[llvm-dev] Switch to ld.bfd tombstone behavior by default

Eric Christopher via llvm-dev
2020-08-05T19:31:59Z

Honestly even though I do understand the debug information I'm with you and
one reason why I'm pushing for the same reset that you are. There are a lot
of threads, it's fairly confusing what has been done where and other than
some fairly widespread breakage among early users of lld (i.e. a short time
from commit to use) it's unclear what the plan is to roll this out
effectively across releases.

Ray: Can you summarize what is where, what you'd like the plan is, and what
is stopping is from doing this in the incremental fashion that's been
requested? I understand that's adding work to an already long thread, but
there's so much going on now that it seems like resetting back to default
for lld and then moving forward is going to be the process that's easiest
understood for everyone.

Thanks!

-eric

On Wed, Aug 5, 2020 at 12:13 PM Hans Wennborg <hans at chromium.org>
wrote:

> My apologies, I didn't realize D84825 now restores the original
> behavior. The review thread is a bit hard to follow :-)
>
> From the change description it still sounds like .debug_ranges &
> .debug_loc tombstones are changing to -2 though? Or is that what we
> had before?
>
> To be clear, I'm not familiar with the technical details here, I'm
> just keen that we end up in a known-good state.
>
> Also the patch is marked "release/11.x only". Normally we land
changes
> on trunk first so it gets proper testing, and then merged to the
> branch. I'd like that to be the case here too.
>
> I'd also like to see Eric or David sign off, since they understand the
> debug info, and I don't.
>
>
> To be honest, after writing this reply I still don't understand why
> we're not just reverting the patches, merging that to 11.x, and
> sending out a new patch where the desired behavior going forward can
> be hashed out.
>
> On Wed, Aug 5, 2020 at 8:47 PM Fāng-ruì Sòng <maskray at google.com>
wrote:
> >
> > As I mentioned in the thread (to many people who don't have time
to
> > read the discussions), pushing https://reviews.llvm.org/D84825
> > restores the original behavior.
> > The same effect as one would get by reverting all related patches. If
> > someone gives me an approval, I'll push it immediately. I already
get
> > verbal LGTM from Peter.
> >
> > > With respect I think the "request for changes" blocked
the change and
> I am not inclined to agree we would otherwise be at an impasse without the
> "request for changes".
> > I would likely get an "Accept Revision" or a just textual
"LGTM", even
> > if there is a conditional request that ".debug_* should use 0 as
well,
> > like GNU ld".
> >
> > On Wed, Aug 5, 2020 at 11:27 AM Hans Wennborg <hans at
chromium.org> wrote:
> > >
> > > +the people who fell off the cc list for some reason
> > >
> > > On Wed, Aug 5, 2020 at 8:24 PM Hans Wennborg <hans at
chromium.org>
> wrote:
> > > >
> > > > Can we please just revert back to what we had before until
the
> > > > discussion about the desired behaviour and how to get there
reaches a
> > > > conclusion?
> > > >
> > > > In particular, I would like to merge that revert to the 11.x
branch.
> > > > At this point in the release process, I'm not keen on
taking any
> patch
> > > > that changes the behavior to something that hasn't been
well tested
> > > > from sitting in trunk for a while.
> > > >
> > > > On Thu, Jul 30, 2020 at 8:55 PM Petr Hosek via llvm-dev
> > > > <llvm-dev at lists.llvm.org> wrote:
> > > > >
> > > > > I can weigh in as a maintainer of the Clang toolchain
on a project
> (Fuchsia) that lives at HEAD. We are still dealing with the fallout of this
> change and our toolchain roll has been blocked since it landed, while
> normally we would roll the toolchain every week. All the issues in Breakpad
