I am developing a contract which allow users to swap tokens on Uniswap V2 pair. What is important thing is to reduce the gas usage of this smart contract.

// SPDX-License-Identifier: MIT

pragma solidity >=0.8.0;

import "./interface/IERC20.sol";
import "./lib/SafeTransfer.sol";

contract Sandwich {
    using SafeTransfer for IERC20;
    // Authorized
    address internal immutable user;

    // transfer(address,uint256)
    bytes4 internal constant ERC20_TRANSFER_ID = 0xa9059cbb;

    // swap(uint256,uint256,address,bytes)
    bytes4 internal constant PAIR_SWAP_ID = 0x022c0d9f;

    // Contructor sets the only user
    receive() external payable {}

    constructor(address _owner) {
        user = _owner;

    // *** Receive profits from contract *** //
    function recoverERC20(address token) public {
        require(msg.sender == user, "shoo");

        Fallback function where you do your frontslice and backslice
        Payload structure (abi encodePacked)
        - token: address        - Address of the token you're swapping
        - pair: address         - Univ2 pair you're sandwiching on
        - amountIn: uint128     - Amount you're giving via swap
        - amountOut: uint128    - Amount you're receiving via swap
        - tokenOutNo: uint8     - Is the token you're giving token0 or token1? (On univ2 pair)
        Note: This fallback function generates some dangling bits
    fallback() external payable {
        // Assembly cannot read immutable variables
        address memUser = user;

        assembly {
            // You can only access teh fallback function if you're authorized
            if iszero(eq(caller(), memUser)) {
                // Ohm (3, 3) makes your code more efficient
                // WGMI
                revert(3, 3)

            // Extract out teh variables
            // We don't have function signatures sweet saving EVEN MORE GAS

            // bytes20
            let token := shr(96, calldataload(0x00))
            // bytes20
            let pair := shr(96, calldataload(0x14))
            // uint128
            let amountIn := shr(128, calldataload(0x28))
            // uint128
            let amountOut := shr(128, calldataload(0x38))
            // uint8
            let tokenOutNo := shr(248, calldataload(0x48))

            // **** calls token.transfer(pair, amountIn) ****

            // transfer function signature
            mstore(0x7c, ERC20_TRANSFER_ID)
            // destination
            mstore(0x80, pair)
            // amount
            mstore(0xa0, amountIn)

            let s1 := call(sub(gas(), 5000), token, 0, 0x7c, 0x44, 0, 0)
            if iszero(s1) {
                // WGMI
                revert(3, 3)

            // ************
                calls pair.swap(
                    tokenOutNo == 0 ? amountOut : 0,
                    tokenOutNo == 1 ? amountOut : 0,
                    new bytes(0)

            // swap function signature
            mstore(0x7c, PAIR_SWAP_ID)
            // tokenOutNo == 0 ? ....
            switch tokenOutNo
            case 0 {
                mstore(0x80, amountOut)
                mstore(0xa0, 0)
            case 1 {
                mstore(0x80, 0)
                mstore(0xa0, amountOut)
            // address(this)
            mstore(0xc0, address())
            // empty bytes
            mstore(0xe0, 0x80)

            let s2 := call(sub(gas(), 5000), pair, 0, 0x7c, 0xa4, 0, 0)
            if iszero(s2) {
                revert(3, 3)

I checked the fallback function with WETH -> A swap and it costs about 110000 gas. And after that I checked fallback function with A -> WETH swap and it costs only 80000 gas.

I want to know why WETH -> A swap needs a lot of gas then A -> WETH swap. And also I want to know how to reduce WETH -> A swap gas usage.

I will be very thankful if someone helps me.

Thank you.

1 Answer 1


Since this code is copied from https://github.com/libevm/subway/tree/master/contracts, you can check yourself that there's another more optimized version written in yul.

Regarding why WETH -> A was more expensive, I guess the cause was new storage slot initialization. During this swap the tokenA.balanceOf(contract) changed from zero to non-zero (a lot of gas). During A -> WETH the balances may have changed from non-zero to non-zero (not much gas) if you didn't swapped all the balance of the contract. You can test this hypothesis by trying another WETH -> A after the first one: it should spend less gas!

By the way, a general trick to spend less gas is never empty out the balances of the contract, try to keep always 1 wei there.

  • Thanks for your response, @0xSanson. I agree with tokenA.balanceOf(contract) changes from zero to non-zero so it will use a lot of gas. You know this contract helps sandwich attack and there are lots of sandwich bots. This is one of the bot but it's not using much gas fee than mine when WETH -> A. etherscan.io/address/0x5AA17fC7F2950ECa85376C3A8CB1509e8e4B39dF I deployed the github.com/libevm/subway/tree/master/contracts and tested. It's using much more gas than the mentioned bot. Commented Sep 13, 2022 at 19:19

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