Intrinsics have the disadvantages of asm (non-portable) but also don't reliably have the advantages of them (compilers are pretty unpredictable about optimizing with them) and they're ugly (especially x86 with its weird Hungarian stuff).

There is just a little bit of intrinsics code in ffmpeg, which I wrote, that does memory copies.

https://github.com/FFmpeg/FFmpeg/blob/master/libavutil/x86/i...

It's like this because we didn't want to hide the memory accesses from the compiler, because that hurts optimization, as well as memory tools like ASan.