Differences from Ethereum


Differences from Ethereum

zkSync Era handles nearly all smart contracts based on the Ethereum Virtual Machine (EVM) and upholds high security standards, minimizing the need for repeated security audits. Nevertheless, it's essential to recognize the following differences.

Best practices

It's highly recommended to review the best practices and key considerations for creating smart contracts on zkSync Era section here.

EVM instructions

CREATE, CREATE2

On zkSync Era, contract deployment is performed using the hash of the bytecode, and the factoryDeps field of EIP712 transactions contains the bytecode. The actual deployment occurs by providing the contract's hash to the ContractDeployer system contract.

To guarantee that create/create2 functions operate correctly, the compiler must be aware of the bytecode of the deployed contract in advance. The compiler interprets the calldata arguments as incomplete input for ContractDeployer, as the remaining part is filled in by the compiler internally. The Yul datasize and dataoffset instructions have been adjusted to return the constant size and bytecode hash rather than the bytecode itself.

The code below should work as expected:

MyContract a = new MyContract();
MyContract a = new MyContract{salt: ...}();

In addition, the subsequent code should also work, but it must be explicitly tested to ensure its intended functionality:

bytes memory bytecode = type(MyContract).creationCode;
assembly {
    addr := create2(0, add(bytecode, 32), mload(bytecode), salt)
}

The following code will not function correctly because the compiler is not aware of the bytecode beforehand:

function myFactory(bytes memory bytecode) public {
   assembly {
      addr := create(0, add(bytecode, 0x20), mload(bytecode))
   }
}

Unfortunately, it's impossible to differentiate between the above cases during compile-time. As a result, we strongly recommend including tests for any factory that deploys child contracts using type(T).creationCode.

Since the deploy and runtime code is merged together on zkSync Era, we do not support type(T).runtimeCode and it always produces a compile-time error.

Address derivation

For zkEVM bytecode, zkSync Era uses a distinct address derivation method compared to Ethereum. The precise formulas can be found in our SDK, as demonstrated below:

export function create2Address(sender: Address, bytecodeHash: BytesLike, salt: BytesLike, input: BytesLike) {
  const prefix = ethers.utils.keccak256(ethers.utils.toUtf8Bytes("zksyncCreate2"));
  const inputHash = ethers.utils.keccak256(input);
  const addressBytes = ethers.utils.keccak256(ethers.utils.concat([prefix, ethers.utils.zeroPad(sender, 32), salt, bytecodeHash, inputHash])).slice(26);
  return ethers.utils.getAddress(addressBytes);
}

export function createAddress(sender: Address, senderNonce: BigNumberish) {
  const prefix = ethers.utils.keccak256(ethers.utils.toUtf8Bytes("zksyncCreate"));
  const addressBytes = ethers.utils
    .keccak256(ethers.utils.concat([prefix, ethers.utils.zeroPad(sender, 32), ethers.utils.zeroPad(ethers.utils.hexlify(senderNonce), 32)]))
    .slice(26);

  return ethers.utils.getAddress(addressBytes);
}

Since the bytecode differs from Ethereum as zkSync uses a modified version of the EVM, the address derived from the bytecode hash will also differ. This means that the same bytecode deployed on Ethereum and zkSync will have different addresses and the Ethereum address will still be available and unused on zkSync. If and when the zkEVM reaches parity with the EVM, the address derivation will be updated to match Ethereum and the same bytecode will have the same address on both chains, deployed bytecodes to different addresses on zkSync could then be deployed to the same the Ethereum-matching addresses on zkSync.

CALL, STATICCALL, DELEGATECALL

For calls, you specify a memory slice to write the return data to, e.g. out and outsize arguments for call(g, a, v, in, insize, out, outsize). In EVM, if outsize != 0, the allocated memory will grow to out + outsize (rounded up to the words) regardless of the returndatasize. On zkSync Era, returndatacopy, similar to calldatacopy, is implemented as a cycle iterating over return data with a few additional checks and triggering a panic if out + outsize > returndatasize to simulate the same behavior as in EVM.

