SIP-46: Address Resolver Gas Improvements

Simple Summary

Cache addresses from resolver inside contracts to reduce gas usage.

Abstract

Ensure that each contract relying on addresses from the AddressResolver caches its own copy of these addresses, thereby reducing the gas usage for users when executing regular transactions like issue, burn, claim and exchange. The cache however must be refreshed on each and every deployment.

Motivation

In our Achernar release (v2.19.8), we introduced SIP-43 - an address lookup contract into the FeePool, Synthetix, Exchanger, Issuer and all Synth contracts, among others. The impact to gas on common user functions like FeePool.claim, Synthetix.exchange and Synthetix.issueSynths was signficant.

Prior to Achernar

Contract	Function	Gas Used
FeePool	claimFees	322,641
Synthetix	burnSynths	352,748
Synthetix	exchange	226,041
Synthetix	issueMaxSynths	342,547
Synthetix	issueSynths	361,070

from https://github.com/Synthetixio/synthetix/runs/353850454

After Achernar

Contract	Function	Gas Used	Diff	%
FeePool	claimFees	497,550	+174,909	+54%
Synthetix	burnSynths	547,257	+193,145	+55%
Synthetix	exchange	565,260	+339,218	+150%
Synthetix	issueMaxSynths	503,953	+161,406	+47%
Synthetix	issueSynths	531,401	+168,223	+46%

from https://github.com/Synthetixio/synthetix/runs/457383033

The biggest impact is to exchange() however that was primarily due to SIP-37 (Fee Reclamation) that was also released in Achernar.

By caching the various required addresses, we can reduce these down and not worry about having to write code in each function to limit how often we lookup the addresses required.

Specification

1. Introduce a local mapping(bytes32 => address) cache into MixinResolver - the functionality included in all contracts that need to communicate via the address resolver
2. Ensure MixinResolver is instantiated with a list of bytes32 addresses that the contract needs to know about
3. Expose a function in MixinResolver.setResolverAndSyncCache() to sync the cache. This is to be called each deployment whenever any new contracts are added.

Preliminary investigations after Caching and other improvements (such as SIP-41):

Contract	Function	Gas Used	Diff from pre-Achenar	%
FeePool	claimFees	422,852	+100,211	+24%
Synthetix	burnSynths	329,877	-22,871	-7%
Synthetix	exchange	473,954	+247,913	+110%
Synthetix	issueMaxSynths	437,013	+94,466	+28%
Synthetix	issueSynths	394,636	+33,566	+9%

from https://github.com/Synthetixio/synthetix/pull/434/checks?check_run_id=480906529

Rationale

1. A local mapping in each MixinResolver will prevent unnecssary CALL opcodes from being used (approximately 1518 gas)
2. Having the list of required address names in each MixinResolver will allow automated checks each deployment to know what needs updating in the AddressResolver
3. After each deployment, determine which MixinResolver contracts need to be resynced based on the list in #2 above

Test Cases

https://github.com/Synthetixio/synthetix/blob/resolver-gas-improvements/test/contracts/MixinResolver.js

https://github.com/Synthetixio/synthetix/blob/resolver-gas-improvements/test/publish/index.js#L944-L1073

Implementation

@Synthetixio/synthetix#434

Copyright

Copyright and related rights waived via CC0.