Inline Assembly: RETURN opcode can't not work. Failed to decode output

Question

I know that: return(p, s): end execution, return data mem[p…(p+s)).

I want to return something in memory but not a multiple of bytes32. For example, I want to return 1 byte:

pragma solidity 0.8.20;
contract Adder {
    function add(uint a, uint b) public pure returns (uint8) {
        assembly {
            mstore(0x0, add(a, b))
            return(0x0, 8)
        }
    }
    function test() public pure returns(uint8){
        return add(1,2);
    }
}

But when I call add(1,2) and test() in Remix, there is something wrong:

Failed to decode output: Error: data out-of-bounds (length=8, offset=32, code=BUFFER_OVERRUN, version=abi/5.7.0)

The doc doesn't say we should use a multiple of bytes32. 1 byte is okay in theory? But What happened?

Answer 1

The error happened because Remix expected a uint8 as output. This is encoded as 32 bytes, so when it tries reading the value it fails ("data out-of-bounds" error).

It's not a blockchain or solidity error. If you were sending a transaction it would go through without reverting. The problem is that Remix will interpret the output as uint8 abi-encoded.

I don't think there's a way in Remix to get the value without attempting the abi-decoding. So the only solution is making a "raw" eth_call and reading the returned value directly.

---

BTW.

        assembly {
            mstore(0x0, add(a, b))
            return(0x0, 8)
        }

The value of add(a, b) will be "aligned to the right". So if you want to return it you have to do:

        assembly {
            mstore(0x0, add(a, b))
            return(24, 32)
        }