EVM-compatible.md

Comparison with EVM

Polyjuice aims at 100% EVM compatibility as a goal, meaning we plan to support all smart contracts supported by the latest Ethereum hardfork version. But in the current version, something is incompatible with EVM.

EVM revision

The maximum EVM revision supported is EVMC_BERLIN.

pCKB

pCKB is a new concept introduced by Polyjuice.

Recall that in Ethereum, the gas of each smart contract is calculated. The transaction fee is calculated then by multiplying gas with specified gas price. In Polyjuice, pCKB is used as the unit for calculating transaction fees. This means while the gas price in Ethereum is ETH/gas(which is denominated in wei, which is 1e-18 ether), in Polyjuice gas price is measured in pCKB/gas. When executing a transaction, Polyjuice will deduct transaction fee using the layer-2 sUDT type denoted by pCKB.

Note when sending a transaction to a smart contract for certain behavior, the value of the transaction is pCKB.

sUDT-ERC20 proxy contract

When you use a sUDT token type, it will be represented in Godwoken as a layer-2 sUDT type. Polyjuice ensures that all the layer-2 sUDT tokens on Godwoken are in compliance with the ERC20 standard by the sUDT-ERC20 Proxy Contract. This contract provides a way for EVM code to interact with ERC20 standard interface to operate sUDT tokens on Godwoken as if they were ERC20 tokens.

In other words, all bridged sUDT tokens have the same ERC20 interface thanks to the 1-to-1 sUDT-ERC20 proxy contract:

Bridged sUDT token list

• mainnet_v1: https://github.com/nervosnetwork/godwoken-info/blob/main/mainnet_v1/bridged-token-list.json
• testnet_v1: https://github.com/nervosnetwork/godwoken-info/blob/main/testnet_v1_1/bridged-token-list.json

Transaction structure

A Polyjuice transaction is essentially just a Godwoken transaction.

When you send an Ethereum transaction, the transaction is converted to Godwoken RawL2Transaction type which is automatically handled by Godwoken Web3.

Behavioral differences of some opcodes

EVM Opcode	Solidity Usage	Behavior in Polyjuice	Behavior in EVM
COINBASE	block.coinbase	address of the block_producer	address of the current block's miner
GASLIMIT	block.gaslimit	12,500,000	current block's gas limit
DIFFICULTY	block.difficulty	2,500,000,000,000,000	current block's difficulty

Restriction of memory usage

Polyjuice runs EVM on ckb-vm. While EVM has no limit on memory usage (despite the limit of 1024 on stack depth for EVM), ckb-vm can use a maximum of 4MB of memory for now. Of which, 3MB for heap space and 1MB for stack space. See more details in here.

For some contracts that consume a lot of memory or that have deep call stacks, this may indicate a potential incompatibility on ckb-vm.

Others

• Transaction context

  • chain_id is defined in Godwoken RollupConfig#chain_id.
  • the block difficulty is always 2500000000000000
  • the gas limit for each block is 12500000; it is not a transaction-level limit. Any transaction can reach the gas limit
  • the size limit for contract's return data is 25KB
  • the size limit for contract's storage is 25KB

• transaction.to MUST be a Contract Address

• The transfer value can not exceed uint128:MAX, because the type of sUDT.amount is uint128

• Pre-compiled contract

  • addition pre-compiled contracts