And just a few shells could be added, to make this optimization right? If the sequence is 1, 4, 10, 23, .. one could start by just adding a pass with 4 and then 1.

https://stackoverflow.com/questions/2539545/fastest-gap-sequ...

I don't know how to select the fallback sort loop size (related to cache line size maybe?); in his github code he uses 16 elements as the threshold.

A very interesting suggestion indeed. Shell sort is an insertion sort though. It owes its better complexity from being able to breaking out of loop early. But these are difficult to predict branches that bubble sort avoids.

Certainly this is worth an experiment

Well, you are the expert :). I misremebered and thought of the base case for shell sort to be a kind of bubble sort.