[m-rev.] diff: improve simplify_info

[Peter Wang]

On Wed, 08 Mar 2017 08:14:15 +1100 (AEDT), "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
>  
>  :- type simplify_info
>      --->    simplify_info(
> -                % The most frequently used fields.
> -                % XXX Move another field here from simplify_sub_info
> -                % to make eight. Boehm will use eight words even for
> -                % seven fields.
> +                % The most frequently used writeable fields, held at eight
> +                % fields (adding even just one more field would cause Boehm
> +                % to allocate a 16 word cell).

Please delete this comment, and other similar comments in the compiler
directory.  They are incorrect.

You can check Boehm GC's size rounding for small objects by printing
the GC_size_map with GC_mercury_write_size_map().  Here are the results
on x86-64.  The i'th number in the list gives the allocation size in
words for a request of i words.  (The zeros are yet to be populated.)

      2    2    4    4    6    6    8    8
     10   10   12   12   14   14   16   16
     18   18   20   20   22   22   24   24
     26   26   28   28   30   30   32   32
     34   34   36   36   38   38   40   40
     42   42   44   44   46   46   48   48
      0    0    0    0    0    0    0    0
      0    0    0    0    0    0    0    0
      0    0    0    0    0    0    0    0
      0    0    0    0    0    0    0    0
      0    0    0    0    0    0    0    0
      0    0    0    0    0    0    0    0
    128  128  128  128  128  128  128  128
    128  128  128  128  128  128  128  128
    128  128  128  128  128  128  128  128
    128  128  128  128  128  128  128  128

Peter