| 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 attachme= nt 135457 [details] Fragment shader with workaround When compiling Mesa against LLVM >=3D 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 =3D uintBitsToFloat(floatBitsToUint(R0).y >=3D 0x4u ? 0xfffffff= fu : 0u); if (bool(floatBitsToUint(R1).x)) The artifacts go away when changing the original 'if' condition to the following: if (floatBitsToUint(R0).y >=3D 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.
Created attachment 135=
458 [details]
Frame 2400 with artifacts
Created attachment =
135459 [details]
Frame 2400 without artifacts
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.
Thanks for the very lucid bug report. I can reproduce this and= am investigating.
I found another pair of equivalent expressions that produce di=
fferent results.
Working as expected:
if ((floatBitsToUint(R0).y >=3D 0x4u ? 0xffffffffu : 0u) =3D=3D 0xff=
ffffffu)
Showing artifacts:
if ((floatBitsToUint(R0).y >=3D 0x4u ? 0xffffffffu : 0u) !=3D 0u)
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 modifica= tion 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 ge= t a better fix which doesn't pessimize a bunch of cases.
Created attachment 13574= 9 [details] [review] patch for related issue
Created attachment 13575= 0 [details] [review] patch to fix this issue Two patches for LLVM attached that fix the issue for me.
Thanks for looking into this. While these two patches certainl= y 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!0MzRQhI6W6Lk0beTv9LrPxDWjNTQveDuXVq3Mz2s= 7-M Note that my original workaround still works with the LLVM patches applied. Tested with updated LLVM-git, revision accb337c76e.
Created attachment =
135780 [details]
Frame 2320 with patched LLVM
Created attachment =
135781 [details]
Frame 2800 with patched LLVM
Any news on this? The two LLVM patches have been pushed ~2 wee= ks ago. Can you still reproduce the issue? Thanks!
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<= /pre>
(In reply to Fabian Maurer from comment #13) > Created attachment 139196 [details] > Video showing minor artifacts >=20 > 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 ju= mps > around. >=20 > 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?
| What | Removed | Added |
|---|---|---|
| Resolution | --- | FIXED |
| Status | NEW | RESOLVED |