Function mi_segment_span_free_coalesce

3rd/mimalloc-2.0.9/src/segment.c:628–667

note: can be called on abandoned segments

// note: can be called on abandoned segments
static mi_slice_t* mi_segment_span_free_coalesce(mi_slice_t* slice, mi_segments_tld_t* tld) {
  mi_assert_internal(slice != NULL && slice->slice_count > 0 && slice->slice_offset == 0);
  mi_segment_t* segment = _mi_ptr_segment(slice);
  bool is_abandoned = mi_segment_is_abandoned(segment);

  // for huge pages, just mark as free but don't add to the queues
  if (segment->kind == MI_SEGMENT_HUGE) {
    mi_assert_internal(segment->used == 1);  // decreased right after this call in `mi_segment_page_clear`
    slice->xblock_size = 0;  // mark as free anyways
    // we should mark the last slice `xblock_size=0` now to maintain invariants but we skip it to 
    // avoid a possible cache miss (and the segment is about to be freed)
    return slice;
  }

  // otherwise coalesce the span and add to the free span queues
  size_t slice_count = slice->slice_count;
  mi_slice_t* next = slice + slice->slice_count;
  mi_assert_internal(next <= mi_segment_slices_end(segment));
  if (next < mi_segment_slices_end(segment) && next->xblock_size==0) {
    // free next block -- remove it from free and merge
    mi_assert_internal(next->slice_count > 0 && next->slice_offset==0);
    slice_count += next->slice_count; // extend
    if (!is_abandoned) { mi_segment_span_remove_from_queue(next, tld); }
  }
  if (slice > segment->slices) {
    mi_slice_t* prev = mi_slice_first(slice - 1);
    mi_assert_internal(prev >= segment->slices);
    if (prev->xblock_size==0) {
      // free previous slice -- remove it from free and merge
      mi_assert_internal(prev->slice_count > 0 && prev->slice_offset==0);
      slice_count += prev->slice_count;
      if (!is_abandoned) { mi_segment_span_remove_from_queue(prev, tld); }
      slice = prev;
    }
  }

  // and add the new free page
  mi_segment_span_free(segment, mi_slice_index(slice), slice_count, true, tld);
  return slice;
}