Memory caching vs direct storage vs pointers

Gas notes

Avoid zero to one storage writes where possible

Initializing a storage variable is one of the most expensive operations a contract can do.

When a storage variable goes from zero to non-zero, the user must pay 22,100 gas total (20,000 gas for a zero to non-zero write and 2,100 for a cold storage access).

This is why the Openzeppelin reentrancy guard registers functions as active or not with 1 and 2 rather than 0 and 1. It only costs 5,000 gas to alter a storage variable from non-zero to non-zero.

Reading & Writing #1

• each function writes to a different stream to avoid overwriting each other

// SPDX-License-Identifier: UNLICENSED
pragma solidity 0.8.24;

contract GasCostsV2 {

    struct Stream {
        // slot 0
        uint256 claimed;
        // slot 1
        uint256 lastClaimedTimestamp;
    }

    mapping(uint256 tokenId => Stream stream) public streams;
        
    function testMemory() public {
        
        //cache
        Stream memory stream = streams[1];

        //doStuff
        stream.claimed += 10 ether;
        stream.lastClaimedTimestamp = block.timestamp;
        
        //update storage
        streams[1] = stream;
    }

    function testStorage() public {
              
        //doStuff
        streams[2].claimed += 10 ether;
        streams[2].lastClaimedTimestamp = block.timestamp;
    }

    
    function testPointers() public {
        
        // pointer
        Stream storage stream = streams[3];
        
        //doStuff
        stream.claimed += 10 ether;
        stream.lastClaimedTimestamp = block.timestamp;
    }
}

Gas used

• storage: 66,540

• memory: 66,621

• pointer: 66,474

Memory cost the most gas, and pointers the least.

Link: https://sepolia.arbiscan.io/address/0xfa4123e66ddbde65a7be3e58c9472095799b479d

Why does caching to memory cost more than storage?

testStorage uses the following opcodes:

• sload to load the value of streams[2].claimed to stack for the incrementation operation

• Two sstore to store the updated value of claimed and timestamp

testMemory uses the following opcodes:

• Two sload to load both uint256 values to memory

• Two sstore to store the updated value of claimed and timestamp

TestMemory has an extra sload due to caching.

Links

• storage: https://dashboard.tenderly.co/tx/arbitrum-sepolia/0x25660453cb8bc3df455bb00e4c43f0c72bcf3650c2df34ab9a538d80808bb5ad
• memory: https://dashboard.tenderly.co/tx/arbitrum-sepolia/0xe73912870f63f515b68e7df515eca894184d996fae3f54ac4de61d267abaff87

Reading & Writing #2

What if we do not increment?

contract GasCostsV2 {

    struct Stream {
        // slot 0
        uint256 claimed;
        // slot 1
        uint256 lastClaimedTimestamp;
    }

    mapping(uint256 tokenId => Stream stream) public streams;
        
    function testMemory() public {
        
        //cache
        Stream memory stream = streams[1];

        //doStuff
        stream.claimed = 10 ether;
        stream.lastClaimedTimestamp = block.timestamp;
        
        //update storage
        streams[1] = stream;
    }

    function testStorage() public {
              
        //doStuff
        streams[2].claimed = 10 ether;
        streams[2].lastClaimedTimestamp = block.timestamp;
    }

    
    function testPointers() public {
        
        // pointer
        Stream storage stream = streams[3];
        
        //doStuff
        stream.claimed = 10 ether;
        stream.lastClaimedTimestamp = block.timestamp;
    }
}

Gas used

• storage: 68,329

• memory: 68,379

• pointer: 68,263

Memory cost the most gas, and pointers the least.

Contract: https://sepolia.arbiscan.io/address/0xe8dc0444916cbfbc70949878a673ad535f481cd7

Why does caching to memory cost more than storage?

Caching to memory costs 50 gas units more than writing to storage directly.

When to cache to memory?

Cache storage variables: write and read storage variables exactly once

// SPDX-License-Identifier: MIT
pragma solidity 0.8.20;

contract Counter1 {
    uint256 public number;

    function increment1() public {
        require(number < 10);
        number = number + 1;
    }
}

contract Counter2 {
    uint256 public number;

    function increment2() public {
        uint256 _number = number;
        require(_number < 10);
        number = _number + 1;
    }
}

• increment1 reads number from storage multiple times; each subsequent SLOAD after the first costs 100 gas

• increment2 caches to avoid repeated SLOADs