EIP1052 - EXTCODEHASH opcode

# Abstract

This EIP specifies a new opcode, which returns the keccak256 hash of a contract's code.

# Motivation

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.

# Specification

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.

# Rationale

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 distincts 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 whenever an account exists.

# Backwards Compatibility

There are no backwards compatibility concerns.

# Test Cases

The EXTCODEHASH of the account without code is c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 what is the keccack256 hash of empty data.
The EXTCODEHASH of non-existent account is 0.
The EXTCODEHASH of an precompiled contract is either c5d246... or 0.
If EXTCODEHASH of A is X, then EXTCODEHASH of A + 2**160 is X.
The EXTCODEHASH of an account that selfdestructed in the current transaction.
The EXTCODEHASH of an account that selfdestructed and later the selfdestruct has been reverted.
The EXTCODEHASH of an account created in the current transaction.
The EXTCODEHASH of an account that has been newly create and later the creation has been reverted.
The EXTCODEHASH of an account that firstly does not exist and later is empty.
The EXTCODEHASH of an empty account that is going to be cleared by the state clearing rule.

# Implementation

TBD