[Mesa-dev] Determinism in the results of llvmpipe?

[Jose Fonseca]

On 17/11/16 07:37, Andrew A. wrote:
> Hello,
>
> I'm using Mesa's software renderer for the purposes of regression
> testing in our graphics software. We render various scenes, save a
> screencap of the framebuffer for each scene, then compare those
> framebuffer captures to previously known-good captures.
>
> Across runs of these tests on the same hardware, the results seem to
> be 100% identical. When running the same tests on a different machine,
> results are *slightly* different. It's very similar within a small
> tolerance, so this is still usable. However, I was hoping for fully
> deterministic behavior, even if the hardware is slightly different.
> Are there some compile time settings or some code that I can change to
> get Mesa's llvmpipe renderer/rasterizer to be fully deterministic in
> its output?
>
> I'm using llvmpipe, and these are the two different CPUs I'm using to
> run the tests:
> Intel(R) Xeon(R) CPU E3-1275 v3
> Intel(R) Xeon(R) CPU X5650

>
> Thanks,
>
> Andrew
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> mesa-dev at lists.freedesktop.org
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llvmpipe changes its behavior in _runtime_ based on the CPU features 
(like SSE AVX, AVX2, etc.)


You could hack u_cpu_detect.c and LLVM source code to mask away CPU 
extra features ans reduce the perceived CPUID flags to the common 
denominator.

In fact, for the two CPUs you mention above, the differences probably go 
away if you set this environment variable:

   LP_NATIVE_VECTOR_WIDTH=128

as it will force llvmpipe to ignore AVX/AVX2/FMA/F16C.


But probably the best is to use x86 virtualization to clamp CPUID and do 
that.  Having a virtual machine image will also solve the problem of 
ensuring all runtime is the same, etc.


https://software.intel.com/en-us/articles/intel-software-development-emulator 
  can also do the same without virtualization (via bianry translation), 
but it might impact performance.


Jose