[Dwarf-Discuss] armcc DWARF

[Keith Walker]

David,

- Why is .debug_abbrev in a group by itself?

So that it can be placed into a COMDAT group and the linker can ensure that only one copy of the .debug_abbrev sections appears in the final image.

armcc generates a fixed table of entries into the .debug_abbrev section, so this allows the linker to ensure only one copy ends up in the final image.   All .debug_info sections in a .o file reference this one .debug_abbrev section.

This is different to many other compilers which generate a .debug_abbrev section which only contains the entries required by a single .debug_info section;  thus each .debug_abbrev section is different.   The resulting table in the final image, which concatenates all the .debug_abbrev sections together, may therefore be larger than having a single table of fixed entries.

- There are two .debug_info not in any group

Armcc partitions up the debugging information in order to allow debugging information to be removed from the final image if the associated code/data is removed.

The debugging information that should always be retained (usually type information) will be placed into sections not in a group.

The debugging information that is associated with code/data that could be discarded by the linker will be placed into the same group section;   so if the code/data is discarded so is the associated debugging information.   The .debug_info entries in a group section will often reference type entries in the .debug_info section which is always retained.

The often happens for C++ code, or if you are generating each function into its own section to allow the linker to discard unused functions.

- Which .debug_line (sec 7 or 8)would be referred to from section 6 debug_line? How would one know?

The magic of relocation entries.     The DW_AT_stmt_list entry in section 6 should have an associated relocation entry whose symbol is associated with the section to which it refers.

Keith

-----Original Message-----
From: Dwarf-Discuss <dwarf-discuss-bounces@lists.dwarfstd.org> On Behalf Of David Anderson via Dwarf-Discuss
Sent: 22 May 2018 22:33
To: dwarf-discuss at lists.dwarfstd.org
Subject: [Dwarf-Discuss] armcc DWARF


I have been given a tiny object file  created by armcc using DWARF and things make no sense to me so far.

dwarfdump (and libdwarf) use the SHT_GROUP section data to associate sections to their group.  This is a .o, not a fully linked executable or shared-library.

(libdwarf assigns group numbers itself. group 1 is  non-dwo .debug sections) (any .dwo sections present would be shown as group 2 if not listed in a section-group header, but no such are present here)

Here is dwarfdump/libdwarf/s reading of the sections and section-group sections.

Section Groups data
   Number of Elf-like sections:   35
   Number of groups           :    5
   Group to print             :    3
   Count of map entries       :   15
   [index]  group section
   [    0]    1     4 .debug_frame
   [    1]    1     5 .debug_info
   [    2]    1     6 .debug_info
   [    3]    1     7 .debug_line
   [    4]    1     8 .debug_line
   [    5]    1     9 .debug_loc
   [    6]    1    10 .debug_macinfo
   [    7]    1    11 .debug_pubnames
   [    8]    3    13 .debug_info
   [    9]    3    14 .debug_line
   [   10]    4    16 .debug_info
   [   11]    5    18 .debug_info
   [   12]    5    19 .debug_line
   [   13]    5    20 .debug_macinfo
   [   14]    6    22 .debug_abbrev

There are two .debug_info not in any group (called group 1 by libdwarf).
There is only one .debug_abbrev and it is in group six.

How is one to make sense of this?  Which .debug_line (sec 7 or 8)would be referred to from section 6 debug_line? How would one know?

Why is .debug_abbrev in a group by itself?

I would appreciate any information on how the sections tie together...
how is a reader supposed to know what goes with what?

Thanks for any hints.
David Anderson

--
Duct tape is like the force. It has a light side, a dark side, and it holds the universe together.... -- Carl Zwanzig

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