Gas Optimizations

[code423n4]

There are multiple variables that can be immutable and their use can be "optimized".

First of all, aToken, rewardsController and poolAddressesProviderRegistry could be immutable: there are no setters for them and all of them are proxies: in case of a change, the implementation address will change, not the proxy address.

In the case of the poolAddressesProviderRegistry:

poolAddressesProviderRegistry is only needed to fetch the pool address, but there is no need to store this address, only the address of the pool (and again, is a proxy). So, there are 2 options:
- Instead of passing thepoolAddressesProviderRegistry, you can pass the poolProviderAddress directly and store its address as immutable.
- You can pass the poolAddressesProviderRegistry, but only store the poolProviderAddress (making the call at the constructor and not in every call).

So, in supplyTokenTo() and redeemToken() there is a call to _pool() which calls _poolProvider() and getPool() --> 4 calls + 1 SLOAD in total. This can be done with 0 calls and 0 SLOAD just storing the pool address at the constructor.

In the case of aToken:

The aToken address does not change (proxy) and its underlying wont never change. In supplyTokenTo() and redeemToken() there is a call to _tokenAddress() that can be skipped (1 SLOAD + 1 CALL) just storing the underlying address as immutable (doing it at the constructor; the aToken address can also be stored as immutable, just for transparency).

Gas comparison reducing the cost as much as possible: https://gist.github.com/miguelmtzinf/fb6d5266fb2fd3d057028b84fc0f66e1
(also provides files with changes)

These changes reduces the gas cost across the board, and makes the most important functions way cheaper:

• supplyTokenTo: 40k less
• redeemToken: 40k less

Bonus track (not applied in the gist):

• The function balanceOfToken() can be view.
• Typo in L154. Instead of (inhereted ERC20), `(inherited ERC20).
• In transferERC20() at L225 you are not using the already built _requireNotAToken() internal function. This function could have a more generic revert message and be used also here.

[PierrickGT]

First of all, aToken, rewardsController and poolAddressesProviderRegistry could be immutable

Duplicate of #1

poolAddressesProviderRegistry is only needed to fetch the pool address

Duplicate of #17

The aToken address does not change (proxy) and its underlying wont never change.

Duplicate of #19

The function balanceOfToken() can be view.

Can't because we inherit from the yield source interface.

Typo in L154. Instead of (inhereted ERC20), `(inherited ERC20).

Duplicate of #22

In transferERC20() at L225 you are not using the already built _requireNotAToken() internal function.

Duplicate of #4