Mz2s7-M

Note that my original workaround still works with the LLVM patches applied.

Tested with updated LLVM-git, revision accb337c76e.

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

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

--- Comment #10 from Philip Rebohle <philip.rebohle@tu-dortmund.de> ---
Created attachment 135780
  --> https://bugs.freedesktop.org/attachment.cgi?id=135780&action=edit
Frame 2320 with patched LLVM

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

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

--- Comment #11 from Philip Rebohle <philip.rebohle@tu-dortmund.de> ---
Created attachment 135781
  --> https://bugs.freedesktop.org/attachment.cgi?id=135781&action=edit
Frame 2800 with patched LLVM

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

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

--- Comment #12 from Samuel Pitoiset <samuel.pitoiset@gmail.com> ---
Any news on this? The two LLVM patches have been pushed ~2 weeks ago. Can you
still reproduce the issue? Thanks!

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

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

--- Comment #13 from Fabian Maurer <dark.shadow4@web.de> ---
Created attachment 139196
  --> https://bugs.freedesktop.org/attachment.cgi?id=139196&action=edit
Video showing minor artifacts

While it's a lot better, it's not fully fixed yet. I added a video to show a
few artifacts, just look at the top middle area where the lightning jumps
around.

Created from this apitrace:
http://www.mediafire.com/file/cch4roemrd2gi1f/AutomataArtifacts.7z

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

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

--- Comment #14 from Timothy Arceri <t_arceri@yahoo.com.au> ---
(In reply to Fabian Maurer from comment #13)
> Created attachment 139196 [details]
> Video showing minor artifacts
> 
> While it's a lot better, it's not fully fixed yet. I added a video to show a
> few artifacts, just look at the top middle area where the lightning jumps
> around.
> 
> Created from this apitrace:
> http://www.mediafire.com/file/cch4roemrd2gi1f/AutomataArtifacts.7z

Those artifacts from the video seem to be gone with recent Mesa (19.1-devel)
and LLVM (9.0-devel). Can you confirm if that this is indeed fixed?

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

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

Timothy Arceri <t_arceri@yahoo.com.au> changed:

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

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