slide 24 · contracts

Advanced contract writing

naive

88k gas

packed

31k gas

costs, vs memory, mappings, inheritance and custom errors.

1Toggle the gas optimisations
2Compare gas before and after
3Apply the checks-effects-interactions order

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Naive cost

63,000 gas

Optimised cost

23,160 gas

Saving

63%

Naive version

 1function sum() external view returns (uint256 t) {
 2    for (uint i = 0; i < items.length; i++) {
 3        t += items[i];   // SLOAD every iteration
 4    }
 5}

Improved version

 1function sum() external view returns (uint256 t) {
 2    uint256[] memory local = items;  // one copy
 3    uint256 len = local.length;      // cached
 4    for (uint i; i < len; ++i) {
 5        t += local[i];   // MLOAD, ~100x cheaper
 6    }
 7}

Why

An SLOAD costs 2,100 gas cold; an MLOAD costs 3. Reading array length inside the loop condition is a storage read on every pass. Copy to memory once, cache the length, and use ++i rather than i++ to skip a temporary.

Advanced is mostly two disciplines: knowing what each opcode costs, and assuming every external call is hostile. Test with Foundry fuzzing, measure with snapshots, and get an audit before holding real value.