Comment by jart

I discovered a really cool ARM64 trick today. One thing about x86 that I've found useful on so many occasions is the PCMPEQB + PMOVMSKB + BSF trick that lets me scan the bytes of a string 10x faster. I couldn't find any information on Google for doing PMOVMSKB with ARM, so I've been studying ARM's "Optimized Routines" codebase where I stumbled upon the answer in their strnlen() implementation. It turns out the trick is to use `shrn dst.8b, src.8h, 4` which turns a 128-bit mask into a 64-bit mask. You can then get the string offset index with fmov, rbit, clz and finally shift by 2.