https://godbolt.org/z/ZyTG9b


#include <altivec.h>

typedef vector unsigned __int128 block;
typedef vector unsigned long long vector2_u64;

block swap_with_shift(block num) {
    return num << 64 | num >> 64;
}

block swap_without_shift(block num) {
    vector unsigned long long ret;
    ret[0] = ((vector2_u64)num)[1];
    ret[1] = ((vector2_u64)num)[0];
    return (block)ret;
}

typedef unsigned __int128 u128;

u128 swap_scalar(u128 in) {
    return in << 64 | in >> 64;
}

swap_with_shift:
        xxpermdi 34,34,34,2
        addi 9,1,-16
        stxvd2x 34,0,9
        ld 8,-8(1)
        ld 9,-16(1)
        mtvsrd 1,8
        mtvsrd 0,9
        xxpermdi 34,0,1,0
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0
swap_without_shift:
        xxpermdi 34,34,34,2
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0
swap_scalar:
        mr 9,3
        mr 3,4
        mr 4,9
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0