[PATCH v5 2/2] Implement pahole-like 'ptype /o' option

Sergio Durigan Junior sergiodj@redhat.com
Wed Dec 13 03:17:00 GMT 2017


This commit implements the pahole-like '/o' option for 'ptype', which
prints the offsets and sizes of struct fields, reporting whenever
there is a hole found.

The output is heavily based on pahole(1), with a few modifications
here and there to adjust it to our reality.  Here's an example:

  (gdb) ptype /o stap_probe
  /* offset    |  size */
  struct stap_probe {
  /*    0      |    40 */    struct probe {
  /*    0      |     8 */        const probe_ops *pops;
  /*    8      |     8 */        gdbarch *arch;
  /*   16      |     8 */        const char *name;
  /*   24      |     8 */        const char *provider;
  /*   32      |     8 */        CORE_ADDR address;
			     } /* total size:   40 bytes */ p;
  /*   40      |     8 */    CORE_ADDR sem_addr;
  /*   48:31   |     4 */    unsigned int args_parsed : 1;
  /* XXX  7-bit hole   */
  /* XXX  7-byte hole  */
  /*   56      |     8 */    union {
  /*                 8 */        const char *text;
  /*                 8 */        VEC_stap_probe_arg_s *vec;
			     } /* total size:    8 bytes */ args_u;
  } /* total size:   64 bytes */

A big part of this patch handles the formatting logic of 'ptype',
which is a bit messy.  I tried to be not very invasive, but I had to
do some cleanups here and there to make life easier.

This patch is the start of a long-term work I'll do to flush the local
patches we carry for Fedora GDB.  In this specific case, I'm aiming at
upstreaming the feature implemented by the 'pahole.py' script that is
shipped with Fedora GDB:

  <https://src.fedoraproject.org/rpms/gdb/blob/master/f/gdb-archer.patch#_311>

This has been regression-tested on the BuildBot.  There's a new
testcase for it, along with an update to the documentation.  I also
thought it was worth mentioning this feature in the NEWS file.

gdb/ChangeLog:
yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	PR cli/16224
	* NEWS (Changes since GDB 8.0): Mention new '/o' flag.
	* c-typeprint.c (OFFSET_SPC_LEN): New define.
	(print_spaces_filtered_with_print_options): New function.
	(output_access_specifier): Take new argument FLAGS.  Modify
	function to call 'print_spaces_filtered_with_print_options'.
	(c_print_type_vtable_offset_marker): New function.
	(c_print_type_union_field_offset): New function.
	(c_print_type_struct_field_offset): New function.
	(c_type_print_base_struct_union): Print offsets and sizes for
	struct/union/class fields.
	* typeprint.c (const struct type_print_options
	type_print_raw_options): Initialize 'print_offsets' and
	'offset_bitpos'.
	(static struct type_print_options default_ptype_flags):
	Likewise.
	(whatis_exp): Handle '/o' option.
	(_initialize_typeprint): Add '/o' flag to ptype's help.
	* typeprint.h (struct type_print_options) <print_offsets>: New
	field.
	<offset_bitpos>: Likewise.

gdb/testsuite/ChangeLog:
yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR cli/16224
	* gdb.base/ptype-offsets.cc: New file.
	* gdb.base/ptype-offsets.exp: New file.

gdb/doc/ChangeLog:
yyyy-mm-dd  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR cli/16224
	* gdb.texinfo (ptype): Add documentation for new flag '/o'.
---
 gdb/NEWS                                 |   3 +
 gdb/c-typeprint.c                        | 248 ++++++++++++++++++++++++++++---
 gdb/doc/gdb.texinfo                      | 115 ++++++++++++++
 gdb/testsuite/gdb.base/ptype-offsets.cc  | 158 ++++++++++++++++++++
 gdb/testsuite/gdb.base/ptype-offsets.exp | 192 ++++++++++++++++++++++++
 gdb/typeprint.c                          |  15 +-
 gdb/typeprint.h                          |   9 ++
 7 files changed, 719 insertions(+), 21 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/ptype-offsets.cc
 create mode 100644 gdb/testsuite/gdb.base/ptype-offsets.exp

diff --git a/gdb/NEWS b/gdb/NEWS
index c6fe297159..67d40d007b 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -3,6 +3,9 @@
 
 *** Changes since GDB 8.0
 
+* The 'ptype' command now accepts a '/o' flag, which prints the
+  offsets and sizes of fields in a struct, like the pahole(1) tool.
+
 * New "--readnever" command line option instructs GDB to not read each
   symbol file's symbolic debug information.  This makes startup faster
   but at the expense of not being able to perform symbolic debugging.
diff --git a/gdb/c-typeprint.c b/gdb/c-typeprint.c
index ce10b5e005..cd21cb2172 100644
--- a/gdb/c-typeprint.c
+++ b/gdb/c-typeprint.c
@@ -32,6 +32,14 @@
 #include "cp-abi.h"
 #include "cp-support.h"
 
