Gas Section Snapshots

Forge can capture gas snapshots over arbitrary sections inside of your test functions. This can be useful to get a granular measurement of how much gas your logic is consuming as both external calls and internal gas usage are measured.

Instead of running a command like forge snapshot or forge test --gas-report, you use the snapshotGas cheatcodes in your tests to capture gas usage as follows:

snapshotGas cheatcodes

Signature

/// Start a snapshot capture of the current gas usage by name.
/// The group name is derived from the contract name.
function startSnapshotGas(string calldata name) external;

/// Start a snapshot capture of the current gas usage by name in a group.
function startSnapshotGas(string calldata group, string calldata name) external;

/// Stop the snapshot capture of the current gas by latest snapshot name, capturing the gas used since the start.
function stopSnapshotGas() external returns (uint256 gasUsed);

/// Stop the snapshot capture of the current gas usage by name, capturing the gas used since the start.
/// The group name is derived from the contract name.
function stopSnapshotGas(string calldata name) external returns (uint256 gasUsed);

/// Stop the snapshot capture of the current gas usage by name in a group, capturing the gas used since the start.
function stopSnapshotGas(string calldata group, string calldata name) external returns (uint256 gasUsed);

/// Snapshot capture an arbitrary numerical value by name.
/// The group name is derived from the contract name.
function snapshotValue(string calldata name, uint256 value) external;

/// Snapshot capture an arbitrary numerical value by name in a group.
function snapshotValue(string calldata group, string calldata name, uint256 value) external;

/// Snapshot capture the gas usage of the last call by name from the callee perspective.
function snapshotGasLastCall(string calldata name) external returns (uint256 gasUsed);

/// Snapshot capture the gas usage of the last call by name in a group from the callee perspective.
function snapshotGasLastCall(string calldata group, string calldata name) external returns (uint256 gasUsed);

Description

snapshotGas* cheatcodes allow you to capture gas usage in your tests. This can be useful to track how much gas your logic is consuming. You can capture the gas usage of the last call by name, capture an arbitrary numerical value by name, or start and stop a snapshot capture of the current gas usage by name.

In order to strictly compare gas usage across test runs, set the FORGE_SNAPSHOT_CHECK environment variable to true before running your tests. This will compare the gas usage of your tests against the last snapshot and fail if the gas usage has changed. By default the snapshots directory will be newly created and its contents removed before each test run to ensure no stale data is present.

It is intended that the snapshots directory created when using the snapshotGas* cheatcodes is checked into version control. This allows you to track changes in gas usage over time and compare gas usage during code reviews.

When running forge clean the snapshots directory will be deleted.

Examples

Capturing the gas usage of a section of code that calls an external contract:

contract SnapshotGasTest is Test {
    uint256 public slot0;

    Flare public flare;

    function setUp() public {
        flare = new Flare();
    }

    function testSnapshotGas() public {
        vm.startSnapshotGas("externalA");
        flare.run(256);
        uint256 gasUsed = vm.stopSnapshotGas();
    }
}

Capturing the gas usage of multiple sections of code that modify the internal state:

contract SnapshotGasTest is Test {
    uint256 public slot0;


    /// Writes to `snapshots/SnapshotGasTest.json` group with name `internalA`, `internalB`, and `internalC`.
    function testSnapshotGas() public {
        vm.startSnapshotGas("internalA");
        slot0 = 1;
        vm.stopSnapshotGas();

        vm.startSnapshotGas("internalB");
        slot0 = 2;
        vm.stopSnapshotGas();

        vm.startSnapshotGas("internalC");
        slot0 = 0;
        vm.stopSnapshotGas();
    }
}

Capturing the gas usage of a section of code that modifies both the internal state and calls an external contract:

contract SnapshotGasTest is Test {
    uint256 public slot0;
    Flare public flare;

    function setUp() public {
        flare = new Flare();
    }

    /// Writes to `snapshots/SnapshotGasTest.json` group with name `combinedA`.
    function testSnapshotGas() public {
        vm.startSnapshotGas("combinedA");
        flare.run(256);
        slot0 = 1;
        vm.stopSnapshotGas();
    }
}

Capturing an arbitrary numerical value (such as the bytecode size of a contract):

```solidity
contract SnapshotGasTest is Test {
    uint256 public slot0;

    /// Writes to `snapshots/SnapshotGasTest.json` group with name `valueA`, `valueB`, and `valueC`.
    function testSnapshotValue() public {
        uint256 a = 123;
        uint256 b = 456;
        uint256 c = 789;

        vm.snapshotValue("valueA", a);
        vm.snapshotValue("valueB", b);
        vm.snapshotValue("valueC", c);
    }
}

Capturing the gas usage of the last call from the callee perspective:

contract SnapshotGasTest is Test {
    Flare public flare;

    function setUp() public {
        flare = new Flare();
    }

    /// Writes to `snapshots/SnapshotGasTest.json` group with name `lastCallA`.
    function testSnapshotGasLastCall() public {
        flare.run(1);
        vm.snapshotGasLastCall("lastCallA");
    }
}

For each of the above examples you can also use the group variant of the cheatcodes to group the snapshots together in a custom group.

contract SnapshotGasTest is Test {
    uint256 public slot0;

    /// Writes to `snapshots/CustomGroup.json` group with name `internalA`, `internalB`, and `internalC`.
    function testSnapshotGas() public {
        vm.startSnapshotGas("CustomGroup", "internalA");
        slot0 = 1;
        vm.stopSnapshotGas();

        vm.startSnapshotGas("CustomGroup", "internalB");
        slot0 = 2;
        vm.stopSnapshotGas();

        vm.startSnapshotGas("CustomGroup", "internalC");
        slot0 = 0;
        vm.stopSnapshotGas();
    }
}