# EIP-2200: Structured Definitions for Net Gas Metering

Authors Wei Tang (@sorpaas) 2019-07-18

## Simple Summary

This is an EIP that implements net gas metering. It’s a combined version of EIP-1283 and EIP-1706, with a structured definition so as to make it interoperable with other gas changes such as EIP-1884.

## Abstract

This EIP provides a structured definition of net gas metering changes for SSTORE opcode, enabling new usages for contract storage, and reducing excessive gas costs where it doesn’t match how most implementation works.

This is a combination of EIP-1283 and EIP-1706.

## Motivation

This EIP proposes a way for gas metering on SSTORE, using information that is more universally available to most implementations, and require as little change in implementation structures as possible.

• Storage slot’s original value.
• Storage slot’s current value.
• Refund counter.

Usages that benefits from this EIP’s gas reduction scheme includes:

• Subsequent storage write operations within the same call frame. This includes reentry locks, same-contract multi-send, etc.
• Exchange storage information between sub call frame and parent call frame, where this information does not need to be persistent outside of a transaction. This includes sub-frame error codes and message passing, etc.

The original definition of EIP-1283 created a danger of a new kind of reentrancy attacks on existing contracts as Solidity by default grants a “stipend” of 2300 gas to simple transfer calls. This danger is easily mitigated if SSTORE is not allowed in low gasleft state, without breaking the backward compatibility and the original intention of EIP-1283.

This EIP also replaces the original EIP-1283 value definitions of gas by parameters, so that it’s more structured, and easier to define changes in the future.

## Specification

Define variables SLOAD_GAS, SSTORE_SET_GAS, SSTORE_RESET_GAS and SSTORE_CLEARS_SCHEDULE. The old and new values for those variables are:

• SLOAD_GAS: changed from 200 to 800.
• SSTORE_SET_GAS: 20000, not changed.
• SSTORE_RESET_GAS: 5000, not changed.
• SSTORE_CLEARS_SCHEDULE: 15000, not changed.

Change the definition of EIP-1283 using those variables. The new specification, combining EIP-1283 and EIP-1706, will look like below. The terms original value, current value and new value are defined in EIP-1283.

Replace SSTORE opcode gas cost calculation (including refunds) with the following logic:

• If gasleft is less than or equal to gas stipend, fail the current call frame with ‘out of gas’ exception.
• If current value equals new value (this is a no-op), SLOAD_GAS is deducted.
• If current value does not equal new value
• If original value equals current value (this storage slot has not been changed by the current execution context)
• If original value is 0, SSTORE_SET_GAS is deducted.
• Otherwise, SSTORE_RESET_GAS gas is deducted. If new value is 0, add SSTORE_CLEARS_SCHEDULE gas to refund counter.
• If original value does not equal current value (this storage slot is dirty), SLOAD_GAS gas is deducted. Apply both of the following clauses.
• If original value is not 0
• If current value is 0 (also means that new value is not 0), remove SSTORE_CLEARS_SCHEDULE gas from refund counter.
• If new value is 0 (also means that current value is not 0), add SSTORE_CLEARS_SCHEDULE gas to refund counter.
• If original value equals new value (this storage slot is reset)
• If original value is 0, add SSTORE_SET_GAS - SLOAD_GAS to refund counter.
• Otherwise, add SSTORE_RESET_GAS - SLOAD_GAS gas to refund counter.

An implementation should also note that with the above definition, if the implementation uses call-frame refund counter, the counter can go negative. If the implementation uses transaction-wise refund counter, the counter always stays positive.

## Rationale

This EIP mostly achieves what a transient storage tries to do (EIP-1087 and EIP-1153), but without the complexity of introducing the concept of “dirty maps”, or an extra storage struct.

