core: add function description

core/vm/instructions.go

@@ -496,6 +496,32 @@ func opExtCodeCopy(pc *uint64, evm *EVM, contract *Contract, memory *Memory, sta
 	return nil, nil
 }
 
+// opExtCodeHash returns the code hash of a specified account.
+// There are several cases when the function is called, while we can relay everything
+// to `state.GetCodeHash` function to ensure the correctness.
+//   (1) Caller tries to get the code hash of a normal contract account, state
+// should return the relative code hash and set it as the result.
+//
+//   (2) Caller tries to get the code hash of a non-existent account, state should
+// return common.Hash{} and zero will be set as the result.
+//
+//   (3) Caller tries to get the code hash for an account without contract code,
+// state should return emptyCodeHash(0xc5d246...) as the result.
+//
+//   (4) Caller tries to get the code hash of a precompiled account, the result
+// should be zero or emptyCodeHash.
+//
+// It is worth noting that in order to avoid unnecessary create and clean,
+// all precompile accounts on mainnet have been transferred 1 wei, so the return
+// here should be emptyCodeHash.
+// If the precompile account is not transferred any amount on a private or
+// customized chain, the return value will be zero.
+//
+//   (5) Caller tries to get the code hash for an account which is marked as suicided
+// in the current transaction, the code hash of this account should be returned.
+//
+//   (6) Caller tries to get the code hash for an account which is marked as deleted,
+// this account should be regarded as a non-existent account and zero should be returned.
 func opExtCodeHash(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
 	slot := stack.peek()
 	slot.SetBytes(evm.StateDB.GetCodeHash(common.BigToAddress(slot)).Bytes())