fix: addmod

[enitrat]

Pull Request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build-related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue Number: Closes #83

What is the new behavior?

• Fixes ADDMOD behavior

Does this introduce a breaking change?

• Yes
• No

Added changes to CHANGELOG.md

• Yes
• No

Other information

ADDMOD

I tried two different implementations for this ADDMOD opcode, leveraging the fact that (x + y) mod N <=> (x mod N) + (y mod N) mod N

The first try was using u256_wide_add, a function that I created that adds two u256s and returns a u512 to avoid overflowing on the a+b addition, and then dividing it by N and returning the remainder.


    fn exec_addmod(ref self: ExecutionContext) {
        let popped = self.stack.pop_n(3);

        let n: u256 = *popped[2];
        let mut result = 0;
        if n != 0 {
            let add_res = u256_wide_add(*popped[0], *popped[1]);
            let (_, r) = u512_safe_div_rem_by_u256(add_res, n.try_into().unwrap());
            result = r;
        }

        self.stack.push(result);
    }
    
    

fn u256_wide_add(a: u256, b: u256) -> u512 {
    let (sum, overflow) = u256_overflowing_add(a, b);

    let limb0 = sum.low;
    let limb1 = sum.high;

    let limb2 = if overflow {
        1
    } else {
        0
    };

    let limb3 = 0;

    u512 { limb0, limb1, limb2, limb3 }
}


Here is the test output

❯ scarb test -f test_exec_addmod_overflow
Running tests for package: kakarot
testing kakarot ...
running 1 tests
[DEBUG]	                               	(raw: 0x3b72c

test kakarot::tests::test_instructions::test_stop_and_arithmetic_operations::test_exec_addmod_overflow ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 78 filtered out;

The other implementation is

    fn exec_addmod(ref self: ExecutionContext) {
        let popped = self.stack.pop_n(3);

        let n: u256 = *popped[2];
        let mut result = 0;
        if n != 0 {
            // (x + y) mod N <=> (x mod N) + (y mod N) mod N
            result = (*popped[0] % n) + (*popped[1] % n) % n;
        }
        self.stack.push(result);
    }







Here is the test output

❯ scarb test -f test_exec_addmod_overflow
Running tests for package: kakarot
testing kakarot ...
running 1 tests
[DEBUG]	                               	(raw: 0x3d5b8

test kakarot::tests::test_instructions::test_stop_and_arithmetic_operations::test_exec_addmod_overflow ... ok
test result: ok. 1 passed; 0 failed; 0 ignored; 78 filtered out;





This second version is ~3% more costly. Therefore, we will keep the first one with u256_wide_add.

[Eikix]

lgtm

[Eikix]

smol conflicts

[greged93]

nit: parenthesis for 2 comments

[greged93]

lgtm!