> that were affecting us seem to have been addressed, but there are issues in
> other tools that we still haven't addressed and we're still
debugging. This
> has been difficult because each of the issues we found so far have
> manifested differently and we had to do bisect multiple times, because we
> weren't sure whether it was introduced by this one or some other
change.
> Furthermore, as the time since our last roll increases while we're
blocked,
> the range over which we have to bisect is also extending making this
> process more and more time consuming.
> > > > >
> > > > > Fangrui suggested passing the appropriate flag in our
build, but
> that's just infeasible for us. I cannot go into details, but there are
too
> many projects, too many repositories, too many build systems involved.
> Doing such a coordinated change would likely take longer than addressing
> all the issues. We have considered passing the flag in the Fuchsia driver
> (which we control) but this doesn't entirely address the issue because
some
> of the failures we're seeing are in the host binaries (we use the same
> toolchain for both the host and the target) so we would need to make the
> same change in the Linux driver as well.
> > > > >
> > > > > I agree that this change is the right long-term
direction, but I
> wish this was communicated better and it was done as an opt-in rather than
> opt-out to give us enough time to iron out all the issues without the
> stress.
> > > > >
> > > > > On Wed, Jul 29, 2020 at 6:32 PM Fāng-ruì Sòng via
llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> > > > >>
> > > > >> On 2020-07-29, Eric Christopher wrote:
> > > > >> >I think the arguments are largely compatibility
for software
> that's already
> > > > >> >deployed and can't easily upgrade, and
wanting to ensure a
> larger time
> > > > >> >frame for migration with a fallback if things
go wrong. A bridge
> basically
> > > > >> >from what we had to where we'd like to be.
> > > > >> >
> > > > >> >I think we also need to make the change in
mainline lld as well
> - as you
> > > > >> >might know it's occasionally easier to
upgrade a linker than to
> upgrade a
> > > > >> >debugger. I don't have any strong opinions
on what we should
> name the
> > > > >> >command line option, but something that makes
it unnecessary to
> actually
> > > > >> >know the values you want to put in would make
it easier.
> > > > >> >-debug-comdat-relocation-filler=compat, bfd,
<something> (yes,
> that's not
> > > > >> >great, but it's at least a start :)
> > > > >> >
> > > > >> >-eric
> > > > >>
> > > > >> .debug_* => 0 looks good to me and I'd like
to make the change
> because some
> > > > >> users in this thread expressed preference, while
some are
> bystanders, and I don't want
> > > > >> to be the only one on the side opposite to the side
of users
> expressing preferece.
> > > > >>
> > > > >> An argument I have in mind but haven't raised
is that I don't
> want releases or
> > > > >> HEAD to change behaviors too many times. This was
one argument I
> did not want
> > > > >> .debug_* => 0 for the upcoming release and
.debug_* => -1 for a
> next one: there are 2
> > > > >> behavior changes rather than one: old LLD: addend,
LLD 11: 0, LLD
> 11: -1.
> > > > >>
> > > > >> >I think we also need to make the change in
mainline lld as well
> - as you
> > > > >> >might know it's occasionally easier to
upgrade a linker than to
> upgrade a
> > > > >>
> > > > >> This is now a new request about HEAD behavior. I
would honestly
> be a bit
> > > > >> concerned about making such kind of back-and-forth
change. This
> would invalidate
> > > > >> some comments people have already contributed to
some projects
> (LLD since D81784
> > > > >> use -1 ......) Do we know who are the other LLD
HEAD users? I
> still want to