Thus, unlike EVM where memory growth occurs before the call itself, on zkSync Era, the necessary copying of return data happens only after the call has ended, leading to a difference in msize() and sometimes zkSync Era not panicking where EVM would panic due to the difference in memory growth.

success := call(gas(), target, 0, in, insize, out, outsize) // grows to 'min(returndatasize(), out + outsize)'
success := call(gas(), target, 0, in, insize, out, 0) // memory untouched
returndatacopy(out, 0, returndatasize()) // grows to 'out + returndatasize()'

Additionally, there is no native support for passing Ether on zkSync Era, so it is handled by a special system contract called MsgValueSimulator. The simulator receives the callee address and Ether amount, performs all necessary balance changes, and then calls the callee.

MSTORE, MLOAD

Unlike EVM, where the memory growth is in words, on zkEVM the memory growth is counted in bytes. For example, if you write mstore(100, 0) the msize on zkEVM will be 132, but on the EVM it will be 160. Note, that also unlike EVM which has quadratic growth for memory payments, on zkEVM the fees are charged linearly at a rate of 1 erg per byte.

The other thing is that our compiler can sometimes optimize unused memory reads/writes. This can lead to different msize compared to Ethereum since fewer bytes have been allocated, leading to cases where EVM panics, but zkEVM will not due to the difference in memory growth.

CALLDATALOAD, CALLDATACOPY

If the offset for calldataload(offset) is greater than 2^32-33 then execution will panic.

Internally on zkEVM, calldatacopy(to, offset, len) there is just a loop with the calldataload and mstore on each iteration. That means that the code will panic if 2^32-32 + offset % 32 < offset + len.

RETURN, STOP

Constructors return the array of immutable values. If you use RETURN or STOP in an assembly block in the constructor on zkSync Era, it will leave the immutable variables uninitialized.

contract Example {
    uint immutable x;

    constructor() {
        x = 45;

        assembly {
            // The statements below are overridden by the zkEVM compiler to return
            // the array of immutables.

            // The statement below leaves the variable x uninitialized.
            // return(0, 32)

            // The statement below leaves the variable x uninitialized.
            // stop()
        }
    }

    function getData() external pure returns (string memory) {
        assembly {
            return(0, 32) // works as expected
        }
    }
}

TIMESTAMP, NUMBER

For more information about blocks on zkSync Era, including the differences between block.timestamp and block.number, check out the blocks on zkSync Documentation.

Changes From the Previous Protocol Version

Modifications were performed on how certain block properties were implemented on zkSync Era. For details on the changes performed visit the announcement on GitHubopen in new window.

COINBASE

Returns the address of the Bootloader contract, which is 0x8001 on zkSync Era.

DIFFICULTY, PREVRANDAO

Returns a constant value of 2500000000000000 on zkSync Era.

BASEFEE

This is not a constant on zkSync Era and is instead defined by the fee model. Most of the time it is 0.25 gwei, but under very high L1 gas prices it may rise.

SELFDESTRUCT

Considered harmful and deprecated in EIP-6049open in new window.

Always produces a compile-time error with the zkEVM compiler.

CALLCODE

Deprecated in EIP-2488open in new window in favor of DELEGATECALL.

Always produces a compile-time error with the zkEVM compiler.

PC

Inaccessible in Yul and Solidity >=0.7.0, but accessible in Solidity 0.6.

Always produces a compile-time error with the zkEVM compiler.

CODESIZE

Deploy codeRuntime code
Size of the constructor argumentsContract size

Yul uses a special instruction datasize to distinguish the contract code and constructor arguments, so we substitute datasize with 0 and codesize with calldatasize in zkSync Era deployment code. This way when Yul calculates the calldata size as sub(codesize, datasize), the result is the size of the constructor arguments.

contract Example {
    uint256 public deployTimeCodeSize;
    uint256 public runTimeCodeSize;

    constructor() {
        assembly {
            deployTimeCodeSize := codesize() // return the size of the constructor arguments
        }
    }

    function getRunTimeCodeSize() external {
        assembly {
            runTimeCodeSize := codesize() // works as expected
        }
    }
}

