> I thought one should generally avoid the vectorized AVX-512 because most processors with it get thermally throttled.
AFAIK, that's only the case for Intel processors; on AMD processors, you can use AVX-512 without fear.
> How does ffmpeg and other binary-distributed software do it? You can't late compile on the platform so presumably they select implementations at runtime?
Yes, that's what they do, either manually, automatically with compiler help (ifunc or similar), or with a full copy of the whole library which is loaded from an alternative directory whenever the necessary hardware features are present (for instance, when AVX-512 and a number of other features is available, the x86-64-v4 subdirectory will be used; run "/lib64/ld-linux-x86-64.so.2 --help" to see which ones would be used on your system).
AFAIK, that's only the case for Intel processors; on AMD processors, you can use AVX-512 without fear.
> How does ffmpeg and other binary-distributed software do it? You can't late compile on the platform so presumably they select implementations at runtime?
Yes, that's what they do, either manually, automatically with compiler help (ifunc or similar), or with a full copy of the whole library which is loaded from an alternative directory whenever the necessary hardware features are present (for instance, when AVX-512 and a number of other features is available, the x86-64-v4 subdirectory will be used; run "/lib64/ld-linux-x86-64.so.2 --help" to see which ones would be used on your system).