The following opcodes trigger a refund
SSTORE[x] = 0 (i.e. deletion)
SSTORE clear refunds
15.000 gas. The accumulated refund can not exceed half the gas used for the current context (i.e. the initial call).
Let's take the following example:
Current state of the contract's storage
And the following execution
SSTORE ; set contents of address 0x0 to 0 (i.e. delete)
This would result in the following gas sum
21000 + 3 + 3 + 5000 (
tx_gas + push_gas + push_gas + sstore_clear_gas). The total amount of which may be refunded is therefor
gas_sum / 2 (
13.003). The amount refunded is
15.000 but since this clearly exceeds the maximum of
13.003 we cap it to that amount.
And the reason why we cap this is so that the miner does not end up paying for the actual execution of the contract :-)
If the refunds weren't capped we would run in to 2 issues:
- It disincentives running any contracts that refund;
- Miners may end up paying for the execution.
It would be disincentive to run these type of transactions if they weren't capped because at the end of the execution the refund is reduced from the amount that which a miner gets (i.e.
gasUsed * tx.price). Capping at say the
gasUsed would render the execution useless to the miner (it doesn't get rewarded for running the contract). This brings us to the second point; having a higher refund than the gas used would therefor end up in a negative gas usage and would therefor have to be reduced from the miner's balance. No miner in their right mind is going to pay you for execution your call.