Unexpected values using get_integer_from_var_name

[ftheirs]

Describe the bug
There appears to be an issue in Cairo-vm when accessing certain variables. The behavior and returned values differ depending on whether the variables are defined using let or local.

Check that if you define a with local instead of let, the hint prints the expected value. As far as I checked, in this case where the expression involves a subtraction, the result instead produces the sum of both terms.
If tmp is defined with let, once again, we get the expected result.

Using the custom_hint example, I made a small modification to define a as an expression containing a variable defined with local. This change results in an incorrect output when attempting to read the variable.

func main() {
    alloc_locals;

    local tmp = 3;
    let a = 17 - tmp;
    // Use custom hint to print the value of a
    %{ print(ids.a) %}
    return ();
}

Current output:

20

To Reproduce
Steps to reproduce the behavior:
1 - Clone this branch: https://github.com/Moonsong-Labs/cairo-vm/tree/ft/get_integer_issue
2 - cd examples/custom_hint/
3- make example

Expected output

14

[FrancoGiachetta]

Hi there!
I'm taking a look at this issue. Seems like like the problem comes from this operation let a = 17 - tmp. If you change the value of tmp to -3 it will print 14 (when it should be 20 due to the law of signs). Seems like it always behaves as an addition.

[ftheirs]

Hey @FrancoGiachetta! Thanks 💪 The main issue regarding the cairo code is that it belongs to the OS and we cannot change that.
While we were making the upgrade to cairo-lang v0.13.3, we've encountered that behavior in this line:
https://github.com/starkware-libs/cairo-lang/blob/8e11b8cc65ae1d0959328b1b4a40b92df8b58595/src/starkware/starknet/core/os/output.cairo#L78

I think that it must be related to ExpressionSimplifier:
https://github.com/starkware-libs/cairo-lang/blob/8e11b8cc65ae1d0959328b1b4a40b92df8b58595/src/starkware/cairo/lang/compiler/expression_simplifier.py#L62-L65

[FrancoGiachetta]

Oh I see. Thanks a lot for the links. I'll notify you as soon as I have any upgrades.

[FrancoGiachetta]

Reading the comment again, I wanted to clarify something just in case. My first comment is just giving an intuition of the problem, perhaps is not the case. We currently taking a look into it to find a solution.

[FrancoGiachetta]

Hi there!
The issue seems to be coming from the parsing of references, currently they are always being taken as positive values. Due to this and since cairo treats subtractions as additions with one of the operands negated, the operation results in 20.
Do you know other cases like this? It would also help to find other cases with multiplications and divisions.

[FrancoGiachetta]

Hi!
I wanted to let you know that the latest version of main should fix the problem. I'll close this issue, pin us if you need anything else!

[FrancoGiachetta]

I'll close this issue as it was solved in the last main version.