compare-windows, but going backwards (or upwards)

The function compare-windows is very handy for comparing two buffers
starting at the selected point in each.  It advances
character-by-character, and stops at the first difference.

It would also be helpful to compare-windows but going backwards.

It seems that surely there must be some function somebody has already
written to do this (???).

I'm using emacs version 30.1.

Thanks in advance for any pointers!

dan