What is the return value of this asembly code?

Question

Taken from this repository

    /// @notice Generate number
function asmbl() public view returns (uint8 b) {
    assembly {
        let c := add(27, 16)
        mstore(0x80, c)
        { let d := add(sload(c), 255) b := d }
        b := add(b, c) }
    }

My understanding is:

1. let c := add(27, 16). c stores now the value 42
2. mstore(0x80, c). c was stored in memoryon address 0x80. Probably an useles statement without an effect. Because the memory is not used afterwards.
3. let d := add(sload(c), 255).

sload(c). loads the length of c??? Not sure about this, but I read somewhere that sloading the varible without an offset returns the length of it. But if true, the sload(c) value is probably 256, because int256

add(sload(c), 255) is the same as add(256, 255) = 511. d = 511

4. b := d. this is simple. value of d is assigned to b.
5. b := add(b, c). b is 511 + 42 = 553
6. 553 is returned by the function

Is this correct?

Answer 1

let c := add(27, 16)                 // 43 -> c
mstore(0x80, c)                      // stores uint256(43) at memory[0x80..0xa0]
{ 
  let d := add(sload(c), 255)        // sload(c) -> 0 (*)
                                     // 255 -> d
  b := d                             // 255 -> b
}
b := add(b, c)                       // 255 + 43 = 298 = 42 -> b (**)

(*) The contract has 12 variables, each occupies a single slot. So most slots above 13 are uninitialized and they will return 0. Some strings are initialized but they are short (less than 32) so no higher slots are used.

(**) b is uint8 so we can't store 298, it higher bits are truncated.

Answer 2

I believe sload(c) loads the value stored at key c. The key is c and storage value is 42. Your mention about length is for dynamic arrays, but I don't think it’s applicable to this snippet here. Hence, d = 42 + 255 = 297, and b = 297 + 42 = 339, which is the value returned by the function.

See:

https://www.evm.codes/#54