We all know the importance of saving gas in smart contracts. In this article, we will try to understand why it is important to structure our read-and-write storage operations appropriately to save more gas.

There are 3 types of storage in EVM at the moment, storage, memory, and stack. In most scenarios, when we want to persist data on a chain we use storage. Data in storage persists even after the end of the transaction lifecycle.

But storage is also one of the most expensive options, hence it is important to structure our code properly when using on-chain storage.

Let's look at an example. Please go through the below code line by line.

// gas values from remix

pragma solidity ^0.8.20;

contract StorageRead {
    uint256 x = 1;  // storage variable
    uint256 y = 2;  // storage variable
    // 2454 gas
    function coldRead() external view returns (uint256 a) {
        a = x;      // cold read
    }
    // 2539 gas
    function coldWarmRead() external view returns (uint256 a) {
        a = x;   // cold read
        a = x;  // warm read
    }
    // 4622 gas
    function coldColdRead() external view returns (uint256 a, uint256 b) {
        a = x;      // cold read
        b = y;  // cold read
    }
    // 2687 gas
    function coldWarmRead2() external view returns (uint256 a,uint256 b) {
        a = x;   // cold read
        b = x;  // warm read
    }
    // 2454 gas
    function coldRead2() external view returns (uint256 a) {
        a = y;      // cold read
    }

}

In the above code, notice the gas cost for each of the functions.

• coldRead- Read from the storage only one time. This refers to cold read.

• coldWarmRead- Read from the storage the first time (cold read) and then read the same storage the second time (warm read).

• coldColdRead- Read from the storage twice (cold read) but different storage slots.

Notice that the gas cost varies a lot when we do two cold reads (coldColdRead) from the storage as compared to one cold read and a warm read (coldWarmRead).

The cost of cold SLOAD is 2100 and the subsequent cost of the same slot is 100.

This is because when a slot is loaded (SLOAD) or some value is stored (SSTORE)at a slot, it will be considered accessed for the rest of the transaction.

Some other notable gas costs,

SSTORE to change the value from 0 to 1 costs 21000 (cold) and 20000 (hot).

SSTORE to change value from 1 to 2 costs 5000 (cold) and 2900 (hot).

Now we will look at another example to understand how we can implement what we learn so far and optimize a bit using memory.

// gas values from remix

pragma solidity ^0.8.20;

contract StorageMemoryRead {
    uint256 x = 1;  // storage variable
    uint256 y = 2;  // storage variable

    // 2993 gas
    function coldRead() external view returns (uint256 a) {
         if(x < 2){
             a = x;  // warm storage read
         }

         if(x==1){ // warm storage read
             a = x+1; // warm storage read
         }
    }

    // 2785 gas
     function coldWithMemory() external view returns (uint256 a) {
         uint256 copy = x; // copy to memory
         if(copy < 2){
             a = copy;
         }

         if(x==1){
             a = copy+1;
         }
    }
}

Let's understand the above code, coldRead, this function uses a lot of SSLOAD opcode by reading the same storage multiple times. The same thing is being achieved by the function, coldWithMemory, but by using memory. This costs less gas as MLOAD & MSTORE only cost 3 gas.