Cairo’s Labeled Jump Exercise I

What does the following code do? (run with –no_end –step=16 to avoid the End of program was not reached error)

func main():
    [ap] = 2; ap++

    my_loop:
    [ap] = [ap - 1] * [ap - 1]; ap++
    [ap] = [ap - 1] + 1; ap++
    jmp my_loop
end

Solution

The code clearly creates an infinity loop and that’s why we need to run it using the flag –step=16 to limit the number of iterations. 

After compiling and running the program we get the output shown below:

$ cairo-compile exercise.cairo --output exercise.json
$ cairo-run --program=exercise.json \
--print_memory --relocate_prints --no_end --steps=16
>>>
Addr  Value
-----------
⋮
1     5189976364521848832
2     2
3     5210805504208502784
4     5198420613823168512
5     1
6     74168662805676031
7     -3
8     21
9     21
10    2
11    4
12    5
13    25
14    26
15    676
16    677
17    458329
18    458330
19    210066388900
20    210066388901

The execution of the code makes the pc counter loop while keeping the ap counter moving forward. This creates a progression that doubles the previous value, adds one, doubles it again, adds one, etc. The loop has no end and thus the function doesn’t even need a return statement (it wouldn’t even reach it).

Reference

The exercise in this article can be found in this section of the Cairo docs.