Issue 120604.1: Alternate debug sections

Author: Jakub Jelinek
Champion: Jakub Jelinek
Date submitted: 2012-06-04
Date revised:
Date closed:
Type: Enhancement
Status: ACCEPTED with modifications
DWARF Version: 5

Appendix E shows how DW_TAG_imported_unit can be used to reduce duplicate
information, but that allows only to reduce duplicate information within
debug information of each shared object or executable.  Often there is
quite a lot of debug information duplication also in between different
executables or shared objects (usually from the same project).


The following proposal adds 2 new forms and 3 new .debug_macro opcodes
that allow reducing duplicate information in between different
executables or shared objects.  Each executable or shared object then has
its own set of debug sections, and in addition to that there can be some
other object with another set of debug sections, which contain information
shared between several executables or shared objects.  How to find this
additional set of debug sections is implementation defined.

It has been implemented in th DWZ utility as a GNU extension.

The current forms used to reference information are references
within the same set of debug sections (same executable, same shared object
or from within the additional set of debug sections within that set).
The new forms allow references from the executable or shared object debug
sections to debug information within the additional set of debug sections.


Here are preliminary DWARF edits for the proposal (on top of the latest


Add at the end:

"Optionally, the debugging information entries that are the same in
 between multiple executables or shared objects may be also contained
 in alternate .debug_info sections in an extra object
 file.  The executable or shared object which contains references to
 those debugging information entries shall contain a .debug_alt section
 with information how to find the extra object file, and the extra
 object file should alongside the .debug_info section contain also
 .debug_alt section."

  Append to end:
  "DW_MACRO_define_indirect_alt        A macro definition (indirect name string in
                    alternate object file)
   DW_MACRO_undef_indirect_alt      A macro undefinition (indirect name string in
                    alternate object file)
   DW_MACRO_transparent_include_alt Include a sequence of entries from given offset of
                    .debug_macro section in alternate object file"

Add new section:  Define and Undefine Using Indirect Strings in Alternate Object File.
  "A DW_MACRO_define_indirect_alt or DW_MACRO_undef_indirect_alt entry has
   two operands.  The first operand encodes the line number of the source line
   on which the relevant defining or undefining macro directives appeared.
   The second operand consists of an offset into a string table contained in
   the .debug_str section of the alternate object file.  The size of the operand is
   given in the section header offset_size.  Apart from the
   encoding of the second operand these entries are equivalent to
   DW_MACRO_define_indirect resp. DW_MACRO_undef_indirect."
  "A DW_MACRO_transparent_include_alt entry has one operand, offset from the
   start of the .debug_macro section in alternate object file.  The size of
   the operand is given in the section header offset size.  Apart from the
   different location where to find the sequence of macro information entries this entry
   type is equivalent to DW_MACRO_transparent_include.  This entry type is aimed
   at sharing duplicate sequences of macro information entries between .debug_macro
   sections from different executables or shared objects.  From within alternate
   .debug_macro section, DW_MACRO_define_indirect and
   DW_MACRO_undef_indirect entry types refer to the alternate .debug_str
   section and DW_MACRO_transparent_include refers to the alternate
   .debug_macro section.".

Add new section:
7.3.6  DWARF Alternate Section Files
In order to further reduce size of the debugging information, it is possible
to move duplicate debug information entries, strings and macro entries from
several executables or shared objects into another object file by some
post-linking utility and the moved entries and strings can be then referenced
from the debugging information of each of those executables or shared objects.

A DWARF alternate section file is itself an object file, using the same object
file format, byte order, and size as the corresponding application executables
or shared libraries. It consists only of a file header, section table, and
a number of DWARF debug information sections.  Both the alternate section file
and all the executables or shared objects that will reference entries or strings
moved to the alternate section file should contain a .debug_alt section.

