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Types with power-of-two size in ggc-page.c extra_order_size_table[]


Looking at ggc-page.c:175ff.,

static const size_t extra_order_size_table[] = {
  sizeof (struct var_ann_d),
  sizeof (struct tree_decl_non_common),
  sizeof (struct tree_field_decl),
  sizeof (struct tree_parm_decl),
  sizeof (struct tree_var_decl),
  sizeof (struct tree_list),
  sizeof (struct tree_ssa_name),
  sizeof (struct function),
  sizeof (struct basic_block_def),
  sizeof (bitmap_element),
  sizeof (bitmap_head),
  RTL_SIZE (2),			/* MEM, PLUS, etc.  */
  RTL_SIZE (9),			/* INSN */

On my openSuSE-11.1 i686-pc-linux-gnu, sizeof (struct basic_block_def)
is 64 and sizeof (bitmap_head) is 16.
Why are these types entered into extra_order_size_table?

ggc-page.c (init_ggc):1567ff.
  for (order = HOST_BITS_PER_PTR; order < NUM_ORDERS; ++order)
      int o;
      int i;

      i = OBJECT_SIZE (order);
      if (i >= NUM_SIZE_LOOKUP)

      for (o = size_lookup[i]; o == size_lookup [i]; --i)
        size_lookup[i] = order;

At order 42, OBJECT_SIZE returns 16 (from extra_order_size_table[10])
and size_lookup[16] down to [9] is changed to 42.
That seems strange to me, given the documentation of size_lookup[],

/* This table provides a fast way to determine ceil(log_2(size)) for
   allocation requests.  The minimum allocation size is eight bytes.  */



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