fprintf: Print number of bit holes in class member types

In addition to paddings (holes at the end of a struct) and byte holes,
inform about bit holes in bitfields in the middle of the structs of
non-pointer members.

For instance, for 'struct task_struct' in the Linux kernel we get this
extra info:

  --- task_struct.before.c        2024-02-09 16:19:34.221134835 -0300
  +++ task_struct.after.c 2024-02-09 17:45:41.137097839 -0300
  @@ -35,6 +35,9 @@
          /* --- cacheline 6 boundary (384 bytes) --- */
          struct sched_rt_entity     rt;                   /*   384    48 */
          struct sched_dl_entity     dl;                   /*   432   224 */
  +
  +       /* XXX last struct has 1 bit hole */
  +
          /* --- cacheline 10 boundary (640 bytes) was 16 bytes ago --- */
          const struct sched_class  * sched_class;         /*   656     8 */
          struct rb_node             core_node;            /*   664    24 */
  @@ -405,12 +408,12 @@
          /* --- cacheline 145 boundary (9280 bytes) --- */
          struct thread_struct       thread __attribute__((__aligned__(64))); /*  9280  4416 */

  -       /* XXX last struct has 1 hole */
  +       /* XXX last struct has 1 hole, 1 bit hole */

          /* size: 13696, cachelines: 214, members: 262 */
          /* sum members: 13518, holes: 21, sum holes: 162 */
          /* sum bitfield members: 82 bits, bit holes: 2, sum bit holes: 46 bits */
  -       /* member types with holes: 4, total of those holes: 6 */
  +       /* member types with holes: 4, total: 6, bit holes: 2, total: 2 */
          /* paddings: 6, sum paddings: 49 */
          /* forced alignments: 2, forced holes: 2, sum forced holes: 88 */
   };

Out of curiosity, lets try one taking advantage that most if not all
distros these days comes with /sys/kernel/btf/vmlinux so that we can ask
just for the type, without specifying where to find it:

  $ pahole sched_dl_entity
  struct sched_dl_entity {
  	struct rb_node             rb_node;              /*     0    24 */
  	u64                        dl_runtime;           /*    24     8 */
  	u64                        dl_deadline;          /*    32     8 */
  	u64                        dl_period;            /*    40     8 */
  	u64                        dl_bw;                /*    48     8 */
  	u64                        dl_density;           /*    56     8 */
  	/* --- cacheline 1 boundary (64 bytes) --- */
  	s64                        runtime;              /*    64     8 */
  	u64                        deadline;             /*    72     8 */
  	unsigned int               flags;                /*    80     4 */
  	unsigned int               dl_throttled:1;       /*    84: 0  4 */
  	unsigned int               dl_yielded:1;         /*    84: 1  4 */
  	unsigned int               dl_non_contending:1;  /*    84: 2  4 */
  	unsigned int               dl_overrun:1;         /*    84: 3  4 */

  	/* XXX 28 bits hole, try to pack */

  	struct hrtimer             dl_timer;             /*    88    64 */

  	/* XXX last struct has 4 bytes of padding */

  	/* --- cacheline 2 boundary (128 bytes) was 24 bytes ago --- */
  	struct hrtimer             inactive_timer;       /*   152    64 */

  	/* XXX last struct has 4 bytes of padding */

  	/* --- cacheline 3 boundary (192 bytes) was 24 bytes ago --- */
  	struct sched_dl_entity *   pi_se;                /*   216     8 */

  	/* size: 224, cachelines: 4, members: 16 */
  	/* sum members: 220 */
  	/* sum bitfield members: 4 bits, bit holes: 1, sum bit holes: 28 bits */
  	/* paddings: 2, sum paddings: 8 */
  	/* last cacheline: 32 bytes */
  };

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

1 files changed, 28 insertions, 6 deletions

diff --git a/dwarves_fprintf.c b/dwarves_fprintf.c
index 1169d359..2fa489a4 100644
--- a/dwarves_fprintf.c
+++ b/dwarves_fprintf.c
@@ -1465,7 +1465,9 @@ out:
 struct member_types_holes {
 	uint16_t nr_paddings;
 	uint16_t nr_with_holes;
+	uint16_t nr_with_bit_holes;
 	uint16_t total_nr_holes;
+	uint16_t total_nr_bit_holes;
 	uint32_t sum_paddings;
 };
 
@@ -1475,7 +1477,7 @@ static size_t class__fprintf_member_type_holes(struct class *class, const struct
 {
 	size_t printed = 0;
 	uint16_t padding;
-	uint8_t nr_holes;
+	uint8_t nr_holes, nr_bit_holes;
 	bool first = true;
 	/*
 	 * We may not yet have looked for holes and paddings in this member's
@@ -1486,8 +1488,9 @@ static size_t class__fprintf_member_type_holes(struct class *class, const struct
 
 	padding = class->padding;
 	nr_holes = class->nr_holes;
+	nr_bit_holes = class->nr_bit_holes;
 
-	if (!padding && !nr_holes)
+	if (!padding && !nr_holes && !nr_bit_holes)
 		return 0;
 
 	if (!(*newline)++) {
@@ -1510,6 +1513,14 @@ static size_t class__fprintf_member_type_holes(struct class *class, const struct
 		holes->total_nr_holes += nr_holes;
 
 		printed += fprintf(fp, "%s %d hole%s", first ? "" : ",", nr_holes, nr_holes != 1 ? "s" : "");
+		first = false;
+	}
+
+	if (nr_bit_holes) {
+		++holes->nr_with_bit_holes;
+		holes->total_nr_bit_holes += nr_bit_holes;
+
+		printed += fprintf(fp, "%s %d bit hole%s", first ? "" : ",", nr_bit_holes, nr_bit_holes != 1 ? "s" : "");
 	}
 
 	return printed + fprintf(fp, " */");
@@ -1873,10 +1884,21 @@ static size_t __class__fprintf(struct class *class, const struct cu *cu,
 		printed += fprintf(fp, "%.*s/* padding: %u */\n",
 				   cconf.indent,
 				   tabs, class->padding);
-	if (member_types_holes.nr_with_holes > 0) {
-		printed += fprintf(fp, "%.*s/* member types with holes: %u, total of those holes: %u */\n",
-				   cconf.indent, tabs,
-				   member_types_holes.nr_with_holes, member_types_holes.total_nr_holes);
+
+	if (member_types_holes.nr_with_holes > 0 || member_types_holes.nr_with_bit_holes > 0) {
+		bool first = true;
+
+		printed += fprintf(fp, "%.*s/* member types with ", cconf.indent, tabs);
+		if (member_types_holes.nr_with_holes > 0) {
+			printed += fprintf(fp, "holes: %u, total: %u",
+					   member_types_holes.nr_with_holes, member_types_holes.total_nr_holes);
+			first = false;
+		}
+		if (member_types_holes.nr_with_bit_holes > 0) {
+			printed += fprintf(fp, "%sbit holes: %u, total: %u", first ? "" : ", ",
+					   member_types_holes.nr_with_bit_holes, member_types_holes.total_nr_bit_holes);
+		}
+		printed += fprintf(fp, " */\n");
 	}
 	if (member_types_holes.nr_paddings > 0)
 		printed += fprintf(fp, "%.*s/* paddings: %u, sum paddings: "