Tombstone values in debug sections (was: Range lists, zero-length functions, linker gc)

[Fangrui Song <maskray@google.com>]

I want to revive the thread, but focus on whether a tombstone value
(-1/-2) in .debug_* can cause trouble to various DWARF consumers (gdb,
debug related tools in elfutils and other utilities I don't know about).

Paul Robinson has proposed that DWARF v6 should reserve a tombstone
value  (the value a relocation referencing a discarded symbol in a
.debug_* section should be resolved to)
http://www.dwarfstd.org/ShowIssue.php?issue=200609.1

Some comments about the proposal:

> - deduplicating different functions with identical content; GNU refers
>   to this as ICF (Identical Code Folding);

ICF (gold --icf={safe,all}) can cause DW_TAG_subprogram with different DW_AT_name to have the same range.

> - functions with no callers; sometimes called dead-stripping or
>   garbage collection.

--gc-sections can lead to tombstone values. A referenced symbol may be
discarded because its containing sections is garbage collected.

> - functions emitted in COMDAT sections, typically C++ template
>   instantiations or inline functions from a header file;

This can cause either tombstone values (STB_LOCAL) or duplicate DIEs (non-STB_LOCAL).


On 2020-06-03, David Blaikie wrote:
>On Tue, Jun 2, 2020 at 8:10 PM Alan Modra <amodra@gmail.com> wrote:
>>
>> On Tue, Jun 02, 2020 at 11:06:10AM -0700, David Blaikie via Binutils wrote:
>> > On Tue, Jun 2, 2020 at 9:50 AM Mark Wielaard <mark@klomp.org> wrote:
>> > > where I
>> > > would argue the compiler simply needs to make sure that if it generates
>> > > code in separate sections it also should create the DWARF separate
>> > > section (groups).
>> >
>> > I don't think that's practical - the overhead, I believe, is too high.
>> > Headers for each section contribution (ELF headers but DWARF headers
>> > moreso - having a separate .debug_addr, .debug_line, etc section for
>> > each function would be very expensive) would make for very large
>> > object files.
>>
>> With a little linker magic I don't see the neccesity of duplicating
>> the DWARF headers.  Taking .debug_line as an example, a compiler could
>> emit the header, opcode, directory and file tables to a .debug_line
>> section with line statements for function foo emitted to
>> .debug_line.foo and for bar to .debug_line.bar, trusting that the
>> linker will combine these sections in order to create an output
>> .debug_line section.  If foo code is excluded then .debug_line.foo
>> info will also be dropped if section groups are used.
>
>I don't think this would apply to debug_addr - where the entries are
>referenced from elsewhere via index, or debug_rnglist where the
>rnglist header (or the debug_info directly) contains offsets into this
>section, so taking chunks out would break those offsets. (or to the
>file/directory name part of debug_line - where you might want to
>remove file/line entries that were eliminated as dead code - but
>that'd throw off the indexes)