CODECOPY

Deploy codeRuntime code (old EVM codegen)Runtime code (new Yul codegen)
Copies the constructor argumentsZeroes memory outCompile-time error
contract Example {
    constructor() {
        assembly {
            codecopy(0, 0, 32) // behaves as CALLDATACOPY
        }
    }

    function getRunTimeCodeSegment() external {
        assembly {
            // Behaves as 'memzero' if the compiler is run with the old (EVM assembly) codegen,
            // since it is how solc performs this operation there. On the new (Yul) codegen
            // `CALLDATACOPY(dest, calldatasize(), 32)` would be generated by solc instead, and
            // `CODECOPY` is safe to prohibit in runtime code.
            // Produces a compile-time error on the new codegen, as it is not required anywhere else,
            // so it is safe to assume that the user wants to read the contract bytecode which is not
            // available on zkEVM.
            codecopy(0, 0, 32)
        }
    }
}

EXTCODECOPY

Contract bytecode cannot be accessed on zkEVM architecture. Only its size is accessible with both CODESIZE and EXTCODESIZE.

EXTCODECOPY always produces a compile-time error with the zkEVM compiler.

DATASIZE, DATAOFFSET, DATACOPY

Contract deployment is handled by two parts of the zkEVM protocol: the compiler front end and the system contract called ContractDeployer.

On the compiler front-end the code of the deployed contract is substituted with its hash. The hash is returned by the dataoffset Yul instruction or the PUSH [$] EVM legacy assembly instruction. The hash is then passed to the datacopy Yul instruction or the CODECOPY EVM legacy instruction, which writes the hash to the correct position of the calldata of the call to ContractDeployer.

The deployer calldata consists of several elements:

ElementOffsetSize
Deployer method signature04
Salt432
Contract hash3632
Constructor calldata offset6832
Constructor calldata length10032
Constructor calldata132N

The data can be logically split into header (first 132 bytes) and constructor calldata (the rest).

The header replaces the contract code in the EVM pipeline, whereas the constructor calldata remains unchanged. For this reason, datasize and PUSH [$] return the header size (132), and the space for constructor arguments is allocated by solc on top of it.

Finally, the CREATE or CREATE2 instructions pass 132+N bytes to the ContractDeployer contract, which makes all the necessary changes to the state and returns the contract address or zero if there has been an error.

If some Ether is passed, the call to the ContractDeployer also goes through the MsgValueSimulator just like ordinary calls.

We do not recommend using CREATE for anything other than creating contracts with the new operator. However, a lot of contracts create contracts in assembly blocks instead, so authors must ensure that the behavior is compatible with the logic described above.

Yul example:

let _1 := 128                                       // the deployer calldata offset
let _2 := datasize("Callable_50")                   // returns the header size (132)
let _3 := add(_1, _2)                               // the constructor arguments begin offset
let _4 := add(_3, args_size)                        // the constructor arguments end offset
datacopy(_1, dataoffset("Callable_50"), _2)         // dataoffset returns the contract hash, which is written according to the offset in the 1st argument
let address_or_zero := create(0, _1, sub(_4, _1))   // the header and constructor arguments are passed to the ContractDeployer system contract

EVM legacy assembly example:

010     PUSH #[$]       tests/solidity/complex/create/create/callable.sol:Callable      // returns the header size (132), equivalent to Yul's datasize
011     DUP1
012     PUSH [$]        tests/solidity/complex/create/create/callable.sol:Callable      // returns the contract hash, equivalent to Yul's dataoffset
013     DUP4
014     CODECOPY        // CODECOPY statically detects the special arguments above and behaves like the Yul's datacopy
...
146     CREATE          // accepts the same data as in the Yul example above

SETIMMUTABLE, LOADIMMUTABLE

zkEVM does not provide any access to the contract bytecode, so the behavior of immutable values is simulated with the system contracts.

