Function mi_os_mem_alloc_aligned

3rd/mimalloc-2.0.9/src/os.c:748–800

Primitive aligned allocation from the OS. This function guarantees the allocated memory is aligned.

// Primitive aligned allocation from the OS.
// This function guarantees the allocated memory is aligned.
static void* mi_os_mem_alloc_aligned(size_t size, size_t alignment, bool commit, bool allow_large, bool* is_large, mi_stats_t* stats) {
  mi_assert_internal(alignment >= _mi_os_page_size() && ((alignment & (alignment - 1)) == 0));
  mi_assert_internal(size > 0 && (size % _mi_os_page_size()) == 0);
  mi_assert_internal(is_large != NULL);
  if (!commit) allow_large = false;
  if (!(alignment >= _mi_os_page_size() && ((alignment & (alignment - 1)) == 0))) return NULL;
  size = _mi_align_up(size, _mi_os_page_size());

  // try first with a hint (this will be aligned directly on Win 10+ or BSD)
  void* p = mi_os_mem_alloc(size, alignment, commit, allow_large, is_large, stats);
  if (p == NULL) return NULL;

  // if not aligned, free it, overallocate, and unmap around it
  if (((uintptr_t)p % alignment != 0)) {
    mi_os_mem_free(p, size, commit, stats);
    _mi_warning_message("unable to allocate aligned OS memory directly, fall back to over-allocation (%zu bytes, address: %p, alignment: %zu, commit: %d)\n", size, p, alignment, commit);
    if (size >= (SIZE_MAX - alignment)) return NULL; // overflow
    const size_t over_size = size + alignment;

#if _WIN32
    // over-allocate uncommitted (virtual) memory
    p = mi_os_mem_alloc(over_size, 0 /*alignment*/, false /* commit? */, false /* allow_large */, is_large, stats);
    if (p == NULL) return NULL;

    // set p to the aligned part in the full region
    // note: this is dangerous on Windows as VirtualFree needs the actual region pointer
    // but in mi_os_mem_free we handle this (hopefully exceptional) situation.
    p = mi_align_up_ptr(p, alignment);

    // explicitly commit only the aligned part
    if (commit) {
      _mi_os_commit(p, size, NULL, stats);
    }
#else
    // overallocate...
    p = mi_os_mem_alloc(over_size, 1, commit, false, is_large, stats);
    if (p == NULL) return NULL;
    // and selectively unmap parts around the over-allocated area. (noop on sbrk)
    void* aligned_p = mi_align_up_ptr(p, alignment);
    size_t pre_size = (uint8_t*)aligned_p - (uint8_t*)p;
    size_t mid_size = _mi_align_up(size, _mi_os_page_size());
    size_t post_size = over_size - pre_size - mid_size;
    mi_assert_internal(pre_size < over_size && post_size < over_size && mid_size >= size);
    if (pre_size > 0)  mi_os_mem_free(p, pre_size, commit, stats);
    if (post_size > 0) mi_os_mem_free((uint8_t*)aligned_p + mid_size, post_size, commit, stats);
    // we can return the aligned pointer on `mmap` (and sbrk) systems
    p = aligned_p;
#endif
  }

  mi_assert_internal(p == NULL || (p != NULL && ((uintptr_t)p % alignment) == 0));
  return p;
}


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