Conv

On-chain repository for MCD rates.

Conv stores all per-second MCD rates for annualized BPSs in a single on-chain repository.

It allows to convert BPS into RAY accumulators and vice versa.

Motivation

Useful for validation using human-friendly notation, which drastically reduces the cognitive overhead when checking rates.

Requirements:

• The rates need to have full precision compared to rates currently used in MCD Spell process.
• Read cost should be reasonable, allowing other components of the system to use it without too much overhead.
• The contract needs to be deployable efficiently (low priority, one time cost).

Design

We explored several ways to store or calculate rates onchain and arrive at this approach, for details see this.

The contract makes use of optimized storage to ease the cost of deployment. Each rate is stored as rate - RAY, so only the relevant part of the rate takes space in storage. Each rate is stored in 8 bytes, so every storage position fits exactly four rates.

On reads, the function btor(bps) will fetch the correct storage slot, fetch the desired rate within it, add one RAY and return the result.

The contract also includes a conversion function rtob(ray) that calculates the annualized BPS rate from the per-second rate.

Limitations

• Since rates are stored in 8 bytes, the max BPS that can be used without reimplementing this contract is 7891.
• EIP-3860: Due to contract size limits on Ethereum mainnet, the ceiling for rates is 6k. On L2s that do not enforce the limit this does not apply.
• Gas cost of deployment: With the current ~36M block gas ceiling on Ethereum mainnet, up to ~5.5k rates can be stored in a single transaction.

Deployments

• 5000bps Ethereum Mainnet: 0xea91A18dAFA1Cb1d2a19DFB205816034e6Fe7e52

Note: The Mainnet deployment above contains a bug fixed in PR #4, that would cause calls to the contract to fail if RATES storage slot is not 0. This does not affect the deployed contract (the storage layout cannot change after deployment) but future deployments should favor the fixed version.

Usage

Build

$ forge build

Test

$ forge test

Gas Snapshots

$ forge snapshot

Deploy

$ forge script script/ConvDeploy.s.sol:ConvDeployScript \
    --rpc-url $ETH_RPC_URL \
    --broadcast