Change ptype/o to print bit offset

Consider this short C example:

    struct inner
    {
      unsigned x;
      unsigned y : 3;
      unsigned z : 3;
    };

    struct outer
    {
      unsigned char o : 3;
      struct inner i __attribute__ ((packed));
    };

When I use "ptype/o" on this, I get:

    (gdb) ptype/o struct outer
    /* offset    |  size */  type = struct outer {
    /*    0: 5   |     1 */    unsigned char o : 3;
    /* XXX  5-bit hole  */
    /*    1      |     8 */    struct inner {
    /*    1      |     4 */        unsigned int x;
    /*    5:29   |     4 */        unsigned int y : 3;
    /*    5:26   |     4 */        unsigned int z : 3;
    /* XXX  2-bit padding  */
    /* XXX  3-byte padding */

				   /* total size (bytes):    8 */
			       } i;

			       /* total size (bytes):    9 */
			     }

In the location of "o" ("0: 5"), the "5" means "there are 5 bits left
relative to the size of the underlying type.

I find this very difficult to follow.  On irc, Sergio said that this
choice came because it is what pahole does.  However, I think it's not
very useful, and maybe is just an artifact of the way that
DW_AT_bit_offset was defined in DWARF 3.

This patch changes ptype/o to print the offset of a bitfield in a more
natural way, that is, using the bit number according to the platform's
bit numbering.

With this patch, the output is now:

    (gdb) ptype/o struct outer
    /* offset    |  size */  type = struct outer {
    /*    0: 0   |     1 */    unsigned char o : 3;
    /* XXX  5-bit hole  */
    /*    1      |     8 */    struct inner {
    /*    1      |     4 */        unsigned int x;
    /*    5: 0   |     4 */        unsigned int y : 3;
    /*    5: 3   |     4 */        unsigned int z : 3;
    /* XXX  2-bit padding  */
    /* XXX  3-byte padding */

				   /* total size (bytes):    8 */
			       } i;

			       /* total size (bytes):    9 */
			     }

This is better, IMO, because now the "offset" of a bitfield is
consistent with the offset of an ordinary member, referring to its
offset from the start of the structure.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tromey@adacore.com>

	* typeprint.c (print_offset_data::update): Print the bit offset,
	not the number of bits remaining.

gdb/doc/ChangeLog
2019-05-08  Tom Tromey  <tromey@adacore.com>

	* gdb.texinfo (Symbols): Document change to ptype/o.

gdb/testsuite/ChangeLog
2019-05-08  Tom Tromey  <tromey@adacore.com>

	* gdb.base/ptype-offsets.exp: Update tests.

1 files changed, 7 insertions, 23 deletions

diff --git a/gdb/typeprint.c b/gdb/typeprint.c
index d1cdfe11cc0..7a44dbdb313 100644
--- a/gdb/typeprint.c
+++ b/gdb/typeprint.c
@@ -131,32 +131,16 @@ print_offset_data::update (struct type *type, unsigned int field_idx,
 
   maybe_print_hole (stream, bitpos, "hole");
 
-  if (TYPE_FIELD_PACKED (type, field_idx))
+  if (TYPE_FIELD_PACKED (type, field_idx)
+      || offset_bitpos % TARGET_CHAR_BIT != 0)
     {
-      /* We're dealing with a bitfield.  Print how many bits are left
-	 to be used.  */
-      unsigned int bitsize = TYPE_FIELD_BITSIZE (type, field_idx);
-      /* The bitpos relative to the beginning of our container
-	 field.  */
-      unsigned int relative_bitpos;
-
-      /* The following was copied from
-	 value.c:value_primitive_field.  */
-      if ((bitpos % fieldsize_bit) + bitsize <= fieldsize_bit)
-	relative_bitpos = bitpos % fieldsize_bit;
-      else
-	relative_bitpos = bitpos % TARGET_CHAR_BIT;
+      /* We're dealing with a bitfield.  Print the bit offset.  */
+      fieldsize_bit = TYPE_FIELD_BITSIZE (type, field_idx);
 
-      /* This is the exact offset (in bits) of this bitfield.  */
-      unsigned int bit_offset
-	= (bitpos - relative_bitpos) + offset_bitpos;
+      unsigned real_bitpos = bitpos + offset_bitpos;
 
-      /* The position of the field, relative to the beginning of the
-	 struct, and how many bits are left to be used in this
-	 container.  */
-      fprintf_filtered (stream, "/* %4u:%2u", bit_offset / TARGET_CHAR_BIT,
-			fieldsize_bit - (relative_bitpos + bitsize));
-      fieldsize_bit = bitsize;
+      fprintf_filtered (stream, "/* %4u:%2u", real_bitpos / TARGET_CHAR_BIT,
+			real_bitpos % TARGET_CHAR_BIT);
     }
   else
     {