• We don’t suffer from the optimization limitation of EIP-1087. EIP-1087 requires keeping a dirty map for storage changes, and implicitly makes the assumption that a transaction’s storage changes are committed to the storage trie at the end of a transaction. This works well for some implementations, but not for others. After EIP-658, an efficient storage cache implementation would probably use an in-memory trie (without RLP encoding/decoding) or other immutable data structures to keep track of storage changes, and only commit changes at the end of a block. For them, it is possible to know a storage’s original value and current value, but it is not possible to iterate over all storage changes without incurring additional memory or processing costs.
• It never costs more gas compared with the current scheme.
• It covers all usages for a transient storage. Clients that are easy to implement EIP-1087 will also be easy to implement this specification. Some other clients might require a little bit extra refactoring on this. Nonetheless, no extra memory or processing cost is needed on runtime.

Regarding SSTORE gas cost and refunds, see Appendix for proofs of properties that this EIP satisfies.

• For absolute gas used (that is, actual gas used minus refund), this EIP is equivalent to EIP-1087 for all cases.
• For one particular case, where a storage slot is changed, reset to its original value, and then changed again, EIP-1283 would move more gases to refund counter compared with EIP-1087.

Examine examples provided in EIP-1087’s Motivation (with SLOAD_GAS being 200):

• If a contract with empty storage sets slot 0 to 1, then back to 0, it will be charged 20000 + 200 - 19800 = 400 gas.
• A contract with empty storage that increments slot 0 5 times will be charged 20000 + 5 * 200 = 21000 gas.
• A balance transfer from account A to account B followed by a transfer from B to C, with all accounts having nonzero starting and ending balances, it will cost 5000 * 3 + 200 - 4800 = 10400 gas.

In order to keep in place the implicit reentrancy protection of existing contracts, transactions should not be allowed to modify state if the remaining gas is lower then the gas stipend given to “transfer”/”send” in Solidity. These are other proposed remediations and objections to implementing them:

• Drop EIP-1283 and abstain from modifying SSTORE cost
• EIP-1283 is an important update
• It was accepted and implemented on test networks and in clients.
• Add a new call context that permits LOG opcodes but not changes to state.
• Adds another call type beyond existing regular/staticcall
• Raise the cost of SSTORE to dirty slots to >=2300 gas
• Makes net gas metering much less useful.
• Reduce the gas stipend
• Makes the stipend almost useless.
• Increase the cost of writes to dirty slots back to 5000 gas, but add 4800 gas to the refund counter
• Still doesn’t make the invariant explicit.
• Requires callers to supply more gas, just to have it refunded
• Add contract metadata specifying per-contract EVM version, and only apply SSTORE changes to contracts deployed with the new version.

## Backwards Compatibility

This EIP requires a hard fork to implement. No gas cost increase is anticipated, and many contracts will see gas reduction.

Performing SSTORE has never been possible with less than 5000 gas, so it does not introduce incompatibility to the Ethereum Mainnet. Gas estimation should account for this requirement.

## Test Cases

Code Used Gas Refund Original 1st 2nd 3rd
0x60006000556000600055 1612 0 0 0 0
0x60006000556001600055 20812 0 0 0 1
0x60016000556000600055 20812 19200 0 1 0
0x60016000556002600055 20812 0 0 1 2
0x60016000556001600055 20812 0 0 1 1
0x60006000556000600055 5812 15000 1 0 0
0x60006000556001600055 5812 4200 1 0 1
0x60006000556002600055 5812 0 1 0 2
0x60026000556000600055 5812 15000 1 2 0
0x60026000556003600055 5812 0 1 2 3
0x60026000556001600055 5812 4200 1 2 1
0x60026000556002600055 5812 0 1 2 2
0x60016000556000600055 5812 15000 1 1 0
0x60016000556002600055 5812 0 1 1 2
0x60016000556001600055 1612 0 1 1 1
0x600160005560006000556001600055 40818 19200 0 1 0 1
0x600060005560016000556000600055 10818 19200 1 0 1 0

## Appendix: Proof

Because the storage slot’s original value is defined as the value when a reversion happens on the current transaction, it’s easy to see that call frames won’t interfere SSTORE gas calculation. So although the below proof is discussed without call frames, it applies to all situations with call frames. We will discuss the case separately for original value being zero and not zero, and use induction to prove some properties of SSTORE gas cost.

