[Bug 93264] Tonga VM Faults since llvm ScheduleDAGInstrs: Rework schedule graph builder.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93264

--- Comment #3 from Nicolai Hähnle <nhaehnle at gmail.com> ---
Time to document some information I've gathered.

I can now confirm that Mesa has nothing to do with it. Something must have gone
wrong with my builds initially, sorry for having caused confusion.

I captured an apitrace for better reproducibility[0] and ran it with shader
dumps enabled and flushing after each draw call in the "interesting" region.

I am going to attach the R600_DEBUG=check_vm dump which I've cross-referenced
with R600_DEBUG=vm to obtain the shaders that were active during the draw call
(file names with prefix llvm-c0a189c.mesa-caf12bebd). I then matched the
shaders to those dumped by a run with a good version of LLVM (commit just
before the bad one, file names with prefix llvm-26ddca1.mesa-caf12bebd).

Clearly, the LLVM changes caused some significant re-ordering of the
instruction schedule, and that somehow, surprisingly, seems to be responsible
for the VM faults.

Another aspect to note is that the shaders are compiled before draw call
174000, while the VM faults happen shortly after draw call 178000. This seems
to suggest that the shaders alone only cause VM faults in conjunction with some
other state. However, the VM faults have always happened in exactly the same
point so far, so it does appear to be deterministic.

[0] The demo always causes VM faults, so I'm not going to upload the giant
trace; however, the timing varies between runs, so it's cleaner to reproduce
using a trace.

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