PowerPC Power8 VSX SIMD optimized filter functions about libpng HOT 21 CLOSED

The issue is still open, but I'm pretty far along on local hardware. Will publish early next week & start the review process.

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I'm taking a whack at this. @glennrp what is the recommended path to get external contributions into libpng?

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You may submit a GIT pull request or just email me a patch.

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Is code in process of being accepted?
Or still open?

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@edelsohn
Should I implement runtime detection of supporting VSX by cpu? If yes please tell me platforms which must be supported (currently linux is the only supported one).

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Does libpng support runtime detection of SSE4 vs AVX vs AVX2 vs AVX512?

The intended target for the issue and bounty is PPC64 Linux support, not other OSes.

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@edelsohn There is no runtime detection support for Intel but there is ones for mips and ARM. Now linux support for ppc64 has been made, so I guess it is okay for this issue.

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Does runtime support work for PPC64 on Linux? I'm unsure how to interpret your comment about "linux support for ppc64 has been made"

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@edelsohn I mean runtime detection of "if current CPU is able to run Altivec && VSX code" . Yes, it is done for PPC64 Linux

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@barkovv @glennrp What is the final speed up and how does this compare with other SIMD architectures of similar width, such as ARM Neon and Intel SSE?

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@edelsohn According to John Bowler's words:

• Analysis of the results suggests a 1/74% speed up from using the Altivec code; this is suspiciously large.
• Final results: about 8% improvement in (just) decode time of typical PNGs

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I saw that in the pull request.

What is the speedup for other SIMD architectures for the same measurements? Is the POWER VSX code achieving equivalent or better speedup?

8% improvement for decode seems small, but I don't know what to compare it against.

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@edelsohn
I've just made tests for Intel. There are some quick results:

libpng-noopt $ ./timepng ../Earth10k.png 
1.519951620
libpng-opt $ ./timepng ../Earth10k.png 
1.192934654

Calculations:

>>> 1.192934654/1.509859932
0.79009623920532

So, according to timepng benchmarking, PowerPC VSX optimisation is on pair with Intel SSE (74% and 79%).

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I can measure this results deeper and with more accuracy if @jbowler will provide some more information about his measure methods and scripts.

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I'm running Earth10k.png through pngcrush for timing. It's not surprising that optimization will make a significant improvement because the image has mostly AVG and PAETH filtering.

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I'm not questioning the result. I greatly appreciate the excellent work on the patch to implement POWER8 VSX optimizations. The milestone for this issue is not some perfect, unrealistic, magical improvement in performance nor some super-human effort to achieve an order of magnitude advantage. The goal for the issue is the implementation of architecture-specific SIMD optimizations for POWER8 VSX that are equivalent in form to Intel SSE, with the hope of achieving equivalent speedup.

I want to reach out to the experts for this important library to get their assessment of the benefit produced by the patch. Is the improvement produced by this patch about right?

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@edelsohn
I don't understand you. My point is that PowerPC VSX optimization are equivalent to Intel SSE one.
Do I need to approve it by profiling? Maybe you want to @glennrp and @jbowler to approve it?
What kind of actions must be perfomed to approve this work?

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@barkovv I want to know if @glennrp agrees with the test methodology. Again, I am not trying to create new hurdles. I simply want to be able to point at an objective test recommended by the libpng community that says, "yes, this is good enough."

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