[PAHOLE]: First stab at implementing --packable

That is to find structs that have combinable holes, trying to pack the struct
by suggesting a move, for now it just prints structs that have holes that can
be combined, but these hints are not guaranteed to generate struct size
reductions, more has to be done and that involves understanding the alignment
rules that depend on the arch being 32 or 64 bits, but it at least reduces the
number of packing candidates.

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

pahole.c

@@ -27,6 +27,14 @@ static size_t decl_exclude_prefix_len;
 static unsigned short nr_holes;
 static unsigned short nr_bit_holes;
 
+static enum {
+	FLAG_show_sizes			= (1 << 0),
+	FLAG_show_nr_members		= (1 << 1),
+	FLAG_show_class_name_len	= (1 << 2),
+	FLAG_show_total_structure_stats = (1 << 3),
+	FLAG_show_packable		= (1 << 4),
+} opts;
+
 struct structure {
 	struct list_head   node;
 	const struct class *class;
@@ -98,6 +106,24 @@ static struct cu *cu__filter(struct cu *cu)
 	return cu;
 }
 
+static int class__packable(const struct class *self)
+{
+	struct class_member *pos;
+
+	if (self->nr_holes == 0 && self->nr_bit_holes == 0)
+		return 0;
+
+	list_for_each_entry(pos, &self->members, tag.node)
+		if (pos->hole != 0 &&
+		    class__find_bit_hole(self, pos, pos->hole * 8) != NULL)
+			return 1;
+		else if (pos->bit_hole != 0 &&
+			 class__find_bit_hole(self, pos, pos->bit_hole) != NULL)
+			return 1;
+
+	return 0;
+}
+
 static struct class *class__to_struct(struct class *class)
 {
 	struct class *typedef_alias;
@@ -127,6 +153,11 @@ static struct class *class__filter(struct class *class)
 		     decl_exclude_prefix_len) == 0))
 		return NULL;
 
+	class__find_holes(class);
+
+	if ((opts & FLAG_show_packable) && !class__packable(class))
+		return NULL;
+
 	return class;
 }
 
@@ -154,6 +185,7 @@ static struct option long_options[] = {
 	{ "exclude",		required_argument,	NULL, 'x' },
 	{ "cu_exclude",		required_argument,	NULL, 'X' },
 	{ "decl_exclude",	required_argument,	NULL, 'D' },
+	{ "packable",		no_argument,		NULL, 'p' },
 	{ NULL, 0, NULL, 0, }
 };
 
@@ -165,6 +197,7 @@ static void usage(void)
 		"   -h, --help                   show usage info\n"
 		"   -B, --bit_holes <nr_holes>   show only structs at least <nr_holes> bit holes\n"
 		"   -H, --holes <nr_holes>       show only structs at least <nr_holes> holes\n"
+		"   -p, --packable		 show only structs that has holes that can be packed\n"
 		"   -c, --cacheline_size <size>  set cacheline size (default=%d)\n"
 		"   -n, --nr_members             show number of members\n"
 		"   -N, --class_name_len         show size of classes\n"
@@ -191,11 +224,9 @@ static int cu_nr_members_iterator(struct cu *cu, void *cookie)
 
 static int sizes_iterator(struct class *class, void *cookie)
 {
-	if (class->size > 0) {
-		class__find_holes(class);
+	if (class->size > 0)
 		printf("%s: %llu %u\n",
 		       class->name, class->size, class->nr_holes);
-	}
 	return 0;
 }
 
@@ -206,7 +237,6 @@ static int cu_sizes_iterator(struct cu *cu, void *cookie)
 
 static int holes_iterator(struct class *class, void *cookie)
 {
-	class__find_holes(class);
 	if ((nr_holes != 0 && class->nr_holes >= nr_holes) ||
 	    (nr_bit_holes != 0 && class->nr_bit_holes >= nr_bit_holes))
 		class__print(class);
@@ -232,7 +262,6 @@ static int cu_class_name_len_iterator(struct cu *cu, void *cookie)
 
 static int class__iterator(struct class *class, void *cookie)
 {
-	class__find_holes(class);
 	class__print(class);
 	return 0;
 }
@@ -249,15 +278,8 @@ int main(int argc, char *argv[])
 	struct cus *cus;
 	char *file_name;
 	char *class_name = NULL;
-	unsigned opts = 0;
-	enum {
-		FLAG_show_sizes = 1,
-		FLAG_show_nr_members,
-		FLAG_show_class_name_len,
-		FLAG_show_total_structure_stats
-	};
 
-	while ((option = getopt_long(argc, argv, "B:c:D:hH:nNstx:X:",
+	while ((option = getopt_long(argc, argv, "B:c:D:hH:nNpstx:X:",
 				     long_options, &option_index)) >= 0)
 		switch (option) {
 		case 'c': cacheline_size = atoi(optarg);  break;
@@ -266,6 +288,7 @@ int main(int argc, char *argv[])
 		case 's': opts |= FLAG_show_sizes;	  break;
 		case 'n': opts |= FLAG_show_nr_members;	  break;
 		case 'N': opts |= FLAG_show_class_name_len;  break;
+		case 'p': opts |= FLAG_show_packable;	  break;
 		case 't': opts |= FLAG_show_total_structure_stats; break;
 		case 'D': decl_exclude_prefix = optarg;
 			  decl_exclude_prefix_len = strlen(decl_exclude_prefix);