Yul prime number

yul-primer.sol

function isPrime(uint256 x) external pure returns (bool ret) {
  ret = true;
  assembly {
    let halfX := add(div(x, 2), 1)
    for {let i := 1} lt(i, halfX) {i := add(i, 1)} {
      if iszero(mod(x, i)) {
        ret := 0
        break
      }
    }
  }
}