Function mi_os_get_aligned_hint

3rd/mimalloc-2.0.9/src/os.c:322–344

#define MI_HINT_MAX  ((uintptr_t)30 << 40) // wrap after 30TiB (area after 32TiB is used for huge OS pages)

static void* mi_os_get_aligned_hint(size_t try_alignment, size_t size)
{
  if (try_alignment <= 1 || try_alignment > MI_SEGMENT_SIZE) return NULL;
  size = _mi_align_up(size, MI_SEGMENT_SIZE);
  if (size > 1*MI_GiB) return NULL;  // guarantee the chance of fixed valid address is at most 1/(MI_HINT_AREA / 1<<30) = 1/4096.
  #if (MI_SECURE>0)
  size += MI_SEGMENT_SIZE;        // put in `MI_SEGMENT_SIZE` virtual gaps between hinted blocks; this splits VLA's but increases guarded areas.
  #endif

  uintptr_t hint = mi_atomic_add_acq_rel(&aligned_base, size);
  if (hint == 0 || hint > MI_HINT_MAX) {   // wrap or initialize
    uintptr_t init = MI_HINT_BASE;
    #if (MI_SECURE>0 || MI_DEBUG==0)       // security: randomize start of aligned allocations unless in debug mode
    uintptr_t r = _mi_heap_random_next(mi_get_default_heap());
    init = init + ((MI_SEGMENT_SIZE * ((r>>17) & 0xFFFFF)) % MI_HINT_AREA);  // (randomly 20 bits)*4MiB == 0 to 4TiB
    #endif
    uintptr_t expected = hint + size;
    mi_atomic_cas_strong_acq_rel(&aligned_base, &expected, init);
    hint = mi_atomic_add_acq_rel(&aligned_base, size); // this may still give 0 or > MI_HINT_MAX but that is ok, it is a hint after all
  }
  if (hint%try_alignment != 0) return NULL;
  return (void*)hint;
}
#else
static void* mi_os_get_aligned_hint(size_t try_alignment, size_t size) {
  MI_UNUSED(try_alignment); MI_UNUSED(size);