Muldiv what is prod0 |= prod1 * twos?

Question

In muldiv, what is this line of code doing?

prod0 |= prod1 * twos

Also can it overflow?

https://github.com/OpenZeppelin/openzeppelin-contracts/blob/f213a10522a7bd808561c5a4b17266065a199dc7/contracts/utils/math/Math.sol#L185

Answer 1

This is a bitwise operation They do not cause an overflow.

prod0 |= prod1 * twos; translated as prod0 = prod0 | (prod1 * twos);

The | operation is a binary OR. It puts a 1 on the result bit for which at least one of the corresponding operand bits is 1.

0 | 0 = 0
1 | 0 = 1
0 | 1 = 1
1 | 1 = 1

Then if you have incoming values:

prod0 = 2 // 00000010 binary
prod1 = 2
twos = 2 

Then the result prod0 = prod0 | (prod1 * twos) is this:

(prod1 * twos) = 4 // 00000100 binary
prod0 = prod0 | 4 = 00000010 | 00000100 = 000000110 = 6