"Intel x86/x64 processors don’t have this instruction, so this is definitely not a portable solution."
This stuck out to me. I know that RISC vs CISC is basically a meaningless distinction nowadays, but I still naively expected that x86 would be more-or-less a strict superset of ARM.
Strictly speaking, AMD's XOP extensions do have an instruction that is close enough: VPPERM. It allows to not only shuffle bytes, like the already mentioned PSHUFB, but also reverse bits within each byte. Therefore, a single VPPERM instruction can reverse up to 128 bits at a time.
Modern ARM has lots of instructions that don’t have direct x86 equivalents. Most are in the vector domain, but there are plenty of non-vector examples too: BFI, BFC, BIC, ORN, RSB, saturating arithmetic, numerous multiply-add variants, etc.
This stuck out to me. I know that RISC vs CISC is basically a meaningless distinction nowadays, but I still naively expected that x86 would be more-or-less a strict superset of ARM.