Write after write

Severity: Gas ID: write-after-write

Flags storage variables that are written to consecutively without the first value ever being read. The first write is dead code; it pays the SSTORE cost but its value is immediately discarded when the second write overwrites it.

What it does

Reports the first assignment to a state variable when that same variable is written a second time before being read. Detection covers:

• Plain = assignments and delete on bare state variable identifiers
• Tuple/destructuring assignments: (x, y) = (1, 2) tracks each component individually
• Pre/post increment and decrement (++x, x--, etc.) — these read then write, so the write they produce can itself become dead if immediately overwritten

The analysis recurses into branch bodies (if/else, loops, try clauses) and modifier bodies with fresh state so intra-body pairs are still caught. Conditional boundaries (&&, ||, ternary) are handled conservatively to avoid false positives across short-circuit paths. Compound assignments (+=, |=, etc.) and index/member writes (mapping[k], struct.field) are excluded to avoid false positives.

Why is this bad?

Every SSTORE costs at least 2,900 gas (warm slot) or 20,000 gas (cold slot). Writing a value to storage and then immediately overwriting it wastes that gas with no observable effect; only the final write matters.

Example

Bad

contract C {
    uint256 public x;

    function bad(uint256 v) external {
        x = 0;   // write-after-write: this value is never read
        x = v;   // second write overwrites the first
    }
}

Good

contract C {
    uint256 public x;

    // Write once with the final value directly.
    function good(uint256 v) external {
        x = v;
    }

    // Reading between writes is fine.
    function goodRead(uint256 v) external returns (uint256 prev) {
        x = 1;
        prev = x; // x is read here
        x = v;
    }

    // Compound assignments read before writing, not flagged.
    function goodCompound() external {
        x = 1;
        x += 1;
    }
}