Opcode LOG3 Data Value Problem

Question

I am trying to get the Event data from LOG3 but the data sometimes on stack5 and sometimes on stack6, can anyone help?

for example, transfer amount = 1 of both two transfer functions, take a look at LOG3 opcode stacks.

event Transfer(address indexed from, address indexed to, uint256 value);


function transfer1(address to, uint tokens) public returns (bool success) {
    emit Transfer(msg.sender, to, tokens);
    return true;
}

function transfer2() public returns (bool success){
    uint256 amount = 1;
    emit Transfer(msg.sender, msg.sender, amount);
    return true;
}

Transfer Amount data located at position 5 at LOG3 Struct Log of transfer1:

[ "0x0000000000000000000000000000000000000000000000000000000000000080", "0x0000000000000000000000000000000000000000000000000000000000000020", "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef", "0x0000000000000000000000005b38da6a701c568545dcfcb03fcb875f56beddc4", "0x0000000000000000000000005b38da6a701c568545dcfcb03fcb875f56beddc4", "0x0000000000000000000000000000000000000000000000000000000000000001", "0x000000000000000000000000000000000000000000000000000000000000004e", "0x000000000000000000000000000000000000000000000000000000002e687de1" ]

Transfer Amount data located at position 6 at LOG3 Struct Log of transfer2:

[ "0x0000000000000000000000000000000000000000000000000000000000000080", "0x0000000000000000000000000000000000000000000000000000000000000020", "0xddf252ad1be2c89b69c2b068fc378daa952ba7f163c4a11628f55a4df523b3ef", "0x0000000000000000000000005b38da6a701c568545dcfcb03fcb875f56beddc4", "0x000000000000000000000000bb4cdb9cbd36b01bd1cbaebf2de08d9173bc095c", "0x0000000000000000000000000000000000000000000000000000000000000000", "0x0000000000000000000000000000000000000000000000000000000000000001", "0x000000000000000000000000bb4cdb9cbd36b01bd1cbaebf2de08d9173bc095c", "0x000000000000000000000000000000000000000000000000000000000000006a", "0x00000000000000000000000000000000000000000000000000000000a9059cbb" ]

How can I know which position to take? Thanks.

Answer 1

Okay so I think I understand your issue now, it's not a problem with solidity, it seems to me like you are misinterpreting the stack at those two points of execution.

The LOG3 opcode (and LOGX in general) take the following inputs from the stack :

• 0x00 - OFFSET : in memory of the data)
• 0x20 - LENGTH : in memory of the data)
• 0x40 - TOPIC 0 : keccak256("Transfer(address,address,uint256)")
• 0x60 - TOPIC 1 : address indexed from
• 0x80 - TOPIC 2 : address indexed to

Therefore, only the first 5 slots in the stack are relevant for you at this point, anything else is for the rest of the execution but doesn't concern the LOGX opcode.

You will find the rest of the data (i.e., value) at MEMORY[OFFSET:OFFSET + LENGTH] and indeed in both cases the value 1 is encoded there on 32 bytes.

Indexed parameters are placed in the topic section, which non indexed parameters are placed in the data section, so from and to are included in the topic section (stack 3 & 4) while value is the only one written to the data section, that's why the memory length is only 0x20 (only one ABI encoded parameter).

I hope that answers your question.