+/* When printing the offsets of a struct and its fields (i.e., 'ptype
+   /o'; type_print_options::print_offsets), we use this many
+   characters when printing the offset information at the beginning of
+   the line.  This is needed in order to generate the correct amount
+   of whitespaces when no offset info should be printed for a certain
+   field.  */
+#define OFFSET_SPC_LEN 23
+
 /* A list of access specifiers used for printing.  */
 
 enum access_specifier
@@ -836,21 +844,36 @@ c_type_print_template_args (const struct type_print_options *flags,
     fputs_filtered (_("] "), stream);
 }
 
+/* Use 'print_spaces_filtered', but take into consideration the
+   type_print_options FLAGS in order to determine how many whitespaces
+   will be printed.  */
+
+static void
+print_spaces_filtered_with_print_options
+  (int level, struct ui_file *stream, const struct type_print_options *flags)
+{
+  if (!flags->print_offsets)
+    print_spaces_filtered (level, stream);
+  else
+    print_spaces_filtered (level + OFFSET_SPC_LEN, stream);
+}
+
 /* Output an access specifier to STREAM, if needed.  LAST_ACCESS is the
    last access specifier output (typically returned by this function).  */
 
 static enum access_specifier
 output_access_specifier (struct ui_file *stream,
 			 enum access_specifier last_access,
-			 int level, bool is_protected, bool is_private)
+			 int level, bool is_protected, bool is_private,
+			 const struct type_print_options *flags)
 {
   if (is_protected)
     {
       if (last_access != s_protected)
 	{
 	  last_access = s_protected;
-	  fprintfi_filtered (level + 2, stream,
-			     "protected:\n");
+	  print_spaces_filtered_with_print_options (level + 2, stream, flags);
+	  fprintf_filtered (stream, "protected:\n");
 	}
     }
   else if (is_private)
@@ -858,8 +881,8 @@ output_access_specifier (struct ui_file *stream,
       if (last_access != s_private)
 	{
 	  last_access = s_private;
-	  fprintfi_filtered (level + 2, stream,
-			     "private:\n");
+	  print_spaces_filtered_with_print_options (level + 2, stream, flags);
+	  fprintf_filtered (stream, "private:\n");
 	}
     }
   else
@@ -867,15 +890,132 @@ output_access_specifier (struct ui_file *stream,
       if (last_access != s_public)
 	{
 	  last_access = s_public;
-	  fprintfi_filtered (level + 2, stream,
-			     "public:\n");
+	  print_spaces_filtered_with_print_options (level + 2, stream, flags);
+	  fprintf_filtered (stream, "public:\n");
 	}
     }
 
   return last_access;
 }
 
-/* Return true is an access label (i.e., "public:", "private:",
+static void
+c_print_type_vtable_offset_marker (struct type *type, unsigned int field_idx,
+				   struct ui_file *stream,
+				   unsigned int offset_bitpos)
+{
+  unsigned int bitpos = TYPE_FIELD_BITPOS (type, field_idx);
+  unsigned int offset = (bitpos + offset_bitpos) / TARGET_CHAR_BIT;
+
+  fprintf_filtered (stream, "/* %4u     |   %4u */ /* vtable */\n", offset,
+		    TYPE_LENGTH (TYPE_FIELD_TYPE (type, field_idx)));
+}
+
+/* Print information about field at index FIELD_IDX of the union type
+   TYPE.  Since union fields don't have the concept of offsets, we
+   just print their sizes.
+
+   The output is strongly based on pahole(1).  */
+
+static void
+c_print_type_union_field_offset (struct type *type, unsigned int field_idx,
+				 struct ui_file *stream)
+{
+  struct type *ftype = check_typedef (TYPE_FIELD_TYPE (type, field_idx));
+
+  fprintf_filtered (stream, "/*              %4u */", TYPE_LENGTH (ftype));
+}
+
+/* Print information about field at index FIELD_IDX of the struct type
+   TYPE.
+
+   ENDPOS is the one-past-the-end bit position of the previous field
+   (where we expect this field to be if there is no hole).  At the
+   end, ENDPOS is updated to the one-past-the-end bit position of the
+   current field.
+
+   OFFSET_BITPOS is the offset value we carry over when we are
+   printing a struct that is inside another struct; this is useful so
+   that the offset is constantly incremented (if we didn't carry it
+   over, the offset would be reset to zero when printing the inner
+   struct).
+
+   The output is strongly based on pahole(1).  */
+
+static void
+c_print_type_struct_field_offset (struct type *type, unsigned int field_idx,
+				  unsigned int *endpos, struct ui_file *stream,
+				  unsigned int offset_bitpos)
+{
+  struct type *ftype = check_typedef (TYPE_FIELD_TYPE (type, field_idx));
+  unsigned int bitpos = TYPE_FIELD_BITPOS (type, field_idx);
+  unsigned int fieldsize_byte = TYPE_LENGTH (ftype);
+  unsigned int fieldsize_bit = fieldsize_byte * TARGET_CHAR_BIT;
+
+  /* We check for *ENDPOS > 0 because there is a specific scenario
+     when *ENDPOS can be zero and BITPOS can be > 0: when we are
+     dealing with a struct/class with a virtual method.  Because of
+     the vtable, the first field of the struct/class will have an
+     offset of sizeof (void *) (the size of the vtable).  If we do not
+     check for *ENDPOS > 0 here, GDB will report a hole before the
+     first field, which is not accurate.  */
+  if (*endpos > 0 && *endpos < bitpos)
+    {
+      /* If ENDPOS is smaller than the current type's bitpos, it means
+	 there's a hole in the struct, so we report it here.  */
+      unsigned int hole = bitpos - *endpos;
+      unsigned int hole_byte = hole / TARGET_CHAR_BIT;
+      unsigned int hole_bit = hole % TARGET_CHAR_BIT;
+
+      if (hole_bit > 0)
+	fprintf_filtered (stream, "/* XXX %2u-bit hole   */\n", hole_bit);
+
+      if (hole_byte > 0)
+	fprintf_filtered (stream, "/* XXX %2u-byte hole  */\n", hole_byte);
+    }
+
+  if (TYPE_FIELD_PACKED (type, field_idx))
+    {
+      /* 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;
+
+      /* 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;
+    }
+  else
+    {
+      /* The position of the field, relative to the beginning of the
+	 struct.  */
+      fprintf_filtered (stream, "/* %4u",
+			(bitpos + offset_bitpos) / TARGET_CHAR_BIT);
+
+      fprintf_filtered (stream, "   ");
+    }
+
+  fprintf_filtered (stream, "   |  %4u */", fieldsize_byte);
+
+  *endpos = bitpos + fieldsize_bit;
+}
+
+/* Return true if an access label (i.e., "public:", "private:",
    "protected:") needs to be printed for TYPE.  */
 
