.text
	.global	bitrev
	.type	bitrev, @function
bitrev:
    mov %rdi,%rax //num = rax = rdi
    xor %rbx,%rbx
    neg %rbx     //mask = rbx
    mov $0x40,%rsi //s = rsi
while:
    mov %rbx,%rcx //mask = rbx = rcx
    sal %rsi,%rbx
    xor %rcx,%rbx // mask ^ (mask << s)
    mov %rbx,%rcx // mask = rbx
    neg %rcx      // ~mask = rcx
    sar %rsi,%rdi // num >> s
    and %rcx,%rdi //mask & (num >> s)
    sal %rsi,%rax
    and %rcx,%rax
    or %rdi,%rax
    mov %rax,%rdi
    sar %rsi     // s = s/2
    jge while
    ret

	.size	bitrev, .-bitrev