Gas Optimizations

[code423n4]

1. Cache array length in loops instead of computing length in every iteration

While looping through array, the array length can be cached to save gas instead of computing length in each array iteration.

for eg.

uint256 len = modules.length;
for(uint256 i = 0; i < len; i++) {
  ...
}

> grep -rn './core' -e 'for.*[.]length'
./core/connext/facets/StableSwapFacet.sol:415:    for (uint8 i = 0; i < _pooledTokens.length; i++) {
./core/connext/facets/RelayerFacet.sol:140:    for (uint256 i; i < _transferIds.length; ) {
./core/connext/facets/RelayerFacet.sol:164:    for (uint256 i; i < _transferIds.length; ) {
./core/connext/libraries/LibDiamond.sol:104:    for (uint256 facetIndex; facetIndex < _diamondCut.length; facetIndex++) {
./core/connext/libraries/LibDiamond.sol:129:    for (uint256 selectorIndex; selectorIndex < _functionSelectors.length; selectorIndex++) {
./core/connext/libraries/LibDiamond.sol:147:    for (uint256 selectorIndex; selectorIndex < _functionSelectors.length; selectorIndex++) {
./core/connext/libraries/LibDiamond.sol:162:    for (uint256 selectorIndex; selectorIndex < _functionSelectors.length; selectorIndex++) {
./core/connext/libraries/SwapUtils.sol:205:    for (uint256 i = 0; i < xp.length; i++) {
./core/connext/libraries/SwapUtils.sol:558:    for (uint256 i = 0; i < balances.length; i++) {
./core/connext/libraries/SwapUtils.sol:591:    for (uint256 i = 0; i < balances.length; i++) {
./core/connext/libraries/SwapUtils.sol:844:    for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:869:      for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:924:    for (uint256 i = 0; i < amounts.length; i++) {
./core/connext/libraries/SwapUtils.sol:1014:      for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:1019:      for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:1039:    for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:1055:    for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/helpers/ConnextPriceOracle.sol:176:    for (uint256 i = 0; i < tokenAddresses.length; i++) {
./core/connext/helpers/StableSwap.sol:81:    for (uint8 i = 0; i < _pooledTokens.length; i++) {

2. Prefix increment (++i) is cheaper than postfix increment (i++)

Using prefix increment is saves small amount of gas than postfix increment because it returns the updated value hence doesn't requires to store intermediate value. This can be more significant in loops where this operation is done multiple times.

for eg.

// before
for(uint256 i = 0; i < len; i++) {
  ...
}

// Replace with
for(uint256 i = 0; i < len; ++i) {
  ...
}

./core/connext/libraries/SwapUtils.sol:425:    for (uint256 i = 0; i < MAX_LOOP_LIMIT; i++) {
./core/connext/libraries/SwapUtils.sol:558:    for (uint256 i = 0; i < balances.length; i++) {
./core/connext/libraries/SwapUtils.sol:591:    for (uint256 i = 0; i < balances.length; i++) {
./core/connext/libraries/SwapUtils.sol:844:    for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:869:      for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:924:    for (uint256 i = 0; i < amounts.length; i++) {
./core/connext/libraries/SwapUtils.sol:1014:      for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:1019:      for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:1039:    for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/libraries/SwapUtils.sol:1055:    for (uint256 i = 0; i < pooledTokens.length; i++) {
./core/connext/helpers/ConnextPriceOracle.sol:176:    for (uint256 i = 0; i < tokenAddresses.length; i++) {
./core/connext/helpers/StableSwap.sol:81:    for (uint8 i = 0; i < _pooledTokens.length; i++) {
...

3.Cache compute of address(this).balance in ternary operator to save gas instead of evaluating it twice

• https://github.com/code-423n4/2022-06-connext/blob/b4532655071566b33c41eac46e75be29b4a381ed/contracts/contracts/core/connext/helpers/SponsorVault.sol#L258

sponsoredFee = sponsoredFee > address(this).balance ? address(this).balance : sponsoredFee; 

[liu-zhipeng]

1, 2 : duplicated
3: fixed

[0xleastwood]

Useful optimisations but also found by many other wardens.