PUSH0 and Cross-Chain Compatibility Challenges

. What is PUSH0 and Why Was It Introduced?

The PUSH0 opcode, introduced in the Ethereum Shanghai hardfork, is designed to push a zero value onto the stack. Before PUSH0, developers had to use alternative methods such as PUSH1 0x00 to achieve this, which required additional bytecode. PUSH0 optimizes this process by reducing the gas cost and bytecode size, making contract execution slightly more efficient.

Benefits of PUSH0:

• Gas Optimization: PUSH0 is cheaper in gas cost compared to previous methods of pushing zero onto the stack.

• Bytecode Efficiency: It reduces the size of the bytecode, making contracts smaller and more optimized.

2. Potential Compatibility Issues

While PUSH0 is an improvement for Ethereum post-Shanghai, the opcode is not supported by all blockchains. Many other EVM-compatible chains like Polygon, Avalanche, Binance Smart Chain, and others may not yet have adopted the Shanghai hardfork at the time of deployment. As a result, contracts that contain the PUSH0 opcode will fail to deploy or function properly on these chains.

For example, here is an excerpt of bytecode from a contract compiled with Solidity 0.8.20, showing the presence of the PUSH0 opcode:

0x5f PUSH0
...

Chains that do not support the Shanghai hardfork will not recognize the PUSH0 opcode, leading to deployment errors or runtime failures.

3. Cross-Chain Deployment Considerations

Many decentralized applications (dApps) aim to be deployed across multiple chains to maximize user reach and liquidity. However, each chain may be at a different stage of adopting Ethereum's latest upgrades. For instance, Ethereum may support PUSH0, but other chains like Polygon or Avalanche may not yet have integrated this feature.

When a smart contract is compiled using Solidity 0.8.20 or later, it defaults to the EVM version that includes PUSH0. If this bytecode is deployed on chains that do not support the Shanghai hardfork, the deployment will fail.

Moreover, using different Solidity versions across chains can result in inconsistent bytecode, breaking the deterministic nature of contract addresses and causing issues with counterfactual deployments, where the contract's address must be predictable across chains.

4. Proof of Concept: Contract Compilation and Deployment Issue

Consider a contract that is compiled using Solidity 0.8.20 and includes the PUSH0 opcode:

// Example contract
pragma solidity 0.8.20;

contract ExampleContract {
    uint256 public value;

    function setValue(uint256 _value) public {
        value = _value;
    }
}

After compiling, the bytecode contains the PUSH0 opcode. If you attempt to deploy this contract on a chain that does not support the Shanghai hardfork, the deployment will fail due to the unrecognized PUSH0 instruction

To avoid the issues caused by PUSH0, you can take the following steps:

1. Use a Compatible Solidity Version: Compile the contracts with Solidity 0.8.19 or earlier, which do not include the PUSH0 opcode. This ensures compatibility with chains that have not yet adopted the Shanghai hardfork.

   In your Solidity contract, specify the compiler version like this:

   pragma solidity 0.8.19;