Unrolling loops

Loop unrolling is the process of transforming a loop into a series of statements, which helps eliminate branching and add additional code optimizations.

When compiling C++ code, loop unrolling occurs automatically.

However, functions in Rust have to explicitly use the #[unroll_for_loops] directive (imported from the unroll crate) for loop unrolling to work.

Consider the following examples.

C++

#include <nil/crypto3/algebra/curves/pallas.hpp>

using namespace nil::crypto3::algebra::curves;
typename pallas::base_field_type::value_type pow_2(typename pallas::base_field_type::value_type a) {
    typename pallas::base_field_type::value_type res = 1;
    for (int i = 0; i < 2; ++i) {
        res *= a;
    }
    return res;
}

[[circuit]] typename pallas::base_field_type::value_type
    field_arithmetic_example(typename pallas::base_field_type::value_type a,
                            typename pallas::base_field_type::value_type b) {

    typename pallas::base_field_type::value_type c = (a + b) * a + b * (a + b) * (a + b);
    const typename pallas::base_field_type::value_type constant = 0x12345678901234567890_cppui255;
    return c * c * c / (b - a) + pow_2(a) + constant;
}

Rust

#![no_main]
#![feature(const_trait_impl)]
#![feature(effects)]

use ark_ff::{Field, MontFp};
use ark_pallas::Fq;
use unroll::unroll_for_loops;

#[unroll_for_loops]
fn pow_2(a: Fq) -> Fq {
    let mut acc = Fq::ONE;
    for i in 0..2 {
        acc = acc * a;
    }

    acc
}

#[circuit]
pub fn field_arithmetic_example(a: Fq, b: Fq) -> Fq {
    let c = (a + b) * a + b * (a + b) * (a + b);
    const CONSTANT: Fq = MontFp!("0x12345678901234567890");
    c * c * c / (b - a) + pow_2(a) + CONSTANT
}