Function mi_segment_check_free

3rd/mimalloc-2.0.9/src/segment.c:1279–1322

Possibly free pages and check if free space is available

// Possibly free pages and check if free space is available
static bool mi_segment_check_free(mi_segment_t* segment, size_t slices_needed, size_t block_size, mi_segments_tld_t* tld) 
{
  mi_assert_internal(block_size < MI_HUGE_BLOCK_SIZE);
  mi_assert_internal(mi_segment_is_abandoned(segment));
  bool has_page = false;
  
  // for all slices
  const mi_slice_t* end;
  mi_slice_t* slice = mi_slices_start_iterate(segment, &end);
  while (slice < end) {
    mi_assert_internal(slice->slice_count > 0);
    mi_assert_internal(slice->slice_offset == 0);
    if (mi_slice_is_used(slice)) { // used page
      // ensure used count is up to date and collect potential concurrent frees
      mi_page_t* const page = mi_slice_to_page(slice);
      _mi_page_free_collect(page, false);
      if (mi_page_all_free(page)) {
        // if this page is all free now, free it without adding to any queues (yet) 
        mi_assert_internal(page->next == NULL && page->prev==NULL);
        _mi_stat_decrease(&tld->stats->pages_abandoned, 1);
        segment->abandoned--;
        slice = mi_segment_page_clear(page, tld); // re-assign slice due to coalesce!
        mi_assert_internal(!mi_slice_is_used(slice));
        if (slice->slice_count >= slices_needed) {
          has_page = true;
        }
      }
      else {
        if (page->xblock_size == block_size && mi_page_has_any_available(page)) {
          // a page has available free blocks of the right size
          has_page = true;
        }
      }      
    }
    else {
      // empty span
      if (slice->slice_count >= slices_needed) {
        has_page = true;
      }
    }
    slice = slice + slice->slice_count;
  }
  return has_page;
}

// Reclaim an abandoned segment; returns NULL if the segment was freed
// set `right_page_reclaimed` to `true` if it reclaimed a page of the right `block_size` that was not full.