Foundry Book

Gas Snapshots

Forge can generate gas snapshots for all your test functions. This can be useful to get a general feel for how much gas your contract will consume, or to compare gas usage before and after various optimizations.

To generate the gas snapshot, run forge snapshot.

This will generate a file called .gas-snapshot by default with all your tests and their respective gas usage.

$ forge snapshot
$ cat .gas-snapshot

ERC20Test:testApprove() (gas: 31162)
ERC20Test:testBurn() (gas: 59875)
ERC20Test:testFailTransferFromInsufficientAllowance() (gas: 81034)
ERC20Test:testFailTransferFromInsufficientBalance() (gas: 81662)
ERC20Test:testFailTransferInsufficientBalance() (gas: 52882)
ERC20Test:testInfiniteApproveTransferFrom() (gas: 90167)
ERC20Test:testMetadata() (gas: 14606)
ERC20Test:testMint() (gas: 53830)
ERC20Test:testTransfer() (gas: 60473)
ERC20Test:testTransferFrom() (gas: 84152)

Filtering

If you would like to specify a different output file, run forge snapshot --snap <FILE_NAME>.

You can also sort the results by gas usage. Use the --asc option to sort the results in ascending order and --desc to sort the results in descending order.

Finally, you can also specify a min/max gas threshold for all your tests. To only include results above a threshold, you can use the --min <VALUE> option. In the same way, to only include results under a threshold, you can use the --max <VALUE> option.

Keep in mind that the changes will be made in the snapshot file, and not in the snapshot being displayed on your screen.

You can also use it in combination with the filters for forge test, such as forge snapshot --match-path contracts/test/ERC721.t.sol to generate a gas snapshot relevant to this test contract.

Comparing gas usage

If you would like to compare the current snapshot file with your latest changes, you can use the --diff or --check options.

--diff will compare against the snapshot and display changes from the snapshot.

It can also optionally take a file name (--diff <FILE_NAME>), with the default being .gas-snapshot.

For example:

$ forge snapshot --diff .gas-snapshot2

Running 10 tests for src/test/ERC20.t.sol:ERC20Test
[PASS] testApprove() (gas: 31162)
[PASS] testBurn() (gas: 59875)
[PASS] testFailTransferFromInsufficientAllowance() (gas: 81034)
[PASS] testFailTransferFromInsufficientBalance() (gas: 81662)
[PASS] testFailTransferInsufficientBalance() (gas: 52882)
[PASS] testInfiniteApproveTransferFrom() (gas: 90167)
[PASS] testMetadata() (gas: 14606)
[PASS] testMint() (gas: 53830)
[PASS] testTransfer() (gas: 60473)
[PASS] testTransferFrom() (gas: 84152)
Test result: ok. 10 passed; 0 failed; finished in 2.86ms
testBurn() (gas: 0 (0.000%))
testFailTransferFromInsufficientAllowance() (gas: 0 (0.000%))
testFailTransferFromInsufficientBalance() (gas: 0 (0.000%))
testFailTransferInsufficientBalance() (gas: 0 (0.000%))
testInfiniteApproveTransferFrom() (gas: 0 (0.000%))
testMetadata() (gas: 0 (0.000%))
testMint() (gas: 0 (0.000%))
testTransfer() (gas: 0 (0.000%))
testTransferFrom() (gas: 0 (0.000%))
testApprove() (gas: -8 (-0.000%))
Overall gas change: -8 (-0.000%)

--check will compare a snapshot with an existing snapshot file and display all the differences, if any. You can change the file to compare against by providing a different file name: --check <FILE_NAME>.

For example:

$ forge snapshot --check .gas-snapshot2

Running 10 tests for src/test/ERC20.t.sol:ERC20Test
[PASS] testApprove() (gas: 31162)
[PASS] testBurn() (gas: 59875)
[PASS] testFailTransferFromInsufficientAllowance() (gas: 81034)
[PASS] testFailTransferFromInsufficientBalance() (gas: 81662)
[PASS] testFailTransferInsufficientBalance() (gas: 52882)
[PASS] testInfiniteApproveTransferFrom() (gas: 90167)
[PASS] testMetadata() (gas: 14606)
[PASS] testMint() (gas: 53830)
[PASS] testTransfer() (gas: 60473)
[PASS] testTransferFrom() (gas: 84152)
Test result: ok. 10 passed; 0 failed; finished in 2.47ms
Diff in "ERC20Test::testApprove()": consumed "(gas: 31162)" gas, expected "(gas: 31170)" gas