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Cycles - AO-Node renders incorrect on some CPU's
Closed, ResolvedPublic

Description

System Information
Windows 10 64bit
Intel Core i7-5820K CPU @ 3.30GHz
32 GB RAM
EVGA GTX 1080 SC ACX 3.0

Blender Version
2.79 0a8af46 win64

Short description of error
The new AO Node renders incorrectly on CPU. GPU rendering is fine. This Bug only happens on some and not all Computers. On the machines where the bug happens, it is reproducible. Screenshots and sample scene are attached.

We testet it on the following machines:
Intel Core i7 5820K @ 3.30GHz - Windows 10 64bit -> BUG
Intel Core i3 8350K - Windows 7 x64 -> BUG
Intel Core i7 920 @ 2.67GHz - Windows 7 64bit Home -> works
Intel X5650 - Windows 7 sp1 x64 pro -> works

CPU (Bug):

GPU:

Blender File:

Exact steps for others to reproduce the error
Open the Blend file and render on CPU.

Details

Type
Bug

Event Timeline

Lukas Stockner (lukasstockner97) triaged this task as Confirmed, Medium priority.

Confirmed by @LazyDodo (LazyDodo), only seems to happen on AVX2-capable processors on Windows.

As a workaround if you *really* need it to work right now, hit Ctrl+Alt+D, enter 256, then go to the new Debug panel in the render settings and disable AVX2.

Thanks for the tip! I disabled AVX2 and everything renders fine.

I can only seem to reproduce this with msvc2013 and avx2, @Falk Müller (xharthok) can you try a build from the bot marked with 'New Compiler' (has vc14 in the filename) and see if it works for you without having disable avx2?

I tried the vc14 version and it works. So this bug seems to be only happening when using Avx2, Windows and the old compiler. Very exclusive little bug. :-)

@Lukas Stockner (lukasstockner97) , @Brecht Van Lommel (brecht) feels like a codegen bug, in a compiler we're soon gonna drop anyhow, not sure if it's worth spending time tracking it down.

I think not many people will experience this bug. Maybe just automaticly disabling avx2 in this case?

LazyDodo (LazyDodo) lowered the priority of this task from Confirmed, Medium to Needs Information from User.Jul 17 2018, 3:01 AM

With msvc2013 being retired today, can you validate that the current buildbot builds are now working properly for you?

Tested it with the latest "blender-2.79-b85be88655c-win64 Jul 17 2018 03:13:53" and it works like a charm! Thanks, for looking into it.

Brecht Van Lommel (brecht) claimed this task.

Thanks for testing.