The .debug_alt section should contain:

1. version (uhalf)
A 2-byte unsigned integer representing the version of the DWARF
information for the compilation unit (see Appendix G). The
value in this field is 5.

2. is_alternate (ubyte)
A 1-byte unsigned integer, which should contain value 1 if it is the
alternate section file other executables or shared objects refer to, or
0 if it is an executable or shared object referring to alternate section object

3. alt_filename (null terminated filename)
If is_alternate is 0, this should contain either absolute filename of the
alternate section object file, or a filename relative to the object file
containing the .debug_alt section.  If is_alternate is 1, the filename
is not needed and can be an empty string.

4. alt_checksum_len (unsigned leb128)
Length of the following checksum field, can be 0 if no checksum is provided.

5. alt_checksum (array of ubyte)
Some checksum or cryptographic hash function of the .debug_info, .debug_str and
.debug_macro sections of the alternate object file, or some unique identifier
which the implementation can choose to verify that the alternate section object
file matches what the debug information in the executables or shared objects

*Debug information entries that refer to executable's or shared
object's addresses shouldn't be moved to alternate section files, that will
unlikely be beneficial as the addresses will be different, and similarly
entries referenced from within expression locations or using loclistptr
form attributes.*

Executable or shared object compilation units then can use
DW_TAG_imported_unit with DW_FORM_ref_alt form DW_AT_import attribute
to import entries from the alternate sections, other DW_FORM_ref_alt
attributes to refer to them and DW_FORM_strp_alt form attributes to
refer to strings that are used by debug information of multiple
executables or shared objects.  Within the alternate section file's
debugging sections DW_FORM_ref_alt or DW_FORM_strp_alt forms should
not be used, and all reference forms referring some other sections
refer the sections in the alternate section file, rather than
the DWARF sections of the executable or shared objects which is
referring to the alternate section object file.

In macro information DW_MACRO_define_indirect_alt or
DW_MACRO_undef_indirect_alt opcodes can refer to strings in the alternate
.debug_str section, or DW_MACRO_transparent_include_alt can refer
to alternate .debug_macro section opcodes.  Within the alternate
.debug_macro section, DW_MACRO_define_indirect and DW_MACRO_undef_indirect
opcodes refer to the alternate .debug_str section, not the one in
the executable or shared object."

  3. Add:
     DW_FORM_ref_alt        offset in .debug_info section in alternate object file
     DW_FORM_strp_alt       offset in .debug_str section in alternate object file

  "There are three types of reference."
  "There are four types of reference."

  "of the target executable or shared object"
  ", or, for references from within alternate .debug_info
  sections, an offset from the beginning of the alternate .debug_info section".

  "that was computed for the type"
  "The fourth type of reference is a reference from within the .debug_info
  section of the executable or shared object to
  a debugging information entry in the .debug_info section of alternate object file.
  This type of reference (DW_FORM_ref_alt) is an offset from the beginning
  of the alternate .debug_info section".

  "second and third"
  "second, third and fourth".

  "across compilation units"
  ", the fourth type for sharing of information across compilation units
  from different executables or shared objects". 

  "or may be represented as an offset into a string table contained in the
  alternate .debug_str section (DW_FORM_strp_alt).  DW_FORM_strp offsets
  from compilation units in alternate .debug_info or .debug_types sections
  refer to the alternate .debug_str section, not .debug_str in the
  executable or shared object".

  In figure 21 add:
  "DW_FORM_ref_alt 0x1c    reference"
  "DW_FORM_strp_alt    0x1d    string"

  In figure 39 add:
  "DW_MACRO_define_indirect_alt        0x08
   DW_MACRO_undef_indirect_alt      0x09
   DW_MACRO_transparent_include_alt 0x0a"

Defered -- 1/21/2014 -- depends on 110722.1.
Revised -- 7/14/2014.
Accepted -- 9/16/2014.  Editorial changes/clarifications:  Better name 
  for debug_alt.  Use "shr" instead of "alt".