Let struct dump_config in objspace fit in a single cache line

[methodmissing]

Another small packed struct change - gets passed around quite a bit in the dump_append* helper methods

Before:

lourens@CarbonX1:~/src/ruby/trunk$ pahole -C dump_config -P ./.ext/x86_64-linux/objspace.so
struct dump_config {
	VALUE                      type;                 /*     0     8 */
	FILE *                     stream;               /*     8     8 */
	VALUE                      string;               /*    16     8 */
	int                        roots;                /*    24     4 */

	/* XXX 4 bytes hole, try to pack */

	const char  *              root_category;        /*    32     8 */
	VALUE                      cur_obj;              /*    40     8 */
	VALUE                      cur_obj_klass;        /*    48     8 */
	size_t                     cur_obj_references;   /*    56     8 */
	/* --- cacheline 1 boundary (64 bytes) --- */
	int                        full_heap;            /*    64     4 */

	/* size: 72, cachelines: 2, members: 9 */
	/* sum members: 64, holes: 1, sum holes: 4 */
	/* padding: 4 */
	/* last cacheline: 8 bytes */
};

After:

lourens@CarbonX1:~/src/ruby/ruby$ pahole -C dump_config -P ./.ext/x86_64-linux/objspace.so
struct dump_config {
	VALUE                      type;                 /*     0     8 */
	FILE *                     stream;               /*     8     8 */
	VALUE                      string;               /*    16     8 */
	int                        roots;                /*    24     4 */
	int                        full_heap;            /*    28     4 */
	const char  *              root_category;        /*    32     8 */
	VALUE                      cur_obj;              /*    40     8 */
	VALUE                      cur_obj_klass;        /*    48     8 */
	size_t                     cur_obj_references;   /*    56     8 */

	/* size: 64, cachelines: 1, members: 9 */
};

[nobu]

They are used as just booleans, and could be bit flags.

[methodmissing]

True, good point - I can change it a bit later today to bit flags.

[methodmissing]

With bit flags for the booleans in a0e50c3 - introduced the flags member to namespace bit flags as per convention in gc.c and other source files :

struct dump_config {
	VALUE                      type;                 /*     0     8 */
	FILE *                     stream;               /*     8     8 */
	VALUE                      string;               /*    16     8 */
	const char  *              root_category;        /*    24     8 */
	VALUE                      cur_obj;              /*    32     8 */
	VALUE                      cur_obj_klass;        /*    40     8 */
	size_t                     cur_obj_references;   /*    48     8 */
	struct {
		unsigned int       roots:1;              /*    56: 0  4 */
		unsigned int       full_heap:1;          /*    56: 1  4 */
	} flags;                                         /*    56     4 */

	/* size: 64, cachelines: 1, members: 8 */
	/* padding: 4 */
};

Still padded to a cacheline, but cleaner pattern. Thanks for the suggestion.

[nobu]

Nested struct for flags is not a common convention, but just some code use it to isolate from bit struct.
So you may place the flags at the same level as the other members, if you prefer.

[methodmissing]

Thanks for the context and explanation again - indifferent for me - what do you prefer as committer?

[nobu]

I think this struct is small enough.

[methodmissing]

OK to merge now without further changes?