introsort_libcxx.h

void
__introsort(_RandomAccessIterator __first, _RandomAccessIterator __last,
            difference_type __depth) {
  // ...
  while (true) {
    if (__depth == 0) {
      // Fallback to heap sort as Introsort suggests.
      _VSTD::__partial_sort<_Compare>(__first, __last, __last);
      return;
    }
    --__depth;
   // ...
}

template <typename _Number>
inline _Number __log2i(_Number __n) {
  _Number __log2 = 0;
  while (__n > 1) {
    __log2++;
    __n >>= 1;
  }
  return __log2;
}
  
void __sort(_RandomAccessIterator __first, _RandomAccessIterator __last) {
  difference_type __depth_limit = 2 * __log2i(__last - __first);
  _VSTD::__introsort(__first, __last, __depth_limit);
}