Comment by dan-robertson

There’s an example in the linked thread. Suppose you write some code like

  if (a+b) % 2 == 0:
    …

And the compiler is able to figure out that a and b are even (that is they both have 0 in the lsb), or if it figures out that a = b, this kind of known-bits tracking allows it to get rid of the branch. The code in the OP can be used to make tracking known-bits fast enough to be practical