Deep Dive — Module 4
Review questions for Module 4. Try answering before expanding.
Q: How is a CREATE2 address computed, and why is it useful?
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Answer
address = keccak256(0xff ++ sender ++ salt ++ keccak256(initcode))[12:]. The 0xff prefix prevents collisions with CREATE addresses. The salt is a user-chosen 32-byte value. This makes the address deterministic — you can compute it before deploying. Use cases: factory patterns (e.g., Uniswap pair creation), counterfactual instantiation (reference a contract before it's deployed), and Account Abstraction (pre-compute wallet addresses for gasless onboarding).
Q: What is the Host trait pattern and why is it important?
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Answer
The Host trait separates EVM execution semantics from state management. The interpreter calls host.code(addr) to read other contracts' code, host.balance(addr) to check balances, and host.emit_log() to write events. In tests, a MockHost uses HashMaps. In production, a real Host reads from disk-backed Merkle tries. This inversion of control makes the interpreter testable, portable, and backend-agnostic — the same pattern revm uses.
Q: What is the gas cost of emitting a LOG4 with 100 bytes of data?
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Answer
gas. The base cost is 375 (G_log), each byte of data costs 8 gas (G_logdata), and each topic costs 375 gas (G_logtopic). See evm.codes — LOG4 for the interactive breakdown.
Q: What is the difference between CODECOPY and EXTCODECOPY?
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CODECOPY copies the current contract's own bytecode into memory. EXTCODECOPY copies another account's bytecode. Both are used in proxy patterns — a proxy reads the implementation contract's code to understand its ABI. CODECOPY is also used in initcode to copy the runtime bytecode into memory before returning it.
Q: Why does contract creation use initcode instead of deploying runtime code directly?
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Answer
Initcode allows constructors — code that runs once at deployment to initialize storage, validate parameters, or compute runtime code dynamically. The separation also enables the proxy pattern: initcode can deploy minimal proxy bytecode that delegates to an implementation contract. Without initcode, every contract would need to be fully static at deployment time.