How to write to an arbitrary storage slot using assembly in Solidity smart contracts?

Question

I am trying to understand the basics of storage manipulation using Solidity assembly (Yul) by changing the storage slot for a variable and trying to read it afterwards to see if it's stored in the correct slot but I don't get the return value I expect when calling seeSlot():

//SPDX-License-Identifier: MIT
pragma solidity 0.8.16;

contract Assembly {
    uint public age = 30;

    function changeStorageSlot(uint newSlot) pure external {
        bytes storage _age;
        assembly {
            _age.slot := newSlot
        }
    }
 
    function seeSlot() external view returns(uint) {
        uint _age = age;
        assembly {
            let sl:= sload(_age)
            sl := age.slot
            return (sl, 0x20)
        }
    }
}

What am I doing wrong?

Answer 1

There are some errors. Let's follow the execution of seeSlot() :

• uint _age = age;

Here age is read from storage and saved in the variable _age on the stack. So _age = 30 .

• let sl:= sload(_age)

Here we are reading from storage at slot _age , so at slot 30. sl is a variable on the stack. sl = 0 since at slot 30 there's nothing.

• sl:= age.slot

The value of sl is overwritten with slot age is saved in storage, so 0 .

• return (sl, 0x20)

return returns the memory from sl to sl+0x20 . Since we haven't saved anything in memory it returns 0 .

The correct function is something like this:

    function seeSlot() external view returns(uint) {
        assembly {
            let sl:= sload(age.slot)   // => sl = 30
            mstore(0x00, sl)           // store in memory at 0x00
            return (0x00, 0x20)        // return first word of memory
        }
    }

Regarding changing the storage slot of a state variable, it can't be done and can lead to bugs in the solidity code.