The ModuleKit has an inbuilt helper to granularly calculate the gas costs of UserOperations. For example, it calculates the gas costs of the various phases of the ERC-4337 flow and L2-L1 calldata gas.

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Setup

Before using this utility, create a folder called gas_calculations in the root folder of the directory (same level as the src folder) and ensure that you have the right permissions settings set in your foundry.toml file: 
fs_permissions = [{ access = "read-write", path = "gas_calculations" }]

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Write the test

In the test function(s) that you want to perform, call instance.log4337Gas("identifier") before a UserOperation is executed. This will log the gas used for the UserOperation and use the identifier as the filename for the gas report. Make sure to use a unique identifier for each UserOp, otherwise the gas reports will be overwritten. For example: 
// Write gas
instance.log4337Gas("testWriteGas");

// Create userOperation
instance.exec({ target: address(1), value: 1 wei, callData: "" });

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Run the tests

To run the tests and log gas, run the following command: 
GAS=true forge test

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Read the gas report

After the tests have finished, the gas reports will be written to the gas_calculations folder in the root of the project. A gas report will look like this: 
{
  "Calldata": {
    "Arbitrum": "6504 gas",
    "OP-Stack": "8859 gas"
  },
  "Phases": {
    "Creation": "187_170 gas",
    "Execution": "37_054 gas",
    "Validation": "35_980 gas"
  },
  "Total": "2_550_948 gas"
}
Note that if you re-run gas calculations, then the newest number will be added and every value will be followed by a (diff: ...) that shows the difference, in gas units between the calculations.

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Convert the gas units

All numbers are in gas units. To calculate the cost in gwei, follow the instructions below:
  • On L1: multiply the total gas used by the gas price in gwei: totalGasUsed * gasPrice
  • On an L2: multiply the total gas used by the L2 gas price in gwei and then add it to to the result of the relevant L2-L1 gas cost muliplied by the L1 gas cost in gwei: totalGasUsed * l2GasPrice + callDataGasUsed * l1GasPrice
Note: if you are calculating the dollar amount of the gas cost, make sure to use the relevant gwei to usd conversions if the L2 has a different gas token to L1.