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

Mark Wielaard
2020-06-19T20:04:52Z

Hi,

On Tue, 2020-06-09 at 13:24 -0700, Fangrui Song via Elfutils-devel wrote:
> 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

I would appreciate having a clear "not valid" marker instead of getting
a possibly bogus (but valid) address. -1 seems a reasonable value.
Although I have seen (and written) code that simply assumes zero is
that value.

Would such an invalid address marker in an DW_AT_low_pc make the whole
program scope under a DIE invalid? What about (addr, loc, rng) base
addresses? Can they contain an invalid marker, does that make the whole
table/range invalid?

I must admit that as a DWARF consumer I am slightly worried that having
a sanctioned "invalid marker" will cause DWARF producers to just not
coordinate and simply assume they can always invalidate anything they
emit. Even if there could be a real solution by coordinating between
compiler/linker who is responsible for producing the valid DWARF
entries (especially when LTO is involved).

> 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.

Cary Coutant wrote up a general Two-Level Line Number Table proposal to
address the issue of having a single machine instruction corresponds to
more than one source statement:
http://wiki.dwarfstd.org/index.php?title=TwoLevelLineTables

Which seems useful in these kind of situations. But I don't know the
current status of the proposal.

Cheers,

Mark