[vitus]

> That only works when y is constant.

Excellent point! That said, that was the case in this particular example.

> This 16-bit modulo was just a final check that the correct number of bytes or bits were being sent (expecting remainder zero), so the denominator was going to be the same every time.

(Libraries like libdivide allow you to memoize the magic numbers for frequently-used denominators, and if on x86 you have floating point operations with more precision than you need for integer division, you can potentially use the FPU instead of the ALU: https://lemire.me/blog/2017/11/16/fast-exact-integer-divisio...)