Discussion topic for Add deprecate CALLCODE EIP by axic · Pull Request #2488 · ethereum/EIPs · GitHub
If we can’t find any real usage of the CALLCODE opcode, I would rather just have that opcode be treated as an undefined opcode so we can re-use it in the future, rather than defining it to have certain behavior. From the EIP it sounds like you have plans to research this further, so I think the results of that research will help inform whether we should do as this EIP suggests, or just free up that opcode and pretend it never existed.
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It was definitely used in some experiments, that’s how DELEGATECALL emerged. I am still torn which is easier: a) having a stub implementation as proposed on EIP-2488, or b) having the current implementation.
But you are correct, more research is needed before concluding on a final proposal.
I would love to see CALLCODE deprecated, and have a slight preference for Micah’s suggestion (vs the current EIP). But either way is fine - just want to stop having to think about its semantics in EVM implementations and specifications.