Comptime and ACIR cast negative values into u128 differently

[rkarabut]

Aim

The program

fn main() -> pub u128 {
    comptime {
        let a = -1;
        a as u128
    }     
}

returns 340282366920938463463374607431768211455, while the program

fn main() -> pub u128 {
    let a = -1;
    a as u128
}

returns 53438638232309528389504892708671455232.

(edit: running in comptime also seemingly causes panics in compiler/noirc_frontend/src/hir/comptime/interpreter/cast.rs:77:17 for some setups)

Expected Behavior

It's not entirely clear to me if such casts should actually fail, but they probably should agree on the result if not.

Bug

With comptime:

nargo execute --force
[test1] Circuit witness successfully solved
[test1] Witness saved to target/test1.gz
[test1] Circuit output: Field(340282366920938463463374607431768211455)

Without comptime:

nargo execute --force
[test1] Circuit witness successfully solved
[test1] Witness saved to target/test1.gz
[test1] Circuit output: Field(53438638232309528389504892708671455232)

(found with comptime_vs_brillig AST fuzzer target)

To Reproduce

Workaround

None

Workaround Description

No response

Additional Context

No response

Project Impact

None

Blocker Context

No response

Nargo Version

No response

NoirJS Version

No response

Proving Backend Tooling & Version

No response

Would you like to submit a PR for this Issue?

None

Support Needs

No response

[rkarabut]

note: for some reason while the programs work when ran with nargo, the AST fuzzer (which is running it with noirc_driver::compile_main) actually panics on the examples of this with

thread 'targets::comptime_vs_brillig::tests::fuzz_with_arbtest' panicked at compiler/noirc_frontend/src/hir/comptime/interpreter/cast.rs:77:17:
attempt to subtract with overflow

[asterite]

On master I get a panic on the first snippet.

I also get a panic for this code:

fn main() -> pub u8 {
    comptime {
        let a = -1;
        a as u8
    }     
}

[rkarabut]

On master I get a panic on the first snippet.

Interesting. I'm also running it on master on my dev machine, nargo execute does return 340282366920938463463374607431768211455, and 255 for your snippet.

[jfecher]

fn main() -> pub u128 {
    comptime {
        unsafe {
            let a = -1;
            a as u128
        }
    }     
}

fn main() -> pub u128 {
    unsafe {
        let a = -1;
        a as u128
    }     
}

Worth noting the unsafe does nothing here in both cases so the discrepancy is between comptime and acir rather than comptime and brillig.

I get a panic on master on the first snippet as well though.

[rkarabut]

Oh yes, I'll fix this. Thank you