.section .text
.global fib_iter

fib_iter:
        push %rbp           # Save old base pointer to stack.
                            # This also makes sure that rsp is
                            #  16-byte aligned.
        movq %rsp, %rbp     # Write new base pointer
        cmpq $1, %rdi       # Compare the argument to 1
        je one              # If n = 1 go to 'one' label.
        cmpq $0, %rdi
        je zero             # If n = 0, go to 'zero' label.


        movq $1, %rcx       # Set minus_one (rcx) to 1
        movq $0, %rdx       # Set minus_two (rdx) to 0
        movq %rcx, %rax     # Set r (rax) to minus_one (rcx)

loop:   addq %rdx, %rax     # r (rax) = minus_one (rcx)
                            #           + minus_two (rdx)
                            # rax is already set to minus_one
                            #  due to the last line in the loop
                            # The first time the loop is run,
                            #  this is handled before the loop.
        movq %rcx, %rdx     # minus_two (rdx) = minus_one (rcx)
        movq %rax, %rcx     # minus_one (rcx) = r (rax)
        decq %rdi           # Decrement the counter by 1.
        cmpq $1, %rdi       # Loop if the counter is over 1:
        jle end             # If counter is 1:
                            #  End and return r (rax)
        jmp loop            # If counter is 0: loop.

zero:   movq $0, %rax       # Return 0.
        jmp end
one:    movq $1, %rax       # Return 1.
end:    leave               # Return the state of the base and
                            #  stack pointers to their
                            #  original state.
        ret                 # Return r (rax)