 static bool
@@ -1032,6 +1172,8 @@ c_type_print_base_struct_union (struct type *type, struct ui_file *stream,
 
       int len = TYPE_NFIELDS (type);
       vptr_fieldno = get_vptr_fieldno (type, &basetype);
+      unsigned int endpos = 0;
+
       for (int i = TYPE_N_BASECLASSES (type); i < len; i++)
 	{
 	  QUIT;
@@ -1041,25 +1183,63 @@ c_type_print_base_struct_union (struct type *type, struct ui_file *stream,
 	     the debug info, they should be artificial.  */
 	  if ((i == vptr_fieldno && type == basetype)
 	      || TYPE_FIELD_ARTIFICIAL (type, i))
-	    continue;
+	    {
+	      if (flags->print_offsets)
+		c_print_type_vtable_offset_marker (type, i, stream,
+						   flags->offset_bitpos);
+	      continue;
+	    }
 
 	  if (need_access_label)
 	    {
 	      section_type = output_access_specifier
 		(stream, section_type, level,
 		 TYPE_FIELD_PROTECTED (type, i),
-		 TYPE_FIELD_PRIVATE (type, i));
+		 TYPE_FIELD_PRIVATE (type, i), flags);
+	    }
+
+	  bool is_static = field_is_static (&TYPE_FIELD (type, i));
+
+	  if (flags->print_offsets)
+	    {
+	      if (!is_static)
+		{
+		  if (TYPE_CODE (type) == TYPE_CODE_STRUCT)
+		    c_print_type_struct_field_offset (type, i, &endpos, stream,
+						      flags->offset_bitpos);
+		  else if (TYPE_CODE (type) == TYPE_CODE_UNION)
+		    c_print_type_union_field_offset (type, i, stream);
+		}
+	      else
+		print_spaces_filtered (OFFSET_SPC_LEN, stream);
 	    }
 
 	  print_spaces_filtered (level + 4, stream);
-	  if (field_is_static (&TYPE_FIELD (type, i)))
+	  if (is_static)
 	    fprintf_filtered (stream, "static ");
+
+	  int newshow = show - 1;
+
+	  if (flags->print_offsets
+	      && (TYPE_CODE (TYPE_FIELD_TYPE (type, i)) == TYPE_CODE_STRUCT
+		  || TYPE_CODE (TYPE_FIELD_TYPE (type, i)) == TYPE_CODE_UNION))
+	    {
+	      /* If we're printing offsets and this field's type is
+		 either a struct or an union, then we're interested in
+		 expanding it.  */
+	      ++newshow;
+
+	      /* Make sure we carry our offset when we expand the
+		 struct.  */
+	      local_flags.offset_bitpos
+		= flags->offset_bitpos + TYPE_FIELD_BITPOS (type, i);
+	    }
+
 	  c_print_type (TYPE_FIELD_TYPE (type, i),
 			TYPE_FIELD_NAME (type, i),
-			stream, show - 1, level + 4,
+			stream, newshow, level + 4,
 			&local_flags);
-	  if (!field_is_static (&TYPE_FIELD (type, i))
-	      && TYPE_FIELD_PACKED (type, i))
+	  if (!is_static && TYPE_FIELD_PACKED (type, i))
 	    {
 	      /* It is a bitfield.  This code does not attempt
 		 to look at the bitpos and reconstruct filler,
@@ -1122,9 +1302,10 @@ c_type_print_base_struct_union (struct type *type, struct ui_file *stream,
 	      section_type = output_access_specifier
 		(stream, section_type, level,
 		 TYPE_FN_FIELD_PROTECTED (f, j),
-		 TYPE_FN_FIELD_PRIVATE (f, j));
+		 TYPE_FN_FIELD_PRIVATE (f, j), flags);
 
