Understanding How the EVM Stores Mappings, Arrays, and Structs in Solidity

The Ethereum Virtual Machine (EVM) is the heart of smart contract execution on the Ethereum blockchain. It manages data in a way that ensures both efficiency and security, especially given the constraints of blockchain environments. In this post, we'll explore how the EVM stores three fundamental data types in Solidity: mappings, arrays, and structs. Understanding this storage layout is crucial for optimizing your smart contracts and minimizing gas costs.

Mapping Storage in EVM

Mappings in Solidity are akin to hash tables in other programming languages. They store key-value pairs, which allows for efficient data retrieval. However, the EVM handles mappings uniquely due to its need to maintain an optimized and secure storage layout.

• Storage Mechanism: Unlike conventional arrays, mappings do not store their data contiguously. Instead, each key-value pair is stored at a location derived by hashing the key with the position of the mapping in storage. For example, for a mapping mapping(uint => uint) map;, the value of map[key] is stored at the location determined by keccak256(h(k) . p), where k is the key and p is the mapping's position in the contract's storage.

• Efficiency: This hashed storage approach makes mappings highly efficient for lookups, as the EVM can quickly compute the storage location of any key-value pair without scanning through an array of elements.

Array Storage in EVM

Arrays in Solidity can be either static (fixed-size) or dynamic (resizable), and each type is stored differently in the EVM.

• Static Arrays: For static arrays, where the size is known at compile time, the EVM stores elements in contiguous slots. For instance, if an array starts at slot 0, its elements are stored in slots 0, 1, 2, etc., based on the array's length.

• Dynamic Arrays: Dynamic arrays are a bit more complex. The first storage slot holds the array's length, and the elements themselves are stored starting at a location computed by keccak256(slot), where slot is the storage position of the length. This setup allows for dynamic resizing of the array without disrupting the storage of other variables.

Struct Storage in EVM

Structs in Solidity are user-defined types that can hold multiple variables of different types. The EVM optimizes the storage of structs by packing and padding variables efficiently.

• Sequential Storage: Variables within a struct are stored sequentially in the order they are defined. The EVM tries to pack multiple variables into a single 32-byte storage slot if possible, reducing the number of storage slots used.

• Padding and Packing: If a struct contains variables smaller than 32 bytes, the EVM packs them into a single slot. For example, two uint8 variables will be stored in the same 32-byte slot. If the variables exceed 32 bytes, the overflow spills into the next slot. This packing mechanism helps save storage space, which in turn reduces gas costs.

Optimizing Storage Layout

Understanding how the EVM stores these data structures allows you to optimize your smart contracts. Here are some best practices:

• Reorder Struct Variables: Group smaller variables together in structs to minimize the number of slots used.
• Minimize Dynamic Arrays: Whenever possible, use static arrays, as they are simpler and cheaper to store.
• Use Mappings Wisely: Since mappings can consume significant storage if used inefficiently, ensure that your keys are unique and necessary.

Conclusion

Optimizing the storage of mappings, arrays, and structs in Solidity is essential for writing efficient smart contracts on the Ethereum blockchain. By understanding the underlying mechanisms of the EVM, you can structure your contracts to minimize gas costs and maximize performance.