Add a warning about getMerkleRoot (#336)

packages/contracts/contracts/optimistic-ethereum/OVM/chain/OVM_CanonicalTransactionChain.sol

@@ -545,6 +545,9 @@ contract OVM_CanonicalTransactionChain is iOVM_CanonicalTransactionChain, Lib_Ad
             blockNumber = lastElement.blockNumber;
         }
 
+        // For efficiency reasons getMerkleRoot modifies the `leaves` argument in place
+        // while calculating the root hash therefore any arguments passed to it must not
+        // be used again afterwards
         _appendBatch(
             Lib_MerkleTree.getMerkleRoot(leaves),
             totalElementsToAppend,

packages/contracts/contracts/optimistic-ethereum/OVM/chain/OVM_StateCommitmentChain.sol

@@ -334,6 +334,9 @@ contract OVM_StateCommitmentChain is iOVM_StateCommitmentChain, Lib_AddressResol
             );
         }
 
+        // For efficiency reasons getMerkleRoot modifies the `_batch` argument in place
+        // while calculating the root hash therefore any arguments passed to it must not
+        // be used again afterwards
         Lib_OVMCodec.ChainBatchHeader memory batchHeader = Lib_OVMCodec.ChainBatchHeader({
             batchIndex: getTotalBatches(),
             batchRoot: Lib_MerkleTree.getMerkleRoot(_batch),

packages/contracts/contracts/optimistic-ethereum/libraries/utils/Lib_MerkleTree.sol

@@ -16,6 +16,7 @@ library Lib_MerkleTree {
      * of leaves passed in is not a power of two, it pads out the tree with zero hashes.
      * If you do not know the original length of elements for the tree you are verifying,
      * then this may allow empty leaves past _elements.length to pass a verification check down the line.
+     * Note that the _elements argument is modified, therefore it must not be used again afterwards
      * @param _elements Array of hashes from which to generate a merkle root.
      * @return Merkle root of the leaves, with zero hashes for non-powers-of-two (see above).
      */