Right Shift not working in inline assembly

Question

I working on in-line assembly in the smart contract.

function rightShift(uint256 x) public pure returns (uint256 result) {
       assembly {
          result := shr(x,1)
       }
    }

Above function always return zero. Am I doing something wrong or it’s bug?

Answer 1

Always double-check the documentation.

shr(x, y) logical shift right y by x bits

With your notation, you are effectively shifting 1 by x bits to the right. For any non-zero value, the result will be zero.

You probably meant to write :

function rightShift(uint256 x) public pure returns (uint256 result) {
   assembly {
      result := shr(1,x)
   }
}

Answer 2

Let's take a look at the definition of the shr Yul instruction:

shr(x, y): logical shift right y by x bits

Your function always returns 0 because you are shifting by x (your variable x) bits, not by 1 bit.

To make it work, you have to rewrite it like this:

function rightShift(uint256 x) public pure returns (uint256 result) {
    assembly {
        result := shr(1, x)
    }
}