Which general purpose sorting algorithm does Swift use? It does not perform well on sorted data

Swift uses Introsort. Looking at the source code we see that the chosen pivot is the first element. The wikipedia page on Introsort says:

(...), one of the critical operations is choosing the pivot: the element around which the list is partitioned. The simplest pivot selection algorithm is to take the first or the last element of the list as the pivot, causing poor behavior for the case of sorted or nearly sorted input.

Thus it is entirely predictable, given the implementation choice, that Swift's sorting performance is worst for sorted inputs.

I have built a complete benchmark for people who want to easily reproduce the OP's claims : https://github.com/lemire/Code-used-on-Daniel-Lemire-s-blog/tree/master/extra/swift/sort

For reference, the GNU ISO C++ standard library uses a median-of-3 pivot (as per the stl_algo.h header).

In the Swift 5 evolution, the IntroSort algorithm was replaced with a modified version TimSort (implemented first in 2002 by Tim Peters for Python) in the 'sort()' method: https://github.com/apple/swift/blob/master/stdlib/public/core/Sort.swift

This is a duplicate of this question it seems: Swift sorting algorithm implementation

Furthermore the reason why it performs so much better when shuffled is probably just because when its shuffled its performance is not hitting the upper bound of NlogN. Sorting the sorted array probably gets closer to that upper limit so its still all in all the same sort. But I dont know this is just theory