Thanks Guy for the explanation, but my implementation is both incrementing element index for slideup and decrementing element index for slidedown (which is symmetrical implementation and simplest from my point of view).

I'm curious why you chose to be symmetrical (no need), and why you
decided incrementing for slideup decrementing for slidedn (I would do
the opposite).

By incrementing for vslidedown, and decrementing for vslideup, it
eliminates the race condition in both directions and allows
overlapping src/dst for both.

However, by supporting both incrementing and decrementing, you are
adding extra hardware that isn't strictly necessary.

Guy