How to read high-level storage variable from inline assembly?

Question

This code does not compile:

pragma solidity >=0.8.0;

contract MyContract {
    uint256 public foo = 314;

    function myFunction() external view returns (uint256 result) {
        assembly {
            result := foo
        }
    }
}

This error is thrown:

TypeError: only local variables are supported. To access storage variables, use the ".slot" and ".offset" suffixes.

I modified my code like this:

result := foo.slot

But the result I got was "1" instead of "314". How can I get access to this storage variable in inline assembly?

Answer 1

See the Access to External Variables, Functions and Libraries section in the docs.

The solution is to use the sload function and pass the slot as the only argument:

assembly {
    result := sload(foo.slot)
}

Caveats:

• Be careful if the value you're reading does not have a span of exactly 256 bits (e.g. address). You cannot assume that the bits not part of the encoding will be zero.
• For dynamic data types like arrays, you have to use .offset and .length besides .slot.