-	      print_spaces_filtered (level + 4, stream);
+	      print_spaces_filtered_with_print_options (level + 4, stream,
+							flags);
 	      if (TYPE_FN_FIELD_VIRTUAL_P (f, j))
 		fprintf_filtered (stream, "virtual ");
 	      else if (TYPE_FN_FIELD_STATIC_P (f, j))
@@ -1143,9 +1324,16 @@ c_type_print_base_struct_union (struct type *type, struct ui_file *stream,
 		       && !is_full_physname_constructor  /* " " */
 		       && !is_type_conversion_operator (type, i, j))
 		{
+		  unsigned int old_po = local_flags.print_offsets;
+
+		  /* Temporarily disable print_offsets, because it
+		     would mess with indentation.  */
+		  local_flags.print_offsets = 0;
 		  c_print_type (TYPE_TARGET_TYPE (TYPE_FN_FIELD_TYPE (f, j)),
 				"", stream, -1, 0,
 				&local_flags);
+		  local_flags.print_offsets = old_po;
+
 		  fputs_filtered (" ", stream);
 		}
 	      if (TYPE_FN_FIELD_STUB (f, j))
@@ -1226,9 +1414,16 @@ c_type_print_base_struct_union (struct type *type, struct ui_file *stream,
 
 	  for (int i = 0; i < TYPE_NESTED_TYPES_COUNT (type); ++i)
 	    {
-	      print_spaces_filtered (level + 4, stream);
+	      unsigned int old_po = semi_local_flags.print_offsets;
+
+	      /* Temporarily disable print_offsets, because it
+		 would mess with indentation.  */
+	      print_spaces_filtered_with_print_options (level + 4, stream,
+							flags);
+	      semi_local_flags.print_offsets = 0;
 	      c_print_type (TYPE_NESTED_TYPES_FIELD_TYPE (type, i),
 			    "", stream, show, level + 4, &semi_local_flags);
+	      semi_local_flags.print_offsets = old_po;
 	      fprintf_filtered (stream, ";\n");
 	    }
 	}
@@ -1254,11 +1449,17 @@ c_type_print_base_struct_union (struct type *type, struct ui_file *stream,
 		  section_type = output_access_specifier
 		    (stream, section_type, level,
 		     TYPE_TYPEDEF_FIELD_PROTECTED (type, i),
-		     TYPE_TYPEDEF_FIELD_PRIVATE (type, i));
+		     TYPE_TYPEDEF_FIELD_PRIVATE (type, i), flags);
 		}
-	      print_spaces_filtered (level + 4, stream);
+	      print_spaces_filtered_with_print_options (level + 4, stream,
+							flags);
 	      fprintf_filtered (stream, "typedef ");
 
+	      unsigned int old_po = semi_local_flags.print_offsets;
+	      /* Temporarily disable print_offsets, because it
+		 would mess with indentation.  */
+	      semi_local_flags.print_offsets = 0;
+
 	      /* We want to print typedefs with substitutions
 		 from the template parameters or globally-known
 		 typedefs but not local typedefs.  */
@@ -1266,13 +1467,21 @@ c_type_print_base_struct_union (struct type *type, struct ui_file *stream,
 			    TYPE_TYPEDEF_FIELD_NAME (type, i),
 			    stream, show - 1, level + 4,
 			    &semi_local_flags);
+	      semi_local_flags.print_offsets = old_po;
 	      fprintf_filtered (stream, ";\n");
 	    }
 	}
 
+      if (flags->print_offsets && level > 0)
+	print_spaces_filtered (OFFSET_SPC_LEN, stream);
+
       fprintfi_filtered (level, stream, "}");
     }
 
+  if (show > 0 && flags->print_offsets)
+    fprintf_filtered (stream, " /* total size: %4u bytes */",
+		      TYPE_LENGTH (type));
+
   do_cleanups (local_cleanups);
 }
 
@@ -1300,7 +1509,6 @@ c_type_print_base (struct type *type, struct ui_file *stream,
 {
   int i;
   int len;
-  int j;
 
   QUIT;
 
