Forge calculates gas as if it's all one tx (affecting ops that depend on warm/cold keys/addresses)

!README.md

Forge calculates gas as if each test is one tx

Sample code

When runing forge test -m testGas --gas-report -vv, it shows the cost of calling x is 261:

╭──────────────────────────────────┬─────────────────┬─────┬────────┬─────┬─────────╮
│ contracts/Gas.sol:Store contract ┆                 ┆     ┆        ┆     ┆         │
╞══════════════════════════════════╪═════════════════╪═════╪════════╪═════╪═════════╡
│ Deployment Cost                  ┆ Deployment Size ┆     ┆        ┆     ┆         │
├╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ 47315                            ┆ 251             ┆     ┆        ┆     ┆         │
├╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ Function Name                    ┆ min             ┆ avg ┆ median ┆ max ┆ # calls │
├╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ x                                ┆ 261             ┆ 261 ┆ 261    ┆ 261 ┆ 2       │
╰──────────────────────────────────┴─────────────────┴─────┴────────┴─────┴─────────╯

After commenting out the x assignment at the constructor, the cost of calling x jumps by 2K

╭──────────────────────────────────┬─────────────────┬──────┬────────┬──────┬─────────╮
│ contracts/Gas.sol:Store contract ┆                 ┆      ┆        ┆      ┆         │
╞══════════════════════════════════╪═════════════════╪══════╪════════╪══════╪═════════╡
│ Deployment Cost                  ┆ Deployment Size ┆      ┆        ┆      ┆         │
├╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ 25214                            ┆ 249             ┆      ┆        ┆      ┆         │
├╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ Function Name                    ┆ min             ┆ avg  ┆ median ┆ max  ┆ # calls │
├╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┤
│ x                                ┆ 2261            ┆ 2261 ┆ 2261   ┆ 2261 ┆ 2       │
╰──────────────────────────────────┴─────────────────┴──────┴────────┴──────┴─────────╯

(Constructor after commenting out:)

    constructor(uint256 _x){
        // x = _x;
    }

Explanation

The reason for the difference is that forge considers each test as one tx, therefore when the constructor initializes x - that makes it a warm key/var and the cost for next sload would be only 100 gas units. When x is not initialized its key is cold and therefore the cost is the 'full price' of 2.1K.

More info on sload cost can be seen here

Gas.sol

//SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.0;


contract Store{
    uint256 public x;
    constructor(uint256 _x){
        x = _x;
    }
}

Gas.t.sol

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.13;

import "forge-std/Test.sol";
import "../contracts/Gas.sol";

contract ContractTest is Test {
    function testGas() public {
        Store s = new Store(5);
        // changing block number and timestamp wouldn't help
        vm.warp(block.timestamp + 30);
        vm.roll(block.number + 1);
        
        uint256 myx = s.x();
        console2.log("x is", myx);
    }
}

Created an issue for this foundry-rs/foundry#2844