MPC: unexpected result when using bit shifts

[sebw42]

Hi, I've noticed an issue with the MPC compiler on the mpc_aws branch.
A combination of bit shifts and multiplications leads to unexpected results in certain cases.
Here is an example program:

int main(__attribute__((private(0))) int x){
    int y = (0 << 0);
    // should be 0
    return ((x * y) << 0);
}

Executing this results in a seemingly random number if we use the css selection scheme.

Compilation command:

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

Execution command:

/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