Dear all!
After a fair amount of testing and tweaking, I am now happy with my
rendering back-end using the highway SIMD library. Vc, which I have used
for intel CPUs until now, has better support for older CPUs: it offers
dedicated AVX (without the '2') support and the 'fallback' mode is
faster than when I use it with highway - but such CPUs are becoming
increasingly rare and using lux with them is no fun, anyway. What tips
the scale for me is AVX512 support. Vc does not produce actual AVX512
instructions, instead it falls back to emitting AVX2 instructions, so
there is not much to be gained in the rendering code which relies on the
SIMD library - some other code which relies on autovectorization may
benefit, but that's not so relevant. highway, on the other side, *does
support AVX512*, and a friend of mine who has a CPU with AVX512 has run
a test of the Vc version against the highway version of the code. The
performance gain was more than 50%!
The binaries on my download page are all done with Vc (except for the
macOS ARM64 build, which also uses highway because Vc does not produce
native ARM NEON code). For those of you who have AVX512 on a windows
machine, I now offer a lux-portable ZIP compiled with the highway
backend, so you can see what a difference the SIMD backend makes! It
would be nice if this binary could see a bit of testing, just to be sure
the change of SIMD backend does not introduce unwanted behaviour which
has escaped my scrutiny! I am considering switching to the highway
backend altogether with the next lux version. Please find the ZIP file here:
https://bitbucket.org/kfj/pv/downloads/lux_for_windows_1.1.5_portable_with_highway.zip
Apart from AVX512, this build supports the fallback ISA (no SIMD),
SSE4.2 and AVX2. The console output will tell you which CPU was detected.
Kay
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