# Simple Summary
REVERT instruction provides a way to stop execution and revert state changes, without consuming all provided gas and with the ability to return a reason.
REVERT instruction will stop execution, roll back all state changes done so far and provide a pointer to a memory section, which can be interpreted as an error code or message. While doing so, it will not consume all the remaining gas.
Currently this is not possible. There are two practical ways to revert a transaction from within a contract: running out of gas or executing an invalid instruction. Both of these options will consume all remaining gas. Additionally, reverting an EVM execution means that all changes, including LOGs, are lost and there is no way to convey a reason for aborting an EVM execution.
On blocks with
block.number >= BYZANTIUM_FORK_BLKNUM, the
REVERT instruction is introduced at
0xfd. It expects two stack items, the top item is the
memory_offset followed by
memory_length. It does not produce any stack elements because it stops execution.
The semantics of
REVERT with respect to memory and memory cost are identical to those of
RETURN. The sequence of bytes given by
memory_length is called "error message" in the following.
The effect of
REVERT is that execution is aborted, considered as failed, and state changes are rolled back. The error message will be available to the caller in the returndata buffer and will also be copied to the output area, i.e. it is handled in the same way as the regular return data is handled.
The cost of the
REVERT instruction equals to that of the
RETURN instruction, i.e. the rollback itself does not consume all gas, the contract only has to pay for memory.
In case there is not enough gas left to cover the cost of
REVERT or there is a stack underflow, the effect of the
REVERT instruction will equal to that of a regular out of gas exception, i.e. it will consume all gas.
In the same way as all other failures, the calling opcode returns
0 on the stack following a
REVERT opcode in the callee.
REVERT is used in the context of a
CREATE2 call, no code is deployed,
0 is put on the stack and the error message is available in the returndata buffer.
The content of the optionally provided memory section is not defined by this EIP, but is a candidate for another Informational EIP.
# Backwards Compatibility
This change has no effect on contracts created in the past unless they contain
0xfd as an instruction.
# Test Cases
- the storage at key
0x0should be left as unset and
- use 20024 gas in total.
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