Question

When using a proxy pattern for upgradeable contracts, which specific EVM opcode is used to forward the function call to the implementation contract while maintaining the original storage context?

Accepted Answer

The specific EVM opcode used to forward function calls while maintaining the original storage context is DELEGATECALL. In the proxy pattern, the proxy contract acts as a permanent entry point for users, holding the contract&#x27;s data in its own storage slots. When a user sends a transaction to the proxy, the proxy uses DELEGATECALL to execute the code residing at the implementation contract address. Unlike a standard CALL, which executes code in the context of the target contract and modifies the target&#x27;s storage, DELEGATECALL executes the implementation&#x27;s code using the proxy&#x27;s own storage, balance, and address. This ensures that even when the implementation contract is updated to a new version, the proxy retains access to the state data accumulated in its own storage slots. The opcode takes the gas limit, the address of the implementation, the memory offset for input data, the length of that data, and the memory offset and length for the return data as arguments. If the call succeeds, the proxy returns the result of the implementation logic to the caller as if it were the proxy itself that executed the function.

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When using a proxy pattern for upgradeable contracts, which specific EVM opcode is used to forward the function call to the implementation contract while maintaining the original storage context?