Bug 73110 - [BISECTED] commit "r600g: use shader-based MSAA resolving when hw-based one cannot be used" crashes some games when R600_LLVM=1 with LLVM < 3.4
Summary: [BISECTED] commit "r600g: use shader-based MSAA resolving when hw-based one c...
Status: RESOLVED FIXED
Alias: None
Product: Mesa
Classification: Unclassified
Component: Drivers/Gallium/r600 (show other bugs)
Version: git
Hardware: All All
: medium normal
Assignee: Default DRI bug account
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Reported: 2013-12-29 08:19 UTC by Alexandre Demers
Modified: 2014-02-03 20:25 UTC (History)
3 users (show)

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Description Alexandre Demers 2013-12-29 08:19:23 UTC 
More than a week ago, after updating mesa, I began experiencing trouble launching Serious Sam 3. Bisecting gave me the following:

696229523d919de15ebc25d0f475bf56d7dad4a9 is the first bad commit
commit 696229523d919de15ebc25d0f475bf56d7dad4a9
Author: Marek Olšák <marek.olsak@amd.com>
Date:   Thu Dec 12 18:09:08 2013 +0100

    r600g: use shader-based MSAA resolving when hw-based one cannot be used
    
    This fixes some MSAA integer tests.

:040000 040000 b04b89764c5edee6dd685e22d47e36a11060583f 147827de9fe4e00bae5cc5844052b4fc45bb396c M	src

However, setting R600_LLVM=0 prevent the crash.

Using an HD6950 (Cayman) on kernel 3.13-rc5 with latest drm and ddx from git on ArchLinux 64bit.
Comment 1 Alexandre Demers 2014-01-07 00:51:34 UTC 
This also affects Awesomenauts, Bastion and Trine 2 in the games I own.
Comment 2 Alexandre Demers 2014-01-10 00:01:12 UTC 
A bit more info. Serious Sam 3 will launch and begin to load. However, once it has loaded, it crashes when the menu is about to appear and I can even hear a fraction of music before it happens.

Marek, can I provide you with something specific? Anything about the shaders I could try to get (with and without R600_LLVM)?
Comment 3 Alex Deucher 2014-01-10 00:27:16 UTC 
I suspect the llvm support for r600 class hardware needs to add support for some additional instructions, etc. that that patch uses.  Vincent or Tom may be able to identify what's missing.
Comment 4 Marek Olšák 2014-01-10 01:25:42 UTC 
The shaders are generated by this code:

http://cgit.freedesktop.org/mesa/mesa/diff/src/gallium/auxiliary/util/u_simple_shaders.c?id=fc21098a95c8f7d0aadbfcc90cb2e9e78d1e2ef3
http://cgit.freedesktop.org/mesa/mesa/diff/src/gallium/auxiliary/util/u_simple_shaders.c?id=5a609fbcb5459fc5cac2e0361a405ea4b884325f

There is nothing special except for the texel fetch instructions which read from MSAA textures. The shaders also contain float<->int and float<->unsigned conversions. I think the latter is not used with GLSL very often.
Comment 5 Alexandre Demers 2014-01-10 17:25:46 UTC 
(In reply to comment #3)
> I suspect the llvm support for r600 class hardware needs to add support for
> some additional instructions, etc. that that patch uses.  Vincent or Tom may
> be able to identify what's missing.

I'm still using LLVM 3.3. I'll try with LLVM 3.4 later if possible just in case it works with that, since LLVM 3.4 has landed in the testing repositories of ArchLinux last night.
Comment 6 Marek Olšák 2014-01-10 19:13:21 UTC 
That might be it. I think LLVM 3.3 doesn't support MSAA texture fetches on R600.
Comment 7 Alexandre Demers 2014-01-11 05:50:34 UTC 
Tested with LLVM 3.4 and it does crash. So LLVM 3.4 is required to support correctly the shader-based MSAA. Would there be a way to work around this issue if a user is using LLVM < 3.4?
Comment 8 Marek Olšák 2014-01-11 10:30:59 UTC 
(In reply to comment #7)
> Tested with LLVM 3.4 and it does crash. So LLVM 3.4 is required to support
> correctly the shader-based MSAA. Would there be a way to work around this
> issue if a user is using LLVM < 3.4?

Did you mean "it doesn't crash"?
Comment 9 Alexandre Demers 2014-01-11 16:47:13 UTC 
(In reply to comment #8)
> (In reply to comment #7)
> > Tested with LLVM 3.4 and it does crash. So LLVM 3.4 is required to support
> > correctly the shader-based MSAA. Would there be a way to work around this
> > issue if a user is using LLVM < 3.4?
> 
> Did you mean "it doesn't crash"?

Yes, sorry. "... LLVM 3.4 and it doesn't crash."
Comment 10 Alexandre Demers 2014-01-14 05:32:49 UTC 
I've been testing with LLVM 3.4. While it doesn't crash anymore, there are some visual glitches. Do you want me to open a different bug about it?
Comment 11 Alexandre Demers 2014-01-31 23:34:39 UTC 
I'm closing this bug since it is fixed by using LLVM 3.4. But my question in comment 7 is still pending.
Comment 12 Marek Olšák 2014-01-31 23:43:21 UTC 
(In reply to comment #7)
> Tested with LLVM 3.4 and it does crash. So LLVM 3.4 is required to support
> correctly the shader-based MSAA. Would there be a way to work around this
> issue if a user is using LLVM < 3.4?

The workaround would be to disable MSAA (and therefore GL 3 too) if the LLVM backend is enabled and the LLVM version is older than 3.4.
Comment 13 Fabio Pedretti 2014-02-01 07:18:54 UTC 
Alternatively, LLVM 3.4 could be required as the minimum version on r600 and radeonsi, there are other bugs I remember only happens with 3.3.
Comment 14 Alexandre Demers 2014-02-03 19:52:20 UTC 
(In reply to comment #13)
> Alternatively, LLVM 3.4 could be required as the minimum version on r600 and
> radeonsi, there are other bugs I remember only happens with 3.3.

Since LLVM 3.4 is officially out, I would go with this solution. Otherwise, could we wrap 696229523d919de15ebc25d0f475bf56d7dad4a9 code in a if (LLVM >= 3.4) kind of condition?
Comment 15 Marek Olšák 2014-02-03 20:25:58 UTC 
No, the code would be too complex.

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