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.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2019-05-08  Tom Tromey  <tromey@adacore.com>
+
+	* typeprint.c (print_offset_data::update): Print the bit offset,
+	not the number of bits remaining.
+
 2019-05-08  Tom Tromey  <tromey@adacore.com>
 
 	* typeprint.c (print_offset_data::maybe_print_hole): Add extra

gdb/doc/ChangeLog

@@ -1,3 +1,7 @@
+2019-05-08  Tom Tromey  <tromey@adacore.com>
+
+	* gdb.texinfo (Symbols): Document change to ptype/o.
+
 2019-05-03  Eli Zaretskii  <eliz@gnu.org>
 
 	* gdb.texinfo (Separate Debug Files): Document how the

gdb/doc/gdb.texinfo

@@ -17861,15 +17861,16 @@ bitfields:
 /* XXX  3-bit hole   */
 /* XXX  4-byte hole  */
 /*    8      |     8 */    int64_t a5;
-/*   16:27   |     4 */    int a6 : 5;
-/*   16:56   |     8 */    int64_t a7 : 3;
+/*   16: 0   |     4 */    int a6 : 5;
+/*   16: 5   |     8 */    int64_t a7 : 3;
+"/* XXX  7-byte padding  */
 
                            /* total size (bytes):   24 */
                          @}
 @end smallexample
 
-Note how the offset information is now extended to also include how
-many bits are left to be used in each bitfield.
+Note how the offset information is now extended to also include the
+first bit of the bitfield.
 @end table
 
 @kindex ptype

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2019-05-08  Tom Tromey  <tromey@adacore.com>
+
+	* gdb.base/ptype-offsets.exp: Update tests.
+
 2019-05-08  Tom Tromey  <tromey@adacore.com>
 
 	* gdb.base/ptype-offsets.exp: Use string_to_regexp.  Fix test

gdb/testsuite/gdb.base/ptype-offsets.exp

@@ -38,7 +38,7 @@ gdb_test "ptype /o struct abc" \
 "/* offset    |  size */  type = struct abc \{" \
 "                         public:" \
 "/*    8      |     8 */    void *field1;" \
-"/*   16:31   |     4 */    unsigned int field2 : 1;" \
+"/*   16: 0   |     4 */    unsigned int field2 : 1;" \
 "/* XXX  7-bit hole   */" \
 "/* XXX  3-byte hole  */" \
 "/*   20      |     4 */    int field3;" \
@@ -63,7 +63,7 @@ gdb_test "ptype /oTM struct abc" \
 "/* offset    |  size */  type = struct abc \{" \
 "                         public:" \
 "/*    8      |     8 */    void *field1;" \
-"/*   16:31   |     4 */    unsigned int field2 : 1;" \
+"/*   16: 0   |     4 */    unsigned int field2 : 1;" \
 "/* XXX  7-bit hole   */" \
 "/* XXX  3-byte hole  */" \
 "/*   20      |     4 */    int field3;" \
@@ -93,7 +93,7 @@ gdb_test "ptype /TMo struct abc" \
 "/* offset    |  size */  type = struct abc \{" \
 "                         public:" \
 "/*    8      |     8 */    void *field1;" \
-"/*   16:31   |     4 */    unsigned int field2 : 1;" \
+"/*   16: 0   |     4 */    unsigned int field2 : 1;" \
 "/* XXX  7-bit hole   */" \
 "/* XXX  3-byte hole  */" \
 "/*   20      |     4 */    int field3;" \
@@ -264,15 +264,15 @@ gdb_test "ptype /o struct poi" \
 gdb_test "ptype /o struct tyu" \
     [string_to_regexp [multi_line \
 "/* offset    |  size */  type = struct tyu \{" \
-"/*    0:31   |     4 */    int a1 : 1;" \
-"/*    0:28   |     4 */    int a2 : 3;" \
-"/*    0: 5   |     4 */    int a3 : 23;" \
+"/*    0: 0   |     4 */    int a1 : 1;" \
+"/*    0: 1   |     4 */    int a2 : 3;" \
+"/*    0: 4   |     4 */    int a3 : 23;" \
 "/*    3: 3   |     1 */    signed char a4 : 2;" \
 "/* XXX  3-bit hole   */" \
 "/* XXX  4-byte hole  */" \
 "/*    8      |     8 */    int64_t a5;" \
-"/*   16:27   |     4 */    int a6 : 5;" \
-"/*   16:56   |     8 */    int64_t a7 : 3;" \
+"/*   16: 0   |     4 */    int a6 : 5;" \
+"/*   16: 5   |     8 */    int64_t a7 : 3;" \
 "/* XXX  7-byte padding  */" \
 "" \
 "                           /* total size (bytes):   24 */" \
@@ -293,8 +293,8 @@ gdb_test "ptype /o struct asd" \
 "" \
 "                                   /* total size (bytes):    8 */" \
 "                               \} f3;" \
-"/*   24:27   |     4 */        int f4 : 5;" \
-"/*   24:26   |     4 */        unsigned int f5 : 1;" \
+"/*   24: 0   |     4 */        int f4 : 5;" \
+"/*   24: 5   |     4 */        unsigned int f5 : 1;" \
 "/* XXX  2-bit hole   */" \
 "/* XXX  1-byte hole  */" \
 "/*   26      |     2 */        short f6;" \
@@ -304,11 +304,11 @@ gdb_test "ptype /o struct asd" \
 "                           \} f7;" \
 "/*   32      |     8 */    unsigned long f8;" \
 "/*   40      |     8 */    signed char *f9;" \
-"/*   48:28   |     4 */    int f10 : 4;" \
-"/*   48:27   |     4 */    unsigned int f11 : 1;" \
-"/*   48:26   |     4 */    unsigned int f12 : 1;" \
-"/*   48:25   |     4 */    unsigned int f13 : 1;" \
-"/*   48:24   |     4 */    unsigned int f14 : 1;" \
+"/*   48: 0   |     4 */    int f10 : 4;" \
+"/*   48: 4   |     4 */    unsigned int f11 : 1;" \
+"/*   48: 5   |     4 */    unsigned int f12 : 1;" \
+"/*   48: 6   |     4 */    unsigned int f13 : 1;" \
+"/*   48: 7   |     4 */    unsigned int f14 : 1;" \
 "/* XXX  7-byte hole  */" \
 "/*   56      |     8 */    void *f15;" \
 "/*   64      |     8 */    void *f16;" \

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;
+      /* We're dealing with a bitfield.  Print the bit offset.  */
+      fieldsize_bit = TYPE_FIELD_BITSIZE (type, field_idx);
 
-      /* 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;
+      unsigned real_bitpos = bitpos + offset_bitpos;
 
-      /* This is the exact offset (in bits) of this bitfield.  */
-      unsigned int bit_offset
-	= (bitpos - relative_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
     {