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 062f01d5ff..9d58f2b7d0 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -17211,6 +17211,121 @@ names are substituted when printing other types.
 @item T
 Print typedefs defined in the class.  This is the default, but the flag
 exists in case you change the default with @command{set print type typedefs}.
+
+@item o
+Print the offsets and sizes of fields in a struct, similar to what the
+@command{pahole} tool does.
+
+For example, given the following declarations:
+
+@smallexample
+struct tuv
+@{
+  int a1;
+  char *a2;
+  int a3;
+@};
+
+struct xyz
+@{
+  int f1;
+  char f2;
+  void *f3;
+  struct tuv f4;
+@};
+
+union qwe
+@{
+  struct tuv fff1;
+  struct xyz fff2;
+@};
+
+struct tyu
+@{
+  int a1 : 1;
+  int a2 : 3;
+  int a3 : 23;
+  char a4 : 2;
+  int64_t a5;
+  int a6 : 5;
+  int64_t a7 : 3;
+@};
+@end smallexample
+
+Issuing a @kbd{ptype /o struct tuv} would print:
+
+@smallexample
+(@value{GDBP}) ptype /o struct tuv
+/* offset    |  size */
+struct tuv @{
+/*    0      |     4 */    int a1;
+/* XXX  4-byte hole  */
+/*    8      |     8 */    char *a2;
+/*   16      |     4 */    int a3;
+@} /* total size:   24 bytes */
+@end smallexample
+
+Notice the format of the first column of comments.  There, you can
+find two parts separated by the @code{|} character: the @emph{offset},
+which indicates where the field is located inside the struct, in
+bytes, and the @emph{size} of the field.  Another interesting line is
+the marker of a @emph{hole} in the struct, indicating that it may be
+possible to pack the struct and make it use less space by reorganizing
+its fields.
+
+It is also possible to print offsets inside an union:
+
+@smallexample
+(@value{GDBP}) ptype /o union qwe
+/* offset    |  size */
+union qwe @{
+/*                24 */    struct tuv @{
+/*    0      |     4 */        int a1;
+/* XXX  4-byte hole  */
+/*    8      |     8 */        char *a2;
+/*   16      |     4 */        int a3;
+                           @} /* total size:   24 bytes */ fff1;
+/*                40 */    struct xyz @{
+/*    0      |     4 */        int f1;
+/*    4      |     1 */        char f2;
+/* XXX  3-byte hole  */
+/*    8      |     8 */        void *f3;
+/*   16      |    24 */        struct tuv @{
+/*   16      |     4 */            int a1;
+/* XXX  4-byte hole  */
+/*   24      |     8 */            char *a2;
+/*   32      |     4 */            int a3;
+                               @} /* total size:   24 bytes */ f4;
+                           @} /* total size:   40 bytes */ fff2;
+@} /* total size:   40 bytes */
+@end smallexample
+
+In this case, since @code{struct tuv} and @code{struct xyz} occupy the
+same space (because we are dealing with an union), the offset is not
+printed for them.  However, you can still examine the offset of each
+of these structures' fields.
+
+Another useful scenario is printing the offsets of a struct containing
+bitfields:
+
+@smallexample
+(@value{GDBP}) ptype /o struct tyu
+/* offset    |  size */
+struct tyu @{
+/*    0:31   |     4 */    int a1 : 1;
+/*    0:28   |     4 */    int a2 : 3;
+/*    0: 5   |     4 */    int a3 : 23;
+/*    3: 3   |     1 */    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;
+@} /* total size:   24 bytes */
+@end smallexample
+
+Note how the offset information is now extended to also include how
+many bits are left to be used in each bitfield.
 @end table
 
