Removal of refunds

Simple Summary

Remove gas refunds for SSTORE and SELFDESTRUCT.

Motivation

Gas refunds for SSTORE and SELFDESTRUCT were originally introduced to motivate application developers to write applications that practice "good state hygiene", clearing storage slots and contracts that are no longer needed. However, they are not widely used for this, and poor state hygiene continues to be the norm. It is now widely accepted that the only solution to state growth is some form of statelessness or state expiry, and if such a solution is implemented, then disused storage slots and contracts would start to be ignored automatically.

Gas refunds additionally have multiple harmful consequences:

• Refunds give rise to GasToken. GasToken has benefits in moving gas space from low-fee periods to high-fee periods, but it also has downsides to the network, particularly in exacerbating state size (as state slots are effectively used as a "battery" to save up gas) and inefficiently clogging blockchain gas usage
• Refunds increase block size variance. The theoretical maximum amount of actual gas consumed in a block is nearly twice the on-paper gas limit (as refunds add gas space for subsequent transactions in a block, though refunds are capped at 50% of a transaction's gas used). This is not fatal, but is still undesirable, especially given that refunds can be used to maintain 2x usage spikes for far longer than EIP 1559 can.

Specification

Parameters

Constant	Value
FORK_BLOCK	TBD

For blocks where block.number >= FORK_BLOCK, the following changes apply.

Do not apply the refund.

The description above is sufficient to describe the change, but for the sake of clarity we enumerate all places where gas refunds are currently used and which should/could be removed within a node implementation.

1. Remove all use of the "refund counter" in SSTORE gas accounting, as defined in EIP 2200. Particularly:

   • If a storage slot is changed and the current value equals the original value, but does not equal the new value, SSTORE_RESET_GAS is deducted (plus COLD_SLOAD_COST if prescribed by EIP 2929 rules), but no modifications to the refund counter are made.
   • If a storage slot is changed and the current value equals neither the new value nor the original value (regardless of whether or not the latter two are equal), SLOAD_GAS is deducted (plus COLD_SLOAD_COST if prescribed by EIP 2929 rules), but no modifications to the refund counter are made.

2. Remove the SELFDESTRUCT refund.

Rationale

A full removal of refunds is the simplest way to solve the issues with refunds; any gains from partial retention of the refund mechanism are not worth the complexity that that would leave remaining in the Ethereum protocol.

Backwards Compatibility

Refunds are currently only applied after transaction execution, so they cannot affect how much gas is available to any particular call frame during execution. Hence, removing them will not break the ability of any code to execute, though it will render some applications economically nonviable.

GasToken in particular will become valueless. DeFi arbitrage bots, which today frequently use either established GasToken schemes or a custom alternative to reduce on-chain costs, would benefit from rewriting their code to remove calls to these no-longer-functional gas storage mechanisms.

Implementation

An implementation can be found here: https://gist.github.com/holiman/460f952716a74eeb9ab358bb1836d821#gistcomment-3642048

Test case changes

• The "original", "1st", "2nd", "3rd" columns refer to the value of storage slot 0 before the execution and after each SSTORE.
• The "Berlin (cold)" column gives the post-Berlin (EIP 2929) gas cost assuming the storage slot had not yet been accessed.
• The "Berlin (hot)" column gives the post-Berlin gas cost assuming the storage slot has already been accessed.
• The "Berlin (hot) + norefund" column gives the post-Berlin gas cost assuming the storage slot has already been accessed, and assuming this EIP has been implemented.

Gas costs are provided with refunds subtracted; if the number is negative this means that refunds exceed gas costs. The 50% refund limit is not applied (due to the implied assumption that this code is only a small fragment of a much larger execution).

If refunds were to be removed, this would be the comparative table

Code	Original	1st	2nd	3rd	Istanbul	Berlin (cold)	Berlin (hot)	Berlin (hot)+norefund
0x60006000556000600055	0	0	0		1612	2312	212	212
0x60006000556001600055	0	0	1		20812	22212	20112	20112
0x60016000556000600055	0	1	0		1612	2312	212	20112
0x60016000556002600055	0	1	2		20812	22212	20112	20112
0x60016000556001600055	0	1	1		20812	22212	20112	20112
0x60006000556000600055	1	0	0		-9188	-9888	-11988	3012
0x60006000556001600055	1	0	1		1612	2312	212	3012
0x60006000556002600055	1	0	2		5812	5112	3012	3012
0x60026000556000600055	1	2	0		-9188	-9888	-11988	3012
0x60026000556003600055	1	2	3		5812	5112	3012	3012
0x60026000556001600055	1	2	1		1612	2312	212	3012
0x60026000556002600055	1	2	2		5812	5112	3012	3012
0x60016000556000600055	1	1	0		-9188	-9888	-11988	3012
0x60016000556002600055	1	1	2		5812	5112	3012	3012
0x60016000556001600055	1	1	1		1612	2312	212	212
0x600160005560006000556001600055	0	1	0	1	21618	22318	20218	40118
0x600060005560016000556000600055	1	0	1	0	-8382	-9782	-11882	5918

Security Considerations

TBD

Copyright

Copyright and related rights waived via CC0.