MPC: unexpected results when comparing negative and positive numbers

[sebw42]

Hello,
I've noticed that the MPC compiler returns unexpected results when both negative and positive numbers are used in a comparison.

Here is an example:

int main(__attribute__((private(0))) int x, __attribute__((private(1))) int y) {
    if (x < y) {
        return 1;
    }
    return 0;
}

When setting x to -1 and y to 0, x < y should be true, but the program returns 0.
With x = -2 and y = -1, it works as expected and returns 1.

Here are the compilation/execution steps I've used (executed in the circ root dir):

export CARGO_MANIFEST_DIR="$PWD"; export RUST_BACKTRACE=1; ./target/release/examples/circ --parties 2 examples/C/mpc/program.c mpc --cost-model "empirical"

/path/to/ABY/build/bin/aby_interpreter -m mpc -f ./scripts/aby_tests/tests/program_c -t ./scripts/aby_tests/test_inputs/program.txt -r 0 & /path/to/ABY/build/bin/aby_interpreter -m mpc -f ./scripts/aby_tests/tests/program_c -t ./scripts/aby_tests/test_inputs/program.txt -r 1

Thank you for having a look!