Final value is the value of a particular storage slot at the end of a transaction. Absolute gas used is the absolute value of gas used minus refund. We use N to represent the total number of SSTORE operations on a storage slot. For states discussed below, refer to State Transition in Explanation section.

Below we do the proof under the assumption that all parameters are unchanged, meaning SLOAD_GAS is 200. However, note that the proof still applies no matter how SLOAD_GAS is changed.

### Original Value Being Zero

When original value is 0, we want to prove that:

• Case I: If the final value ends up still being 0, we want to charge 200 * N gases, because no disk write is needed.
• Case II: If the final value ends up being a non-zero value, we want to charge 20000 + 200 * (N-1) gas, because it requires writing this slot to disk.

#### Base Case

We always start at state A. The first SSTORE can:

• Go to state A: 200 gas is deducted. We satisfy Case I because 200 * N == 200 * 1.
• Go to state B: 20000 gas is deducted. We satisfy Case II because 20000 + 200 * (N-1) == 20000 + 200 * 0.

#### Inductive Step

• From A to A. The previous gas cost is 200 * (N-1). The current gas cost is 200 + 200 * (N-1). It satisfy Case I.
• From A to B. The previous gas cost is 200 * (N-1). The current gas cost is 20000 + 200 * (N-1). It satisfy Case II.
• From B to B. The previous gas cost is 20000 + 200 * (N-2). The current gas cost is 200 + 20000 + 200 * (N-2). It satisfy Case II.
• From B to A. The previous gas cost is 20000 + 200 * (N-2). The current gas cost is 200 - 19800 + 20000 + 200 * (N-2). It satisfy Case I.

### Original Value Not Being Zero

When original value is not 0, we want to prove that:

• Case I: If the final value ends up unchanged, we want to charge 200 * N gases, because no disk write is needed.
• Case II: If the final value ends up being zero, we want to charge 5000 - 15000 + 200 * (N-1) gas. Note that 15000 is the refund in actual definition.
• Case III: If the final value ends up being a changed non-zero value, we want to charge 5000 + 200 * (N-1) gas.

#### Base Case

We always start at state X. The first SSTORE can:

• Go to state X: 200 gas is deducted. We satisfy Case I because 200 * N == 200 * 1.
• Go to state Y: 5000 gas is deducted. We satisfy Case III because 5000 + 200 * (N-1) == 5000 + 200 * 0.
• Go to state Z: The absolute gas used is 5000 - 15000 where 15000 is the refund. We satisfy Case II because 5000 - 15000 + 200 * (N-1) == 5000 - 15000 + 200 * 0.

#### Inductive Step

• From X to X. The previous gas cost is 200 * (N-1). The current gas cost is 200 + 200 * (N-1). It satisfy Case I.
• From X to Y. The previous gas cost is 200 * (N-1). The current gas cost is 5000 + 200 * (N-1). It satisfy Case III.
• From X to Z. The previous gas cost is 200 * (N-1). The current absolute gas cost is 5000 - 15000 + 200 * (N-1). It satisfy Case II.
• From Y to X. The previous gas cost is 5000 + 200 * (N-2). The absolute current gas cost is 200 - 4800 + 5000 + 200 * (N-2). It satisfy Case I.
• From Y to Y. The previous gas cost is 5000 + 200 * (N-2). The current gas cost is 200 + 5000 + 200 * (N-2). It satisfy Case III.
• From Y to Z. The previous gas cost is 5000 + 200 * (N-2). The current absolute gas cost is 200 - 15000 + 5000 + 200 * (N-2). It satisfy Case II.
• From Z to X. The previous gas cost is 5000 - 15000 + 200 * (N-2). The current absolute gas cost is 200 + 10200 + 5000 - 15000 + 200 * (N-2). It satisfy Case I.
• From Z to Y. The previous gas cost is 5000 - 15000 + 200 * (N-2). The current absolute gas cost is 200 + 15000 + 5000 - 15000 + 200 * (N-2). It satisfy Case III.
• From Z to Z. The previous gas cost is 5000 - 15000 + 200 * (N-2). The current absolute gas cost is 200 + 5000 - 15000 + 200 * (N-2). It satisfy Case II.

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