 @kindex ptype
diff --git a/gdb/testsuite/gdb.base/ptype-offsets.cc b/gdb/testsuite/gdb.base/ptype-offsets.cc
new file mode 100644
index 0000000000..aadcace517
--- /dev/null
+++ b/gdb/testsuite/gdb.base/ptype-offsets.cc
@@ -0,0 +1,158 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* This file will be used to test 'ptype /o' on x86_64 only.  */
+
+#include <stdint.h>
+
+/* A struct with many types of fields, in order to test 'ptype
+   /o'.  */
+
+struct abc
+{
+  /* Virtual destructor.  */
+  virtual ~abc ()
+  {}
+
+  /* 8-byte address.  Because of the virtual destructor above, this
+     field's offset will be 8.  */
+  void *field1;
+
+  /* No hole here.  */
+
+  /* 4-byte int bitfield of 1-bit.  */
+  unsigned int field2 : 1;
+
+  /* 31-bit hole here.  */
+
+  /* 4-byte int.  */
+  int field3;
+
+  /* No hole here.  */
+
+  /* 1-byte char.  */
+  char field4;
+
+  /* 7-byte hole here.  */
+
+  /* 8-byte int.  */
+  uint64_t field5;
+
+  /* We just print the offset and size of a union, ignoring its
+     fields.  */
+  union
+  {
+    /* 8-byte address.  */
+    void *field6;
+
+    /* 4-byte int.  */
+    int field7;
+  } field8;
+
+  /* Empty constructor.  */
+  abc ()
+  {}
+};
+
+/* This struct will be nested inside 'struct xyz'.  */
+
+struct tuv
+{
+  int a1;
+
+  char *a2;
+
+  int a3;
+};
+
+/* This struct will be nested inside 'struct pqr'.  */
+
+struct xyz
+{
+  int f1;
+
+  char f2;
+
+  void *f3;
+
+  struct tuv f4;
+};
+
+/* A struct with a nested struct.  */
+
+struct pqr
+{
+  int ff1;
+
+  struct xyz ff2;
+
+  char ff3;
+};
+
+/* A union with two nested structs.  */
+
+union qwe
+{
+  struct tuv fff1;
+
+  struct xyz fff2;
+};
+
+/* A struct with an union.  */
+
+struct poi
+{
+  int f1;
+
+  union qwe f2;
+
+  uint16_t f3;
+
+  struct pqr f4;
+};
+
+/* A struct with bitfields.  */
+
+struct tyu
+{
+  int a1 : 1;
+
+  int a2 : 3;
+
+  int a3 : 23;
+
+  char a4 : 2;
+
+  int64_t a5;
+
+  int a6 : 5;
+
+  int64_t a7 : 3;
+};
+
+int
+main (int argc, char *argv[])
+{
+  struct abc foo;
+  struct pqr bar;
+  union qwe c;
+  struct poi d;
+  struct tyu e;
+  uint8_t i;
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/ptype-offsets.exp b/gdb/testsuite/gdb.base/ptype-offsets.exp
new file mode 100644
index 0000000000..04a887a703
--- /dev/null
+++ b/gdb/testsuite/gdb.base/ptype-offsets.exp
@@ -0,0 +1,192 @@
+# This testcase is part of GDB, the GNU debugger.
+
+# Copyright 2017 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+standard_testfile .cc
+
+# Test only works on x86_64 LP64 targets.  That's how we guarantee
+# that the expected holes will be present in the struct.
+if { !([istarget "x86_64-*-*"] && [is_lp64_target]) } {
+    untested "test work only on x86_64 lp64"
+    return 0
+}
+
+if { [prepare_for_testing "failed to prepare" $testfile $srcfile \
+	  { debug c++ optimize=-O0 }] } {
+    return -1
+}
+
+# Test general offset printing, ctor/dtor printing, union, formatting.
+gdb_test "ptype /o struct abc" \
+    [multi_line \
+"type = struct abc {" \
+"/\\\* offset    |  size \\\*/" \
+"                         public:" \
+"/\\\*    8      |     8 \\\*/    void \\\*field1;" \
+"/\\\*   16:31   |     4 \\\*/    unsigned int field2 : 1;" \
+"/\\\* XXX  7-bit hole   \\\*/" \
+"/\\\* XXX  3-byte hole  \\\*/" \
+"/\\\*   20      |     4 \\\*/    int field3;" \
+"/\\\*   24      |     1 \\\*/    char field4;" \
+"/\\\* XXX  7-byte hole  \\\*/" \
+"/\\\*   32      |     8 \\\*/    uint64_t field5;" \
+"/\\\*   40      |     8 \\\*/    union {" \
+"/\\\*                 8 \\\*/        void \\\*field6;" \
+"/\\\*                 4 \\\*/        int field7;" \
+"                           } /\\\* total size:    8 bytes \\\*/ field8;" \
+"" \
+"                           abc\\(void\\);" \
+"                           ~abc\\(\\);" \
+"} /\\\* total size:   48 bytes \\\*/"] \
+    "ptype offset struct abc"
+
+# Test nested structs.
+gdb_test "ptype /o struct pqr" \
+    [multi_line \
+"type = struct pqr {" \
+"/\\\* offset    |  size \\\*/" \
+"/\\\*    0      |     4 \\\*/    int f1;" \
+"/\\\* XXX  4-byte hole  \\\*/" \
+"/\\\*    8      |    16 \\\*/    struct xyz {" \
+"/\\\*    8      |     4 \\\*/        int f1;" \
+"/\\\*   12      |     1 \\\*/        char f2;" \
+"/\\\* XXX  3-byte hole  \\\*/" \
+"/\\\*   16      |     8 \\\*/        void \\\*f3;" \
+"/\\\*   24      |    24 \\\*/        struct tuv {" \
+"/\\\*   24      |     4 \\\*/            int a1;" \
+"/\\\* XXX  4-byte hole  \\\*/" \
+"/\\\*   32      |     8 \\\*/            char *a2;" \
+"/\\\*   40      |     4 \\\*/            int a3;" \
+"                               } /\\\* total size:   24 bytes \\\*/ f4;" \
+"                           } /\\\* total size:   40 bytes \\\*/ ff2;" \
+"/\\\*   48      |     1 \\\*/    char ff3;" \
+"} /\\\* total size:   56 bytes \\\*/"] \
+    "ptype offset struct pqr"
+
+# Test that the offset is properly reset when we are printing an union
+# and go inside two inner structs.
+# This also tests a struct inside a struct inside an union.
+gdb_test "ptype /o union qwe" \
+    [multi_line \
+"/\\\* offset    |  size \\\*/" \
+"/\\\*                24 \\\*/    struct tuv {" \
+"/\\\*    0      |     4 \\\*/        int a1;" \
+"/\\\* XXX  4-byte hole  \\\*/" \
+"/\\\*    8      |     8 \\\*/        char \\\*a2;" \
+"/\\\*   16      |     4 \\\*/        int a3;" \
+"                           } /\\\* total size:   24 bytes \\\*/ fff1;" \
+"/\\\*                40 \\\*/    struct xyz {" \
+"/\\\*    0      |     4 \\\*/        int f1;" \
+"/\\\*    4      |     1 \\\*/        char f2;" \
+"/\\\* XXX  3-byte hole  \\\*/" \
+"/\\\*    8      |     8 \\\*/        void \\\*f3;" \
