Solidity Deep Dive (2): Is Memory Always Cheaper Than Storage?

The Question

While studying the Magic Dragon DAO contracts, I noticed something odd in the _removeZeroStakes function. The Stake struct variable s is declared with the storage keyword inside a for loop, even though it is only read (not written):

function _removeZeroStakes() internal {
    bool shouldRecurse = stakes.length > 0;

    for (uint256 i = 0; i < stakes.length; i++) {
        _updateStakeDepositAmount(i);

        Stake storage s = stakes[i]; // Why storage, not memory?

        if (s.amount == 0) {
            _removeStake(i);
            break;
        }

        if (i == stakes.length - 1) {
            shouldRecurse = false;
        }
    }

    if (shouldRecurse) {
        _removeZeroStakes();
    }
}

Shouldn’t memory always be cheaper since the data doesn’t need to persist on-chain?

The Answer: Not Always

When you assign a storage reference to a memory variable, the EVM must:

1. Allocate new memory space (additional gas cost).
2. Copy the struct contents from storage into memory.

Each subsequent access uses MLOAD (cheap), but the upfront allocation cost is non-trivial.

When you declare a storage reference, there is:

• No memory allocation cost.
• Just a single SLOAD per field access.

Rule of Thumb

Access Count	Recommended Keyword
≤ 4	storage
> 4	memory

With four or fewer accesses, the overhead of memory allocation outweighs the per-access savings of MLOAD over SLOAD. Beyond that threshold, memory becomes cheaper overall. [1]

In the example above, only one field (s.amount) is accessed, so Stake storage s is indeed the gas-optimal choice.

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Reference

[1] Switching from storage to memory increases the gas cost — Ethereum Stack Exchange

2026 Update Note

• This post was migrated from the original blog and language-polished in 2026.
• Gas costs for SLOAD and memory expansion have been adjusted by multiple EIPs since 2022 (e.g., EIP-2929 cold/warm access). The general principle still holds, but exact break-even points may differ on current mainnet.