Does an SSTORE where the new value is the same as the existing value cost gas?

Question

For example (in Vyper):

@external
def foo(bar: uint256):
  self.baz = bar

foo(3)
foo(3) # Is the gas cost here still 5000?

Is it better to do this instead?

def foo(bar: uint256):
  if self.baz != bar: # Will this check save gas from unnecessary SSTORE's
      self.baz = bar

Answer 1

The following test implies that the cost of storing the same value is 800 gas units:

Solidity Contract:

pragma solidity 0.6.12;

contract MyContract {
    uint256 public gasUsed;
    uint256 public storageSlot;
    function func(uint256 x) public {
        storageSlot = x;
        uint256 gasLeft = gasleft();
        storageSlot = x;
        gasUsed = gasLeft - gasleft();
    }
}

Truffle 5.x Test:

const MyContract = artifacts.require("MyContract");

contract("MyContract", accounts => {
    it("test", async () => {
        const myContract = await MyContract.new();
        for (let x = 0; x < 10; x++) {
            await myContract.func(x);
            const gasUsed = await myContract.gasUsed();
            console.log(gasUsed.toString());
        }
    });
});

The printout is 816 for every iteration.

Assuming that the gasleft() operation in the last line of the contract function spends 16 gas units, the cost of storing the same value appears to be 800 gas units.

Answer 2

Thank you to the answers by @technicallyty and @goodvibration, and @goodvibration's continuation and approach is correct.

Yes, storing the same value costs 800 gas.

EIP-2200 and the relevant code is actually the next function gasSStoreEIP2200

More precisely the code:

if current == value { // noop (1)
    return params.SloadGasEIP2200, nil
}

which is commented as

// 1. If current value equals new value (this is a no-op), SLOAD_GAS is deducted.

And SloadGasEIP2200 is 800:

SloadGasEIP2200 uint64 = 800  // Cost of SLOAD after EIP 2200 (part of Istanbul)

---

More information

Here are the full comments on gasSStoreEIP2200 based on EIP-2200:

// 0. If *gasleft* is less than or equal to 2300, fail the current call.
// 1. If current value equals new value (this is a no-op), SLOAD_GAS is deducted.
// 2. If current value does not equal new value:
//   2.1. If original value equals current value (this storage slot has not been changed by the current execution context):
//     2.1.1. If original value is 0, SSTORE_SET_GAS (20K) gas is deducted.
//     2.1.2. Otherwise, SSTORE_RESET_GAS gas is deducted. If new value is 0, add SSTORE_CLEARS_SCHEDULE to refund counter.
//   2.2. If original value does not equal current value (this storage slot is dirty), SLOAD_GAS gas is deducted. Apply both of the following clauses:
//     2.2.1. If original value is not 0:
//       2.2.1.1. If current value is 0 (also means that new value is not 0), subtract SSTORE_CLEARS_SCHEDULE gas from refund counter.
//       2.2.1.2. If new value is 0 (also means that current value is not 0), add SSTORE_CLEARS_SCHEDULE gas to refund counter.
//     2.2.2. If original value equals new value (this storage slot is reset):
//       2.2.2.1. If original value is 0, add SSTORE_SET_GAS - SLOAD_GAS to refund counter.
//       2.2.2.2. Otherwise, add SSTORE_RESET_GAS - SLOAD_GAS gas to refund counter.

gasSStoreEIP2200 is the current function, and gasSStore is for historical. (As @MrClottom says in his answer: "When a new node syncs and verifies all transactions it also has to be able to understand old consensus rules...")

Original answer below was missing EIP-2200 and was based on gasSStore.

The Geth source code (master branch Nov 2020) and EIP-1283 state:

// The new gas metering is based on net gas costs (EIP-1283): // //

1. If current value equals new value (this is a no-op), 200 gas is deducted. // 2. If current value does not equal new value // 2.1. If original value equals current value (this storage slot has not been changed by the current execution context) // 2.1.1. If original value is 0, 20000 gas is deducted. // 2.1.2. Otherwise, 5000 gas is deducted. If new value is 0, add 15000 gas to refund counter. // 2.2. If original value does not equal current value (this storage slot is dirty), 200 gas is deducted. Apply both of the following clauses. // 2.2.1. If original value is not 0 // 2.2.1.1. If current value is 0 (also means that new value is not 0), remove 15000 gas from refund counter. We can prove that refund counter will never go below 0. // 2.2.1.2. If new value is 0 (also means that current value is not 0), add 15000 gas to refund counter. //
   2.2.2. If original value equals new value (this storage slot is reset) // 2.2.2.1. If original value is 0, add 19800 gas to refund counter. // 2.2.2.2. Otherwise, add 4800 gas to refund counter. value := common.Hash(y.Bytes32()) if current == value { // noop (1) return params.NetSstoreNoopGas, nil }

Answer 3

The gas cost would be 200. There is actually a comment in the source code of Geth referring to this exact behavior. In protocol_params.go on line 46 we see the following line of code:

NetSstoreNoopGas uint64 = 200 // Once per SSTORE operation if the value doesn't change.

Source: https://github.com/ethereum/go-ethereum/blob/master/params/protocol_params.go