+"/\\\*   16      |    24 \\\*/        struct tuv {" \
+"/\\\*   16      |     4 \\\*/            int a1;" \
+"/\\\* XXX  4-byte hole  \\\*/" \
+"/\\\*   24      |     8 \\\*/            char \\\*a2;" \
+"/\\\*   32      |     4 \\\*/            int a3;" \
+"                               } /\\\* total size:   24 bytes \\\*/ f4;" \
+"                           } /\\\* total size:   40 bytes \\\*/ fff2;" \
+"} /\\\* total size:   40 bytes \\\*/"] \
+    "ptype offset union qwe"
+
+# Test printing a struct that contains an union, and that also
+# contains a struct.
+gdb_test "ptype /o struct poi" \
+    [multi_line \
+"/\\\* offset    |  size \\\*/" \
+"/\\\*    0      |     4 \\\*/    int f1;" \
+"/\\\* XXX  4-byte hole  \\\*/" \
+"/\\\*    8      |    40 \\\*/    union qwe {" \
+"/\\\*                24 \\\*/        struct tuv {" \
+"/\\\*    8      |     4 \\\*/            int a1;" \
+"/\\\* XXX  4-byte hole  \\\*/" \
+"/\\\*   16      |     8 \\\*/            char \\\*a2;" \
+"/\\\*   24      |     4 \\\*/            int a3;" \
+"                               } /\\\* total size:   24 bytes \\\*/ fff1;" \
+"/\\\*                40 \\\*/        struct xyz {" \
+"/\\\*    8      |     4 \\\*/            int f1;" \
+"/\\\*   12      |     1 \\\*/            char f2;" \
+"/\\\* XXX  3-byte hole  \\\*/" \
+"/\\\*   16      |     8 \\\*/            void \\\*f3;" \
+"/\\\*   24      |    24 \\\*/            struct tuv {" \
+"/\\\*   24      |     4 \\\*/                int a1;" \
+"/\\\* XXX  4-byte hole  \\\*/" \
+"/\\\*   32      |     8 \\\*/                char \\\*a2;" \
+"/\\\*   40      |     4 \\\*/                int a3;" \
+"                                   } /\\\* total size:   24 bytes \\\*/ f4;" \
+"                               } /\\\* total size:   40 bytes \\\*/ fff2;" \
+"                           } /\\\* total size:   40 bytes \\\*/ f2;" \
+"/\\\*   48      |     2 \\\*/    uint16_t f3;" \
+"/\\\* XXX  6-byte hole  */" \
+"/\\\*   56      |    56 \\\*/    struct pqr {" \
+"/\\\*   56      |     4 \\\*/        int ff1;" \
+"/\\\* XXX  4-byte hole  \\\*/" \
+"/\\\*   64      |    40 \\\*/        struct xyz {" \
+"/\\\*   64      |     4 \\\*/            int f1;" \
+"/\\\*   68      |     1 \\\*/            char f2;" \
+"/\\\* XXX  3-byte hole  \\\*/" \
+"/\\\*   72      |     8 \\\*/            void \\\*f3;" \
+"/\\\*   80      |    24 \\\*/            struct tuv {" \
+"/\\\*   80      |     4 \\\*/                int a1;" \
+"/\\\* XXX  4-byte hole  \\\*/" \
+"/\\\*   88      |     8 \\\*/                char \\\*a2;" \
+"/\\\*   96      |     4 \\\*/                int a3;" \
+"                                   } /\\\* total size:   24 bytes \\\*/ f4;" \
+"                               } /\\\* total size:   40 bytes \\\*/ ff2;" \
+"/\\\*  104      |     1 \\\*/        char ff3;" \
+"                           } /\\\* total size:   56 bytes \\\*/ f4;" \
+"} /\\\* total size:  112 bytes \\\*/"] \
+    "ptype offset struct poi"
+
+# Test printing a struct with several bitfields, laid out in various
+# ways.
+#
+# Because dealing with bitfields and offsets is difficult, it can be
+# tricky to confirm that the output of the this command is accurate.
+# A nice way to do that is to use GDB's "x" command and print the
+# actual memory layout of the struct.  In order to differentiate
+# betweent bitfields and non-bitfield variables, one can assign "-1"
+# to every bitfield in the struct.  An example of the output of "x"
+# using "struct tyu" is:
+#
+#   (gdb) x/24xb &e
+#   0x7fffffffd540: 0xff    0xff    0xff    0x1f    0x00    0x00    0x00    0x00
+#   0x7fffffffd548: 0xff    0xff    0xff    0xff    0xff    0xff    0xff    0xff
+#   0x7fffffffd550: 0xff    0x00    0x00    0x00    0x00    0x00    0x00    0x00
+#
+# Thanks to Pedro Alves for coming up with this method (which can be
+# used to inspect the other cases, of course).
+gdb_test "ptype /o struct tyu" \
+    [multi_line \
+"/\\\* offset    |  size \\\*/" \
+"struct tyu {" \
+"/\\\*    0:31   |     4 \\\*/    int a1 : 1;" \
+"/\\\*    0:28   |     4 \\\*/    int a2 : 3;" \
+"/\\\*    0: 5   |     4 \\\*/    int a3 : 23;" \
+"/\\\*    3: 3   |     1 \\\*/    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;" \
+"} /\\\* total size:   24 bytes \\\*/"] \
+    "ptype offset struct tyu"
+
+# Test that we don't print any header when issuing a "ptype /o" on a
+# non-struct, non-union, non-class type.
+gdb_test "ptype /o int" "int" "ptype offset int"
+gdb_test "ptype /o uint8_t" "char" "ptype offset uint8_t"
diff --git a/gdb/typeprint.c b/gdb/typeprint.c
index 9d9d6f5a49..225e94b1bb 100644
--- a/gdb/typeprint.c
+++ b/gdb/typeprint.c
@@ -42,6 +42,8 @@ const struct type_print_options type_print_raw_options =
   1,				/* raw */
   1,				/* print_methods */
   1,				/* print_typedefs */
+  0,				/* print_offsets */
+  0,				/* offset_bitpos */
   0,				/* print_nested_type_limit  */
   NULL,				/* local_typedefs */
   NULL,				/* global_table */
@@ -55,6 +57,8 @@ static struct type_print_options default_ptype_flags =
   0,				/* raw */
   1,				/* print_methods */
   1,				/* print_typedefs */
+  0,				/* print_offsets */
+  0,				/* offset_bitpos */
   0,				/* print_nested_type_limit  */
   NULL,				/* local_typedefs */
   NULL,				/* global_table */
@@ -440,6 +444,9 @@ whatis_exp (const char *exp, int show)
 		case 'T':
 		  flags.print_typedefs = 1;
 		  break;
+		case 'o':
+		  flags.print_offsets = 1;
+		  break;
 		default:
 		  error (_("unrecognized flag '%c'"), *exp);
 		}
@@ -499,6 +506,11 @@ whatis_exp (const char *exp, int show)
 	real_type = value_rtti_type (val, &full, &top, &using_enc);
     }
 
