Deprecate CALLCODE in a somewhat backwards compatible way, by making it always return failure.
CALLCODE was part of the Frontier release of Ethereum. In the first few weeks/months it became clear
that it cannot accomplish its intended design goal. This was rectified with introducing DELEGATECALL
(EIP-7) in the Homestead update (early 2016).
CALLCODE became never utilized, but it still puts a burden on EVM implementations.
Disabling it will not improve the situation for any client whose goal is to sync from genesis, but would help light clients or clients planning to sync from a later point in time.
If block.number >= FORK_BLOCK, the CALLCODE (0xf2) instruction always returns 0, which signals failure.
It would be possible just to remove the opcode and exceptionally abort if it is encountered. However, by returning failure, the contract has a chance to act on it and potentially recover.
This is a breaking change and has a potential to break contracts. The author expects no contracts of any value should be affected.
TODO: validate this claim.
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