Make nonce sequential

src/Gateway.sol

@@ -54,11 +54,6 @@ contract Gateway is IGateway, IExecutor, IUpgradable, GatewayEIP712 {
     using RouteStore for RouteStore.NetworkInfo;
     using NetworkIDHelpers for NetworkID;
 
-    /**
-     * @dev Non-zero value used to initialize the `prevMessageHash` storage
-     */
-    bytes32 internal constant FIRST_MESSAGE_PLACEHOLDER = bytes32(uint256(2 ** 256 - 1));
-
     /**
      * @dev Selector of `GmpCreated` event.
      * keccak256("GmpCreated(bytes32,bytes32,address,uint16,uint256,uint256,bytes)");
@@ -75,7 +70,7 @@ contract Gateway is IGateway, IExecutor, IUpgradable, GatewayEIP712 {
     mapping(bytes32 => GmpInfo) private _messages;
 
     // Hash of the previous GMP message submitted.
-    bytes32 public prevMessageHash;
+    uint256 internal nonce;
 
     // Replay protection mechanism, stores the hash of the executed messages
     // messageHash => shardId
@@ -97,12 +92,12 @@ contract Gateway is IGateway, IExecutor, IUpgradable, GatewayEIP712 {
     // EIP-712 typed hash
     function initialize(address proxyAdmin, TssKey[] calldata keys, Network[] calldata networks) external {
         require(PROXY_ADDRESS == address(this) || msg.sender == FACTORY, "only proxy can be initialize");
-        require(prevMessageHash == 0, "already initialized");
+        require(nonce == 0, "already initialized");
         ERC1967.setAdmin(proxyAdmin);
 
-        // Initialize the prevMessageHash with a non-zero value to avoid the first GMP to spent more gas,
+        // Initialize the `nonce` as one to avoid the first GMP to spent more gas,
         // once initialize the storage cost 21k gas, while alter it cost just 2800 gas.
-        prevMessageHash = FIRST_MESSAGE_PLACEHOLDER;
+        nonce = 1;
 
         // Register networks
         RouteStore.getMainStorage().initialize(networks, NetworkID.wrap(NETWORK_ID));
@@ -115,6 +110,10 @@ contract Gateway is IGateway, IExecutor, IUpgradable, GatewayEIP712 {
         emit KeySetChanged(bytes32(0), revoked, keys);
     }
 
+    function prevMessageHash() external view returns (uint256) {
+        return nonce;
+    }
+
     function gmpInfo(bytes32 id) external view returns (GmpInfo memory) {
         return _messages[id];
     }
@@ -303,30 +302,21 @@ contract Gateway is IGateway, IExecutor, IUpgradable, GatewayEIP712 {
         GmpSender source = msg.sender.toSender(false);
 
         // Salt is equal to the previous message id (EIP-712 hash), this allows us to establish a sequence and eaily query the message history.
-        bytes32 prevHash = prevMessageHash;
-
-        // if the messageHash is the first message, we use a zero salt
-        uint256 salt = BranchlessMath.ternary(prevHash == FIRST_MESSAGE_PLACEHOLDER, 0, uint256(prevHash));
-
-        // Create GMP message and update prevMessageHash
-        bytes memory payload;
-        {
-            GmpMessage memory message =
-                GmpMessage(source, NETWORK_ID, destinationAddress, routeId, executionGasLimit, salt, data);
-            prevHash = message.eip712hash();
-            prevMessageHash = prevHash;
-            payload = message.data;
-        }
+        uint256 nextNonce = nonce++;
+
+        // Create GMP message and update nonce
+        GmpMessage memory message =
+                GmpMessage(source, NETWORK_ID, destinationAddress, routeId, executionGasLimit, nextNonce, data);
 
         // Emit `GmpCreated` event without copy the data, to simplify the gas estimation.
-        _emitGmpCreated(prevHash, source, destinationAddress, routeId, executionGasLimit, salt, payload);
+        _emitGmpCreated(message.eip712hash(), source, destinationAddress, routeId, executionGasLimit, nextNonce, message.data);
     }
 
     /**
      * @dev Emit `GmpCreated` event without copy the data, to simplify the gas estimation.
      */
     function _emitGmpCreated(
-        bytes32 prevHash,
+        bytes32 messageID,
         GmpSender source,
         address destinationAddress,
         uint16 destinationNetwork,
@@ -348,8 +338,8 @@ contract Gateway is IGateway, IExecutor, IUpgradable, GatewayEIP712 {
             mstore(add(ptr, 0x60), 0x80) // data offset
             let size := and(add(mload(payload), 31), 0xffffffe0)
             size := add(size, 160)
-            log4(ptr, size, GMP_CREATED_EVENT_SELECTOR, prevHash, source, destinationAddress)
-            mstore(0, prevHash)
+            log4(ptr, size, GMP_CREATED_EVENT_SELECTOR, messageID, source, destinationAddress)
+            mstore(0, messageID)
             return(0, 32)
         }
     }

src/utils/GasUtils.sol

@@ -25,7 +25,7 @@ library GasUtils {
     /**
      * @dev Base cost of the `IGateway.submitMessage` method.
      */
-    uint256 internal constant SUBMIT_BASE_COST = 23449;
+    uint256 internal constant SUBMIT_BASE_COST = 23449 + 19;
 
     /**
      * @dev Compute the gas cost of memory expansion.

test/Example.t.sol

@@ -109,7 +109,7 @@ contract ExampleTest is Test {
             dest: address(dstToken),
             destNetwork: DEST_NETWORK_ID,
             gasLimit: 100_000,
-            salt: 0,
+            salt: 1,
             data: abi.encode(MockERC20.CrossChainTransfer({from: ALICE, to: BOB, amount: 100}))
         });
 

test/Gateway.t.sol

@@ -420,7 +420,7 @@ contract GatewayBase is Test {
             dest: address(bytes20(keccak256("dummy_address"))),
             destNetwork: DEST_NETWORK_ID,
             gasLimit: 0,
-            salt: 0,
+            salt: 1,
             data: new bytes(messageSize)
         });
 
@@ -631,13 +631,13 @@ contract GatewayBase is Test {
             dest: address(receiver),
             destNetwork: DEST_NETWORK_ID,
             gasLimit: 100_000,
-            salt: 0,
+            salt: 1,
             data: abi.encodePacked(uint256(100_000))
         });
         bytes32 id = gmp.eip712hash();
 
         // Check the previous message hash
-        assertEq(gateway.prevMessageHash(), bytes32(uint256(2 ** 256 - 1)), "wrong previous message hash");
+        assertEq(gateway.prevMessageHash(), 1, "wrong previous message hash");
 
         CallOptions memory ctx = CallOptions({
             from: gmpSender.toAddress(),
@@ -675,7 +675,7 @@ contract GatewayBase is Test {
         assertEq(ctx.executionCost, expectedCost, "unexpected execution gas cost in first call");
 
         // Now the second GMP message should have the salt equals to previous gmp hash
-        gmp.salt = uint256(id);
+        gmp.salt += 1;
         id = gmp.eip712hash();
 
         // Expect event