+  if (flags.print_offsets &&
+      (TYPE_CODE (type) == TYPE_CODE_STRUCT
+       || TYPE_CODE (type) == TYPE_CODE_UNION))
+    fprintf_filtered (gdb_stdout, "/* offset    |  size */\n");
+
   printf_filtered ("type = ");
 
   if (!flags.raw)
@@ -759,7 +771,8 @@ Available FLAGS are:\n\
   /m    do not print methods defined in a class\n\
   /M    print methods defined in a class\n\
   /t    do not print typedefs defined in a class\n\
-  /T    print typedefs defined in a class"));
+  /T    print typedefs defined in a class\n\
+  /o    print offsets and sizes of fields in a struct (like pahole)\n"));
   set_cmd_completer (c, expression_completer);
 
   c = add_com ("whatis", class_vars, whatis_command,
diff --git a/gdb/typeprint.h b/gdb/typeprint.h
index a2058b0120..08333b3762 100644
--- a/gdb/typeprint.h
+++ b/gdb/typeprint.h
@@ -35,6 +35,15 @@ struct type_print_options
   /* True means print typedefs in a class.  */
   unsigned int print_typedefs : 1;
 
+  /* True means to print offsets, a la 'pahole'.  */
+  unsigned int print_offsets : 1;
+
+  /* The offset to be applied to bitpos when PRINT_OFFSETS is true.
+     This is needed for when we are printing nested structs and want
+     to make sure that the printed offset for each field carries over
+     the offset of the outter struct.  */
+  unsigned int offset_bitpos;
+
   /* The number of nested type definitions to print.  -1 == all.  */
   int print_nested_type_limit;
 
-- 
2.14.3



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