Gas Tokens in Bridges

[dekz]

Description

When a user makes a trade against a Bridge like Curve (10e5 gas). They spend approximately (10e5 * 8e9) * 1e-18 * $150 $1.20 in gas. We can reduce this by approximately 40% by freeing gas tokens if we have an available balance. So the user only pays $0.72 to fill on Curve.

Calculations are from gastoken.io

// GST2
var FREE_BASE = 14154;
var FREE_TOKEN = 6870;
var REIMBURSE = 24000;
Math.floor((gasUsed + FREE_BASE) / (2 * REIMBURSE - FREE_TOKEN));

bridge	gas consumed	GST2 freed
Uniswap	3e5	0.03
Oasis	5e5	0.12
Curve	10e5	0.24

freeFrom

Rather than have each bridge use its own balance, we chose to use freeFromUpTo to allow a EOA (GST Collector) to hold the GST balance, preventing a drain if GST2 was traded via a Bridge. This keeps the balances of tokens traded distinct from the gst balance. The collector can whitelist certain contracts, and remove contracts if required. If there is no allowance set it does not revert.

We also don't loose any unspent (and sent) tokens when we re-deploy bridges.

Process

• GST Collector holds the balance of GST2.
• User deploys new Bridge Contract specifying the GST Collector as the address to free from
• Until Collector approves the Bridge Contract, 0 GST will be freed
• Collector approves Bridge Contract

Notes

• This does not refund on a revert
• This does not cover 0x overhead (only bridge overhead)

[hysz]

Nice! (~‾▿‾)~

[coveralls]

Coverage increased (+0.05%) to 79.638% when pulling 1e17841 on feature/gas-token-bridge into 99b0a95 on development.

[dorothy-zbornak]

This tries to refund the entire amount of gas consumed, right? Maybe we should just try to refund half. I believe a quirk of the EVM is that refunds are capped at half the gas consumed, so any excess might be wasted.

[dekz]

Yes this calculation gets us to maximum efficiency. You can also play with the calculator (https://gastoken.io/) to confirm.

gasUsed = 2,000,000
value = 48 =  (gasUsed + 14154) / 41130 

SELFDESTRUCT = -24000 (refund)
SELFDESTRUCT_COST = 700 (CALL) + 5000 (SUICIDE)
refund = -878,400 = SELFDESTRUCT * 48 + SELFDESTRUCT_COST * 48


newGasUsed = 1,121,600 > gasUsed * 0.5

note: also a bit of book keeping w.r.t balance updates is excluded