On Wed, Feb 28, 2018 at 06:22:33PM +0000, Eric Wong wrote:
> Duy Nguyen <[email protected]> wrote:
> > which saves 12 bytes (or another 74 MB). 222 MB total is plenty of
> > space to keep some file cache from being evicted.
>
> Nice! I can definitely benefit from lower memory usage when
> packing. Fwiw, I use pahole with other projects to help find
> packing opportunities:
>
> git://git.kernel.org/pub/scm/devel/pahole/pahole.git
Yes it's a wonderful tool.
> > @@ -14,11 +26,10 @@ struct object_entry {
> > void *delta_data; /* cached delta (uncompressed) */
> > unsigned long delta_size; /* delta data size (uncompressed) */
> > unsigned long z_delta_size; /* delta data size (compressed) */
> > - enum object_type type;
> > - enum object_type in_pack_type; /* could be delta */
> > uint32_t hash; /* name hint hash */
> > - unsigned int in_pack_pos;
> > unsigned char in_pack_header_size;
> > + unsigned type:3; /* enum object_type */
> > + unsigned in_pack_type:3; /* enum object_type - could be delta */
>
> For C99 compilers, enums can be bitfields. I introduced the
> following macro into Ruby a few weeks ago to remain compatible
> with non-C99 compilers:
>
> /*
> * For declaring bitfields out of non-unsigned int types:
> * struct date {
> * BITFIELD(enum months) month:4;
> * ...
> * };
> */
> #if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
> # define BITFIELD(type) type
> #else
> # define BITFIELD(type) unsigned int
> #endif
I tried this and got
In file included from builtin/pack-objects.c:20:0:
./pack-objects.h:49:19: l?i: ?type? is narrower than values of its type
[-Werror]
enum object_type type:TYPE_BITS;
^~~~
The compiler is not wrong. What it does not realize is pack-objects
code never uses out-of-range values (OBJ_BAD and OBJ_ANY) but I don't
see how I could suppress this warning. So I went back to non-enum
bitfields.
--
Duy
