A Practical Introduction To Solidity Assembly: Part 1

In the previous part (Part 0) we covered some pre-requisite knowledge of EVM. In this part, we will write the Box contract partially in assembly - the function bodies of retrieve() and store(). Here is our Box contract in Solidity:

pragma solidity ^0.8.7;

contract Box {
    uint256 private _value;

    event NewValue(uint256 newValue);

    function store(uint256 newValue) public {
        _value = newValue;
        emit NewValue(newValue);
    }

    function retrieve() public view returns (uint256) {
        return _value;
    }
}

You can interleave Solidity statements with assembly { } blocks and can write inline assembly within that code block. As mentioned previously language used for assembly for EVM is Yul.

Prelude

Before you move further, I want to clear up some possible confusion. When you compile a contract it is compiled to bytecode. Bytecode is nothing but a long sequence of bytes, something like:

608060405234801561001....36f6c63430008070033

What does it represent? Well, it is nothing but list of tiny, 1 byte, instructions appended together. These tiny instructions are called opcodes. EVM understands/interprets these opcodes during execution of any operation in the contract.

In the above bytecode, for example, the first instruction is 60 (1 byte), which is code for opcode PUSH1. You can look up all of the EVM opcodes on evm.codes. These opcodes comprise of “EVM dialect”.

While writing assembly code, in Yul, you’ll notice that Yul provides functions like add, sload, store, mstore etc. which are similar to opcodes of EVM dialect - ADD, SLOAD, SSTORE, MLOAD respectively. But, EVM dialect is not exactly the same as Yul’s. Even though most opcodes from EVM dialect are available in Yul dialect, but not all. E.g. PUSH1 opcode has no corresponding Yul function. You can check all such Yul functions on the docs.

Even though while writing Yul you get fine-grain control, Yul still manages some of the even more low level things like local variables and control flow. So, the order of level of control would be like:

Solidity < Yul (assembly) < bytecode (opcodes)

If you would want to go even beyond Yul, you’d be writing raw bytecode for the EVM and won’t need the compiler 🤖.

Note: From now on I may use the term “opcode” to refer to Yul function that corresponds to EVM opcode in the context, since all common EVM opcodes are provided by Yul with similar function signature.

Inline Assembly

Now, let’s use inline assembly to write retrieve() and store() function bodies of Box in assembly. We simply use assembly { } blocks. These blocks have access to outside local variables too.

First, focus on retrieve() function body. All it does is:

  • Read _value state variable from storage
  • Return the read value

You want opcodes to perform this. evm.codes is a great reference to search for EVM opcodes. Have a look there and search for our first opcode: sload.

sload

sload takes one input:

And returns the read value to call stack.

Go ahead read the slot 0 (_value is in slot 0):

assembly {
    let v := sload(0) // _value is at slot #0
}

Now how do we return it? Right! The return opcode.

return

It takes two inputs:

  • offset which is the location of where the value starts from in memory
  • size which is the number of bytes starting from offset.

But v returned by sload is in call-stack not memory for return to work with! So we need to move it to memory first. Enter mstore.

mstore

It also takes two inputs:

  • offset which is location (in memory array) where the value should be stored
  • value which is bytes to store (which is v for us).

Proceeding:

assembly {
    let v := sload(0) // read from slot #0
    mstore(0x80, v) // store v at position 0x80 in memory
    return(0x80, 32) // v is 32 bytes (uint256)
}

That’s it we converted retrieve() body to assembly!

Note: If you’re curious as to why I specifically chose 0x80 position in memory to store to store the value, it’s because Solidity reserves four 32 byte slots (i.e. 0x00 to 0x7f) for special purposes. So free memory starts from 0x80 initially. While it’s ok to just put 0x80 to store new variable in our very simple case, any more complex operation requires you to keep track of pointer to free memory and manage it. Read more in the docs.

function retrieve() public view returns(uint256) {
    assembly {
        // load value at slot 0 of storage
        let v := sload(0) 

        // store into memory at 0x80
        mstore(0x80, v)

        // return value at 0 memory address of size 32 bytes
        return(0x80, 32) 
    }
}

The store() function body can be written on the similar lines. I encourage you to reference the opcodes and try to write it on your own before moving further!

Alright, here is the store() function body in assembly:

function store(uint256 newValue) public {
    assembly {
        // store value at slot 0 of storage
        sstore(0, newValue)

        // emit event
        mstore(0x80, newValue)
        log1(0x80, 0x20,0xac3e966f295f2d5312f973dc6d42f30a6dc1c1f76ab8ee91cc8ca5dad1fa60fd)
    }
}

This time we used sstore opcode to store new value - the newValue param, at the same storage slot (slot 0) where our _value state variable is stored. Hence overwriting the old value with new value.

To emit the event (NewValue) with new value data, we used log1 opcode. First two parameters to it are offset in memory and size of the data like other opcodes.

So, we first moved newValue to memory at 0x80 position with mstore. Then, passed 0x80 as offset and 0x20 (32 in decimal) as size to log1 opcode. Now, you might be wondering what is that third argument we passed in to log1. It is topic - sort of a label for event, like the name - NewValue. The passed in argument is nothing but the hash of the event signature:

bytes32(keccak256("NewValue(uint256)"))

// 0xac3e966f295f2d5312f973dc6d42f30a6dc1c1f76ab8ee91cc8ca5dad1fa60fd

Finally, our Box contract looks like this now:

pragma solidity ^0.8.7;

contract Box {
    uint256 public value;

    function retrieve() public view returns(uint256) {
        assembly {
            // load value at slot 0 of storage
            let v := sload(0) 

            // store into memory at 0
            mstore(0x80, v)

            // return value at 0 memory address of size 32 bytes
            return(0x80, 32) 
        }
    }

    function store(uint256 newValue) public {
        assembly {
            // store value at slot 0 of storage
            sstore(0, newValue)

            // emit event
            mstore(0x80, newValue)
            log1(0x80, 0x20,0xac3e966f295f2d5312f973dc6d42f30a6dc1c1f76ab8ee91cc8ca5dad1fa60fd)
        }
    }
}

In the next, part we’ll be writing the Box in pure assembly. Buckle up! 😎

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