|Author||Peter Murray, Nate Welch, Joe Messerman|
Table of Contents
To simply and cheaply clone contract functionality in an immutable way, this standard specifies a minimal bytecode implementation that delegates all calls to a known, fixed address.
By standardizing on a known minimal bytecode redirect implementation, this standard allows users and third party tools (e.g. Etherscan) to (a) simply discover that a contract will always redirect in a known manner and (b) depend on the behavior of the code at the destination contract as the behavior of the redirecting contract. Specifically, tooling can interrogate the bytecode at a redirecting address to determine the location of the code that will run - and can depend on representations about that code (verified source, third-party audits, etc). This implementation forwards all calls and 100% of the gas to the implementation contract and then relays the return value back to the caller. In the case where the implementation reverts, the revert is passed back along with the payload data (for revert with message).
This standard supports use-cases wherein it is desirable to clone exact contract functionality with a minimum of side effects (e.g. memory slot stomping) and with low gas cost deployment of duplicate proxies.
The exact bytecode of the standard clone contract is this:
363d3d373d3d3d363d73bebebebebebebebebebebebebebebebebebebebe5af43d82803e903d91602b57fd5bf3 wherein the bytes at indices 10 - 29 (inclusive) are replaced with the 20 byte address of the master functionality contract.
A reference implementation of this can be found at the optionality/clone-factory github repo.
The goals of this effort have been the following:
- inexpensive deployment (low gas to deploy clones)
- support clone initialization in creation transaction (through factory contract model)
- simple clone bytecode to encourage directly bytecode interrogation (see CloneProbe.sol in the clone-factory project)
- dependable, locked-down behavior - this is not designed to handle upgradability, nor should it as the representation we are seeking is stronger.
- small operational overhead - adds a single call cost to each call
- handles error return bubbling for revert messages
There are no backwards compatibility issues. There may be some systems that are using earlier versions of the proxy contract bytecode. They will not be compliant with this standard.
Test cases include:
- invocation with no arguments
- invocation with arguments
- invocation with fixed length return values
- invocation with variable length return values
- invocation with revert (confirming reverted payload is transferred)
Tests for these cases are included in the reference implementation project.
Deployment bytecode is not included in this specification. One approach is defined in the proxy-contract reference implementation.
The disassembly of the standard deployed proxy contract code (from r2 and edited to include stack visualization)
| 0x00000000 36 calldatasize cds | 0x00000001 3d returndatasize 0 cds | 0x00000002 3d returndatasize 0 0 cds | 0x00000003 37 calldatacopy | 0x00000004 3d returndatasize 0 | 0x00000005 3d returndatasize 0 0 | 0x00000006 3d returndatasize 0 0 0 | 0x00000007 36 calldatasize cds 0 0 0 | 0x00000008 3d returndatasize 0 cds 0 0 0 | 0x00000009 73bebebebebe. push20 0xbebebebe 0xbebe 0 cds 0 0 0 | 0x0000001e 5a gas gas 0xbebe 0 cds 0 0 0 | 0x0000001f f4 delegatecall suc 0 | 0x00000020 3d returndatasize rds suc 0 | 0x00000021 82 dup3 0 rds suc 0 | 0x00000022 80 dup1 0 0 rds suc 0 | 0x00000023 3e returndatacopy suc 0 | 0x00000024 90 swap1 0 suc | 0x00000025 3d returndatasize rds 0 suc | 0x00000026 91 swap2 suc 0 rds | 0x00000027 602b push1 0x2b 0x2b suc 0 rds | ,=< 0x00000029 57 jumpi 0 rds | | 0x0000002a fd revert | `-> 0x0000002b 5b jumpdest 0 rds \ 0x0000002c f3 return
NOTE: as an effort to reduce gas costs as much as possible, the above bytecode depends on EIP-211 specification that
returndatasize returns zero prior to any calls within the call-frame.
returndatasize uses 1 less gas than
Proxy deployment can be further optimized by installing the master contract at a vanity contract deployment address with leading zero-bytes. By generating a master contract vanity address that includes Z leading 0 bytes in its address, you can shorten the proxy bytecode by replacing the
push20 opcode with
pushN (where N is 20 - Z) followed by the N non-zero address bytes. The revert jump address is decremented by Z in this case. Here is an example where Z = 4:
| 0x00000000 36 calldatasize cds | 0x00000001 3d returndatasize 0 cds | 0x00000002 3d returndatasize 0 0 cds | 0x00000003 37 calldatacopy | 0x00000004 3d returndatasize 0 | 0x00000005 3d returndatasize 0 0 | 0x00000006 3d returndatasize 0 0 0 | 0x00000007 36 calldatasize cds 0 0 0 | 0x00000008 3d returndatasize 0 cds 0 0 0 | 0x00000009 6fbebebebebe. push16 0xbebebebe 0xbebe 0 cds 0 0 0 | 0x0000001a 5a gas gas 0xbebe 0 cds 0 0 0 | 0x0000001b f4 delegatecall suc 0 | 0x0000001c 3d returndatasize rds suc 0 | 0x0000001d 82 dup3 0 rds suc 0 | 0x0000001e 80 dup1 0 0 rds suc 0 | 0x0000001f 3e returndatacopy suc 0 | 0x00000020 90 swap1 0 suc | 0x00000021 3d returndatasize rds 0 suc | 0x00000022 91 swap2 suc 0 rds | 0x00000023 6027 push1 0x27 0x27 suc 0 rds | ,=< 0x00000025 57 jumpi 0 rds | | 0x00000026 fd revert | `-> 0x00000027 5b jumpdest 0 rds \ 0x00000028 f3 return
This saves 4 bytes of proxy contract size (savings on each deployment) and has zero impact on runtime gas costs.
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