Comment by userbinator

ARM64 has many registers but I believe the lack of suitably large immediate values and, apparently, compilers that are willing to use them all across functions, puts it at a disadvantage here. Assuming we want the return value in eax and the leading count comes in cl, this can be done branchlessly and data-lessly on x86 as follows:

    mov eax, 0x00043201
    test cl, 8
    setz al
    shl cl, 2
    shr eax, cl
    and eax, 15

Something similar may be possible on ARM64, but I suspect it will definitely be more than 19 bytes ;-)