Incorrect gas cost for SSTORE opcode when using fork feature

Executing transactions that use SSTORE opcode costs less gas when using fork feature.

Expected Behavior

SSTORE that sets the value v in the slot s should cost 20k gas when the current value in slot s is 0x0000000000000000000000000000000000000000000000000000000000000000 and v is not 0x0000000000000000000000000000000000000000000000000000000000000000.

Current Behavior

SSTORE mentioned above costs 5k gas.

Possible Solution

This is probably an issue with the gas calculator, SSTORE should cost 5k gas when the current value in the slot is not 0x0000000000000000000000000000000000000000000000000000000000000000

Steps to Reproduce (for bugs)

1. ganache-cli -f mainnet_node --networkId 1
2. truffle migrate
3. Saving migration to the chain costs 27363 gas, but it should cost 42363 gas.

Your Environment

• Version used: Ganache CLI v6.10.1-beta.2 (ganache-core: 2.11.3-beta.0)
• Operating System and version: macOS Catalina 10.15.4

[nebojsa94]

Hey @seesemichaelj, sorry for the delays,

It was pretty hard to find the actual issue that led to the incorrect SSTORE gas usage, but I think I figured it out 😃

We can use the test in forking.js on line 179 with modified Example.sol where the constructor function doesn’t set the value property, the gas usage in the receipt in this test should be 42535, but atm it is 27535, the reason is that the SSTORE gas calculator doesn’t realize that the original value is empty but instead goes to storage-slot-update, this is because original field has the value [0] instead of [].

StateManager is responsible for storage fetching during the execution of the transaction, this line will call ForkedStorageTrie implementation which is going to invoke web3.eth.getStorageAt that is going to return 0x0, unfortunately, this is going to encode into [0] and won’t trigger slot creation.

The fix should be pretty straight forward, just make sure 0x0 encodes to [] buffer

[nebojsa94]

I can confirm that the issue is still present if the state variable is set to the default value in the declaration statement; in this case, if you remove = 0 in uint256 public x = 0; the SSTORE opcode will cost 20k, which is correct behavior.