[NLA-01C] Inefficient `mapping` Lookups

[dkuppitz]

NLA-01C: Inefficient mapping Lookups

Type	Severity	Location
Gas Optimization		NonblockingLzAppUpgradeable.sol:L172, L176

Description:

The linked statements perform key-based lookup operations on mapping declarations from storage multiple times for the same key redundantly.

Example:

bytes32 payloadHash = $.failedMessages[srcChainId][srcAddress][nonce];
require(payloadHash != bytes32(0), "NonblockingLzApp: no stored message");
require(keccak256(payload) == payloadHash, "NonblockingLzApp: invalid payload");
// clear the stored message
$.failedMessages[srcChainId][srcAddress][nonce] = bytes32(0);

Recommendation:

As the lookups internally perform an expensive keccak256 operation, we advise the lookups to be cached wherever possible to a single local declaration that either holds the value of the mapping in case of primitive types or holds a storage pointer to the struct contained.

[dkuppitz]

Resolved in 9d292c1.

[dkuppitz]

Post-audit recommendation implemented in 66bcf24.