[rbultje]

> If it's so heavy in assembly, the fact that ffmpeg works on my Mac seems like a miracle. Is it ported by hand?

Not ported, but rather re-implemented. So: yes.

A bit more detail: during build, on x86, the FFmpeg binary would include hand-written AVX2 (and SSSE3, and AVX512, etc.) implementations of CPU-intensive functions, and on Arm, the FFmpeg binary would include hand-written Neon implementations (and a bunch of extensions; e.g. dotprod) instead.

At runtime (when you start the FFmpeg binary), FFmpeg "asks" the CPU what instruction sets it supports. Each component (decoder, encoder, etc.) - when used - will then set function pointers (for CPU-intensive tasks) which are initialized to a C version, and these are updated to the Neon or AVX2 version depending on what's included in the build and supported by this specific device.

So in practice, all CPU-intensive tasks for components in use will run hand-written Neon code for you, and hand-written AVX2 for me. For people on obscure devices, it will run the regular C fallback.