Gas messen

Gas optimization without measurement is guesswork. The compiler, opcode costs, and access patterns interact in non-obvious ways, so an intuitive change can sometimes make things worse.

Always profile before and after. A change that does not reduce measured gas is not an optimization, no matter how clever it looks.

Foundry's forge test --gas-report prints a per-function table of min, average, median, and max gas usage across all your tests. It is the fastest way to spot expensive functions.

forge snapshot writes the gas cost of every test to a .gas-snapshot file. Commit it to version control, then run forge snapshot --diff to see exactly how a change affected gas.

This turns gas regressions into reviewable diffs in pull requests.

For fine-grained measurement, read gasleft() before and after a call inside a test. The difference is the gas consumed by that block of code.

Running tests with -vvvv shows a full execution trace including the gas cost of each external call and opcode group. This reveals where gas is spent, not just the total.

The Solidity optimizer changes runtime and deployment gas. The runs parameter is a hint about how often functions are called.

• Low runs (e.g. 1) optimizes for cheap deployment
• High runs (e.g. 10000) optimizes for cheap repeated calls

Set it in foundry.toml and always measure with the optimizer enabled, since it reflects production behavior.

To compare two approaches, write a test that exercises each under identical conditions and inspect the gas report side by side.

Keep inputs the same and isolate the change to one variable, otherwise you cannot attribute the gas difference to the optimization you made.

Gas reports can mislead because the first access in a test is cold and later ones are warm. A function called once in setup may show a higher cold cost than the same function later.

Look at min, median, and max columns together rather than a single average, and design tests that reflect real-world access patterns.

On-chain transaction receipts report the actual gasUsed. Etherscan and block explorers display this for every transaction, letting you validate your local measurements against mainnet reality.

The eth_estimateGas RPC method gives a pre-flight estimate, though it adds a safety margin and may differ from the real execution cost.

Wire forge snapshot --check into continuous integration. It fails the build if any test's gas exceeds the committed snapshot beyond a tolerance.

This prevents accidental gas regressions from slipping into production and makes gas a first-class quality metric.

A solid gas workflow:

• Write tests covering the hot paths
• Record a baseline with forge snapshot
• Make one optimization at a time
• Run forge snapshot --diff to confirm the gain
• Gate regressions in CI with --check

Discipline beats cleverness.

You learned to measure gas:

• forge test --gas-report for per-function tables
• forge snapshot --diff to track changes
• gasleft() for fine-grained measurement
• Verbose traces reveal where gas goes
• Gate regressions in CI with --check

This completes the Gas Optimization course. Next: the Foundry toolkit in depth.