4

I've seen mulmod used in Mikhail Vladimirov's Math in Solidity series:

function fullMul(uint256 x, uint256 y) public pure returns (uint256 l, uint256 h) {
    uint256 mm = mulmod(x, y, uint256(-1));
    l = x * y;
    h = mm - l;
    if (mm < l) h -= 1;
}

What does it do exactly?

4

As per the Solidity cheatsheet, mulmod is a function available at the global scope.

mulmod(uint x, uint y, uint k) returns (uint): compute (x * y) % k where the multiplication is performed with arbitrary precision and does not wrap around at 2**256. Assert that k != 0 starting from version 0.5.0.

Some examples:

  1. mulmod(3, 4, 5) is equal to 2.
  2. mulmod(2**256 - 1, 1, type(uint256).max) is equal to 0.
  3. mulmod(2**255, 2**255, type(uint256).max) is equal to 1.

Notice that even if 2**255 * 2**255 overflows, the contract call doesn't revert in Solidity v0.8.

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