Function mi_segment_span_allocate

3rd/mimalloc-2.0.9/src/segment.c:676–723

Note: may still return NULL if committing the memory failed

// Note: may still return NULL if committing the memory failed
static mi_page_t* mi_segment_span_allocate(mi_segment_t* segment, size_t slice_index, size_t slice_count, mi_segments_tld_t* tld) {
  mi_assert_internal(slice_index < segment->slice_entries);
  mi_slice_t* const slice = &segment->slices[slice_index];
  mi_assert_internal(slice->xblock_size==0 || slice->xblock_size==1);

  // commit before changing the slice data
  if (!mi_segment_ensure_committed(segment, _mi_segment_page_start_from_slice(segment, slice, 0, NULL), slice_count * MI_SEGMENT_SLICE_SIZE, tld->stats)) {
    return NULL;  // commit failed!
  }

  // convert the slices to a page
  slice->slice_offset = 0;
  slice->slice_count = (uint32_t)slice_count;
  mi_assert_internal(slice->slice_count == slice_count);
  const size_t bsize = slice_count * MI_SEGMENT_SLICE_SIZE;
  slice->xblock_size = (uint32_t)(bsize >= MI_HUGE_BLOCK_SIZE ? MI_HUGE_BLOCK_SIZE : bsize);
  mi_page_t*  page = mi_slice_to_page(slice);
  mi_assert_internal(mi_page_block_size(page) == bsize);

  // set slice back pointers for the first MI_MAX_SLICE_OFFSET entries
  size_t extra = slice_count-1;
  if (extra > MI_MAX_SLICE_OFFSET) extra = MI_MAX_SLICE_OFFSET;
  if (slice_index + extra >= segment->slice_entries) extra = segment->slice_entries - slice_index - 1;  // huge objects may have more slices than avaiable entries in the segment->slices
  
  mi_slice_t* slice_next = slice + 1;
  for (size_t i = 1; i <= extra; i++, slice_next++) {
    slice_next->slice_offset = (uint32_t)(sizeof(mi_slice_t)*i);
    slice_next->slice_count = 0;
    slice_next->xblock_size = 1;
  }

  // and also for the last one (if not set already) (the last one is needed for coalescing and for large alignments)
  // note: the cast is needed for ubsan since the index can be larger than MI_SLICES_PER_SEGMENT for huge allocations (see #543)
  mi_slice_t* last = slice + slice_count - 1;
  mi_slice_t* end = (mi_slice_t*)mi_segment_slices_end(segment);
  if (last > end) last = end;
  if (last > slice) {
    last->slice_offset = (uint32_t)(sizeof(mi_slice_t) * (last - slice));
    last->slice_count = 0;
    last->xblock_size = 1;
  }
  
  // and initialize the page
  page->is_reset = false;
  page->is_committed = true;
  segment->used++;
  return page;
}

static void mi_segment_slice_split(mi_segment_t* segment, mi_slice_t* slice, size_t slice_count, mi_segments_tld_t* tld) {
  mi_assert_internal(_mi_ptr_segment(slice) == segment);