From: Mark Wielaard <mark@klomp.org>
To: Fangrui Song <maskray@google.com>,
gdb@sourceware.org, elfutils-devel@sourceware.org,
binutils@sourceware.org
Cc: David Blaikie <dblaikie@gmail.com>, Alan Modra <amodra@gmail.com>
Subject: Re: Tombstone values in debug sections (was: Range lists, zero-length functions, linker gc)
Date: Fri, 19 Jun 2020 22:04:50 +0200 [thread overview]
Message-ID: <69e4e7a60b23bff32d88b3edd2a718cf2f6e8cdc.camel@klomp.org> (raw)
In-Reply-To: <20200609202414.2olgwq2jniweeyr6@google.com>
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
next prev parent reply other threads:[~2020-06-19 20:04 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-31 18:55 Range lists, zero-length functions, linker gc Fangrui Song
2020-05-31 19:15 ` Fangrui Song
2020-05-31 20:10 ` Mark Wielaard
2020-05-31 20:47 ` Fangrui Song
2020-05-31 22:11 ` Mark Wielaard
2020-05-31 23:17 ` David Blaikie
2020-05-31 20:49 ` David Blaikie
2020-05-31 22:29 ` Mark Wielaard
2020-05-31 22:36 ` David Blaikie
2020-06-01 9:31 ` Mark Wielaard
2020-06-01 20:18 ` David Blaikie
2020-06-02 16:50 ` Mark Wielaard
2020-06-02 18:06 ` David Blaikie
2020-06-03 3:10 ` Alan Modra
2020-06-03 4:06 ` Fangrui Song
2020-06-03 21:50 ` David Blaikie
2020-06-09 20:24 ` Tombstone values in debug sections (was: Range lists, zero-length functions, linker gc) Fangrui Song
2020-06-19 20:04 ` Mark Wielaard [this message]
2020-06-20 1:02 ` David Blaikie
2020-06-19 12:00 ` Range lists, zero-length functions, linker gc Mark Wielaard
2020-06-20 0:46 ` David Blaikie
2020-06-24 22:21 ` Mark Wielaard
2020-06-25 23:45 ` David Blaikie
2020-05-31 21:33 ` David Blaikie
2020-06-01 16:25 ` Andrew Burgess
2021-09-27 14:00 Tombstone values in debug sections (was: Range lists, zero-length functions, linker gc) Fangrui Song
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