How is uniswap assembly create2 function working?

Question

I was going through uniswap code trying to understand the code and most of it is pretty clear but I do have a few questions.

in this function:

function createPair(address tokenA, address tokenB) external returns (address pair) {
    require(tokenA != tokenB, 'UniswapV2: IDENTICAL_ADDRESSES');
    (address token0, address token1) = tokenA < tokenB ? (tokenA, tokenB) : (tokenB, tokenA);
    require(token0 != address(0), 'UniswapV2: ZERO_ADDRESS');
    require(getPair[token0][token1] == address(0), 'UniswapV2: PAIR_EXISTS'); // single check is sufficient
    bytes memory bytecode = type(UniswapV2Pair).creationCode;
    bytes32 salt = keccak256(abi.encodePacked(token0, token1));
    assembly {
        pair := create2(0, add(bytecode, 32), mload(bytecode), salt)
    }
    IUniswapV2Pair(pair).initialize(token0, token1);
    getPair[token0][token1] = pair;
    getPair[token1][token0] = pair; // populate mapping in the reverse direction
    allPairs.push(pair);
    emit PairCreated(token0, token1, pair, allPairs.length);

There is the assembly line. According to solidity docs this deploys a new contract but I don't understand how it works where it gets the code from and so on.

So is it possible to "translate" this into solidity somehow? Thanks a lot!

Answer 1

It makes use of the create2 opcode allowing you to deploy a contract to an address determinable by its bytecode and salt.

Uniswap V2 was written in Solidity 0.5 that didn't have a way to produce the create2 opcode directly from the language. So you had to use a low-lewel assemly to actually use this opcode.

The current version 0.8 allows passing the salt param producing the create2 (instead of the regular create) opcode.

pragma solidity ^0.8;

contract UniswapV2Pair {
}

contract MyContract {
    function createPair() external {
        bytes32 salt = 0x1234567890123456789012345678901234567890123456789012345678901234;
        address pair = address(
            new UniswapV2Pair{salt: salt}()
        );
    }
}

Uniswap uses a combination of the pair token addresses as the salt, and the bytecode is always the same. Which effectively allows to deploy just one contract for each unique pair combination.

Example:

• Tokens 0x123 and 0x456 would always result in UniswapV2Pair contract address 0xabc.
• But once you change the salt, it changes the deployed contract address. So tokens 0x123 and 0x789 would always result in UniswapV2Pair contract address 0xdef.