What is the proper use of inline assembly revert, and how do I retrieve returndata passed to it?

Question

I stumbled on this EIP which describes the revert opcode that looks like it allows you to "throw" more specific errors by passing a memory pointer with the opcode (also described briefly in the docs). Note in the docs that return and revert receive the same input format.

So far, I have this function as an example:

function throwError(bytes _error) constant returns (bytes32) {
    assembly {
        calldatacopy(0xff, 0, calldatasize)
        return(add(0xff, 68), 32)
    }
}

It works as expected, copying the string passed as function parameter and returning it. However, when I change return to revert like this:

function throwError(bytes _error) constant returns (bytes32) {
    assembly {
        calldatacopy(0xff, 0, calldatasize)
        revert(add(0xff, 68), 32)
    }
}

All I get back is a normal invalid opcode error from web3, truffle, and remix when testing with testrpc. I don't see any reason why this wouldn't work; is there something I'm missing here?

Answer 1

revert is not yet implemented in the EVM so it does indeed currently result in an invalid opcode (it's genuinely an invalid op-code in this case, not a proxy for throw). It will be part of the Metropolis update.