EIP-1052 adds the EXTCODEHASH opcode that returns the keccak256 hash of a contract's bytecode. For non-existent or empty accounts, it returns 0. For accounts without code, it returns the hash of empty data. The operation costs 400 gas, same as BALANCE.
Many contracts need to verify if an address has specific code without copying the entire bytecode. Before EXTCODEHASH, contracts used expensive EXTCODECOPY. This enables cheap verification of implementation contracts in proxies, whitelisting by code hash, and checking if an address is a contract.
This EIP specifies a new opcode, which returns the keccak256 hash of a contract's code.
Many contracts need to perform checks on a contract's bytecode, but do not necessarily need the bytecode itself. For instance, a contract may want to check if another contract's bytecode is one of a set of permitted implementations, or it may perform analyses on code and whitelist any contract with matching bytecode if the analysis passes.
Contracts can presently do this using the EXTCODECOPY (0x3c) opcode, but this is expensive, especially for large contracts, in cases where only the hash is required. As a result, we propose a new opcode, EXTCODEHASH, which returns the keccak256 hash of a contract's bytecode.
A new opcode, EXTCODEHASH, is introduced, with number 0x3f. The EXTCODEHASH
takes one argument from the stack, zeros the first 96 bits
and pushes to the stack the keccak256 hash of the code of the account
at the address being the remaining 160 bits.
In case the account does not exist or is empty (as defined by EIP-161) 0 is pushed to the stack.
In case the account does not have code the keccak256 hash of empty data
(i.e. c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470)
is pushed to the stack.
The gas cost of the EXTCODEHASH is 400.
As described in the motivation section, this opcode is widely useful, and saves on wasted gas in many cases.
The gas cost is the same as the gas cost for the BALANCE opcode because the
execution of the EXTCODEHASH requires the same account lookup as in BALANCE.
Only the 20 last bytes of the argument are significant (the first 12 bytes are
ignored) similarly to the semantics of the BALANCE (0x31), EXTCODESIZE (0x3b) and
EXTCODECOPY (0x3c).
The EXTCODEHASH distinguishes accounts without code and non-existing accounts.
This is consistent with the way accounts are represented in the state trie.
This also allows smart contracts to check whether an account exists.
There are no backwards compatibility concerns.
EXTCODEHASH of the account without code is c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470
what is the keccak256 hash of empty data.EXTCODEHASH of non-existent account is 0.EXTCODEHASH of a precompiled contract is either c5d246... or 0.EXTCODEHASH of A is X, then EXTCODEHASH of A + 2**160 is X.EXTCODEHASH of an account that selfdestructed in the current transaction.EXTCODEHASH of an account that selfdestructed and later the selfdestruct has been reverted.EXTCODEHASH of an account created in the current transaction.EXTCODEHASH of an account that has been newly created and later the creation has been reverted.EXTCODEHASH of an account that firstly does not exist and later is empty.EXTCODEHASH of an empty account that is going to be cleared by the state clearing rule.TBD
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