dec      rax

mov      rdx,rax
shr      rdx,0x1
or       rax,rdx

mov      rdx,rax
shr      rdx,0x2
or       rax,rdx

mov      rdx,rax
shr      rdx,0x4
or       rax,rdx

mov      rdx,rax
shr      rdx,0x8
or       rax,rdx

mov      rdx,rax
shr      rdx,0x10
or       rax,rdx

mov      rdx,rax
shr      rdx,0x20
or       rax,rdx

inc      rax