  1. The deploy code, also known as constructor, assembles the array of immutables in the auxiliary heap. Each array element consists of an index and a value. Indexes are allocated sequentially by zksolc for each string literal identifier allocated by solc.
  2. The constructor returns the array as the return data to the contract deployer.
  3. The array is passed to a special system contract called ImmutableSimulator, where it is stored in a mapping with the contract address as the key.
  4. In order to access immutables from the runtime code, contracts call the ImmutableSimulator to fetch a value using the address and value index. In the deploy code, immutable values are read from the auxiliary heap, where they are still available.

The element of the array of immutable values:

struct Immutable {
    uint256 index;
		uint256 value;
}

Yul example:

mstore(128, 1)                                   // write the 1st value to the heap
mstore(160, 2)                                   // write the 2nd value to the heap

let _2 := mload(64)
let _3 := datasize("X_21_deployed")              // returns 0 in the deploy code
codecopy(_2, dataoffset("X_21_deployed"), _3)    // no effect, because the length is 0

// the 1st argument is ignored
setimmutable(_2, "3", mload(128))                // write the 1st value to the auxiliary heap array at index 0
setimmutable(_2, "5", mload(160))                // write the 2nd value to the auxiliary heap array at index 32

return(_2, _3)                                   // returns the auxiliary heap array instead

EVM legacy assembly example:

053     PUSH #[$]       <path:Type>               // returns 0 in the deploy code
054     PUSH [$]        <path:Type>
055     PUSH            0
056     CODECOPY                                  // no effect, because the length is 0
057     ASSIGNIMMUTABLE 5                         // write the 1st value to the auxiliary heap array at index 0
058     ASSIGNIMMUTABLE 3                         // write the 2nd value to the auxiliary heap array at index 32
059     PUSH #[$]       <path:Type>
060     PUSH            0
061     RETURN                                    // returns the auxiliary heap array instead

Libraries

We rely on the solc optimizer for library inlining, so a library may only be used without deployment if it has been inlined by the optimizer.

The addresses of deployed libraries must be set in the project configuration. These addresses then replace their placeholders in IRs: linkersymbol in Yul and PUSHLIB in EVM legacy assembly.

All linking happens at compile-time. Deploy-time linking is not supported.

Precompiles

Some EVM cryptographic precompiles (notably pairings and RSA) aren't currently available. However, pairing is prioritized to allow deployment of both Hyperchains and protocols like Aztec/Dark Forest without modifications.

Ethereum cryptographic primitives like ecrecover, keccak256, and sha256 are supported as precompiles. No actions are required from your side as all the calls to the precompiles are done by the compilers under the hood.

Native AA vs EIP 4337

The native account abstraction of zkSync and Ethereum's EIP 4337 aim to enhance accounts' flexibility and user experience, but they differ in critical aspects listed below:

  1. Implementation Level: zkSync's account abstraction is integrated at the protocol level; however, EIP 4337 avoids the implementation at the protocol level.
  2. Account Types: on zkSync Era, smart contract accounts and paymasters are first-class citizens. Under the hood, all accounts (even EOAs) behave like smart contract accounts; all accounts support paymasters.
  3. Transaction Processing: EIP 4337 introduces a separate transaction flow for smart contract accounts, which relies on a separate mempool for user operations, and Bundlers - nodes that bundle user operations and sends them to be processed by the EntryPoint contract, resulting in two separate transaction flows. In contrast, on zkSync Era there is a unified mempool for transactions from both Externally Owned Accounts (EOAs) and smart contract accounts. On zkSync Era, the Operator takes on the role of bundling transactions, irrespective of the account type, and sends them to the Bootloader (similar to the EntryPoint contract), which results in a single mempool and transaction flow.
  4. Paymasters support: zkSync Era allows both EOAs and smart contract accounts to benefit from paymasters thanks to its single transaction flow. On the other hand, EIP 4337 does not support paymasters for EOAs because paymasters are only implemented in the new transaction flow for smart contract accounts.

ecrecover

In contrast to Ethereum, zkSync Era ecrecover always return a zero address for the zero digests. Be careful with adapting crypto primitives that rely on that, specifically, it affects secp256k1 mul verification via ecrecoveropen in new window.