Solc: Troubleshooting Inline Assembly Compilation Errors on Linux

Within the assembly code, _data is the memory address of the start of the array data.

However, the first memory word (32 bytes = 0x20 bytes) is reserved for the length of the array, so we need to step over this. Thus, _data[0] is at memory address _data + 0x20. In the code, this looks like add(_data, 0x20).

The array elements follow consecutively, numbered from 0, each occupying one 32 byte (0x20 byte) word. Thus, element [N] is offset N * 0x20 from the start of the array data that we found above. In the code this offset is mul(i, 0x20).

Putting it all together, _data[i] is found at (_data + 0x20) + (i * 0x20), which is the expression (add(add(_data, 0x20), mul(i, 0x20))).

The net effect of the loop is to add up all the array elements into o_sum. It is likely done in assembly to avoid the overhead of array bounds checking that Solidity always inserts.

Add the following line at the end of your assembly -

sstore(value.slot, sum)

That will store sum value into the slot of contract variable value.

You can also remove the mstore and local var sum and write directly to the variable -

contract Test {

    uint256 public value;

    function add(uint a, uint b) external {
        assembly {
            sstore(value.slot, add(a,b))
        }
    } 
}

Take into consideration this doesn't handle arithmetic overflow, this is not safe to use if input number a + b is larger than uint256.