x.high >> 1 double.high |= x.low << 63 double.low = x.low >> 1 // If the most-significant bit was set before shifting then it, // conceptually, becomes a term of x^128. This is greater than the // irreducible polynomial so the result has to be reduced. The // irreducible polynomial is 1+x+x^2+x^7+x^128. We can subtract that to // eliminate the term at x^128 which also means subtracting the other // four terms. In characteristic 2 fields, subtraction == addition == // XOR. if msbSet { double.low ^=...

x.high >> 1 double.high |= x.low << 63 double.low = x.low >> 1 // If the most-significant bit was set before shifting then it, // conceptually, becomes a term of x^128. This is greater than the // irreducible polynomial so the result has to be reduced. The // irreducible polynomial is 1+x+x^2+x^7+x^128. We can subtract that to // eliminate the term at x^128 which also means subtracting the other // four terms. In characteristic 2 fields, subtraction == addition == // XOR. if msbSet { double.low ^=...