state: Simplify code modification indicator in StateDiff

test/state/account.hpp

@@ -73,6 +73,9 @@ struct Account
     /// The account has been created in the current transaction.
     bool just_created = false;
 
+    // This account's code has been modified.
+    bool code_changed = false;
+
     evmc_access_status access_status = EVMC_ACCESS_COLD;
 
     [[nodiscard]] bool is_empty() const noexcept

test/state/host.cpp

@@ -377,26 +377,30 @@ evmc::Result Host::create(const evmc_message& msg) noexcept
                    evmc::Result{EVMC_FAILURE};
     }
 
-    if (!code.empty() && code[0] == 0xEF)
+    if (!code.empty())
     {
-        if (m_rev >= EVMC_OSAKA)
+        if (code[0] == 0xEF)
         {
-            // Only EOFCREATE/EOF-creation-tx is allowed to deploy code starting with EF.
-            // It must be valid EOF, which was validated before execution.
-            if (msg.kind != EVMC_EOFCREATE)
+            if (m_rev >= EVMC_OSAKA)
+            {
+                // Only EOFCREATE/EOF-creation-tx is allowed to deploy code starting with EF.
+                // It must be valid EOF, which was validated before execution.
+                if (msg.kind != EVMC_EOFCREATE)
+                    return evmc::Result{EVMC_CONTRACT_VALIDATION_FAILURE};
+                assert(validate_eof(m_rev, ContainerKind::runtime, code) ==
+                       EOFValidationError::success);
+            }
+            else if (m_rev >= EVMC_LONDON)
+            {
+                // EIP-3541: Reject EF code.
                 return evmc::Result{EVMC_CONTRACT_VALIDATION_FAILURE};
-            assert(
-                validate_eof(m_rev, ContainerKind::runtime, code) == EOFValidationError::success);
-        }
-        else if (m_rev >= EVMC_LONDON)
-        {
-            // EIP-3541: Reject EF code.
-            return evmc::Result{EVMC_CONTRACT_VALIDATION_FAILURE};
+            }
         }
-    }
 
-    new_acc->code_hash = keccak256(code);
-    new_acc->code = code;
+        new_acc->code_hash = keccak256(code);
+        new_acc->code = code;
+        new_acc->code_changed = true;
+    }
 
     return evmc::Result{result.status_code, gas_left, result.gas_refund, msg.recipient};
 }

test/state/state.cpp

@@ -130,7 +130,7 @@ StateDiff State::build_diff(evmc_revision rev) const
 
         // Output only the new code.
         // TODO: Output also the code hash. It will be needed for DB update and MPT hash.
-        if (m.just_created && !m.code.empty())
+        if (m.code_changed)
             a.code = m.code;
 
         for (const auto& [k, v] : m.storage)

test/state/state_diff.hpp

@@ -30,8 +30,8 @@ struct StateDiff
         /// TODO: Currently it is not guaranteed the value is different from the initial one.
         uint256 balance;
 
-        /// New account code. If empty, it means the code has not changed.
-        bytes code;
+        /// New or modified account code. If bytes are empty, it means the code has been cleared.
+        std::optional<bytes> code;
 
         /// The list of the account's storage modifications: key => new value.
         /// The value 0 means the storage entry is deleted.

test/state/test_state.cpp

@@ -34,8 +34,8 @@ void TestState::apply(const state::StateDiff& diff)
         auto& a = (*this)[m.addr];
         a.nonce = m.nonce;
         a.balance = m.balance;
-        if (!m.code.empty())
-            a.code = m.code;  // TODO: Consider taking rvalue ref to avoid code copy.
+        if (m.code.has_value())
+            a.code = *m.code;  // TODO: Consider taking rvalue ref to avoid code copy.
         for (const auto& [k, v] : m.modified_storage)
         {
             if (v)