> > > > >> remind people that so far the only problematic
behaviors were
> caused by
> > > > >> .debug_line: -1, not any other .debug_*. With lack
of more input,
> I'd want to
> > > > >> avoid changes mitigating hypothetical issues. It is
possible that
> making more
> > > > >> revert-restore changes could make the situation
more confusing.
> Ultimately,
> > > > >> this is an open source project and we make changes
for the
> interests of
> > > > >> thecommunity.  Maybe it is time for more folks to
share opinions?
> > > > >>
> > > > >> If I had to make a name, I would call it
> --dead-reloc-in-debug={bfd,new}, not -z
> > > > >> as we don't expect GNU ld to adopt. The option
comes with a large
> notice that
> > > > >> this option will go away. Moreover, the user has to
look up the
> semantics of
> > > > >> 'bfd'. And the 'bfd' semantics may
get stale if GNU ld updates
> its behaviors.
> > > > >>
> > > > >> We can also take it from another perspective, do
the opt-in
> options look
> > > > >> intimidating just because they are too long?
> > > > >>
> > > > >> -z
'dead-reloc-in-nonalloc=.debug_ranges=0xfffffffffffffffe'
> > > > >> -z
'dead-reloc-in-nonalloc=.debug_loc=0xfffffffffffffffe'
> > > > >> -z
'dead-reloc-in-nonalloc=.debug_*=0xffffffffffffffff'
> > > > >>
> > > > >> We can support signed integers:
> > > > >>
> > > > >> -z
'dead-reloc-in-nonalloc=.debug_ranges=-2'
> > > > >> -z 'dead-reloc-in-nonalloc=.debug_loc=-2'
> > > > >> -z 'dead-reloc-in-nonalloc=.debug_*=-1'
> > > > >>
> > > > >> Three options seem too long?
> > > > >>
> > > > >> We can support comma separated list:
> > > > >>
> > > > >> -z
> 'dead-reloc-in-nonalloc=.debug_ranges=-2,.debug_loc=-2,.debug_*=-1'
> > > > >>
> > > > >> I'm glad that people raised a request for a
easy-to-toggle
> option, but I'd only
> > > > >> be more comfortable accepting it if there are
sufficiently strong
> reasons for
> > > > >> it, with the acknowledgement that the option will
be another GNU
> ld/gold/LLD
> > > > >> difference (-z dead-reloc-in-nonalloc= has somewhat
a larger
> chance that it can
> > > > >> be adopted by other linkers) and will go away in
two or three
> releases.
> > > > >>
> > > > >> >On Tue, Jul 28, 2020 at 10:00 PM Fāng-ruì Sòng
<
> maskray at google.com> wrote:
> > > > >> >
> > > > >> >> Created https://reviews.llvm.org/D84825 to
be used for
> release/11.x
> > > > >> >>
> > > > >> >> I haven't seen a strong argument for
changing other .debug_*
> but in
> > > > >> >> any case I don't want to continue
debating on this topic.
> > > > >> >>
> > > > >> >> * .debug_ranges & .debug_loc: -2
(lld<11: 0+addend)
> > > > >> >> * .debug_*: 0 (lld<11: 0+addend, lld
HEAD: -1)
> > > > >> >>
> > > > >> >> On Mon, Jul 27, 2020 at 12:47 PM David
Blaikie <
> dblaikie at gmail.com> wrote:
> > > > >> >> >
> > > > >> >> >
> > > > >> >> >
> > > > >> >> > On Mon, Jul 27, 2020 at 9:11 AM
Robinson, Paul <
> paul.robinson at sony.com>
> > > > >> >> wrote:
> > > > >> >> >>
> > > > >> >> >> > I still think that we do bfd
locs with a decent option to
> change for
> > > > >> >> at least the current release and sources
and then, once we're
> a little more
> > > > >> >> certain we have everything that might want
to parse dwarf (say
> by working
> > > > >> >> with dwarf-discuss), we can change the
default.
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> Given what’s been found, I think
Eric/Dave are correct, use
> bfd
> > > > >> >> behavior by default with an option to do
the new thing.  The
> option can be
> > > > >> >> coded to let Sony (SCE debugger tuning)
have it on by default.
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> Bringing it up on dwarf-discuss
seems like a good idea too,
> you can
> > > > >> >> refer to
http://dwarfstd.org/ShowIssue.php?issue=200609.1 as
> part of that.
> > > > >> >> >
> > > > >> >> >
> > > > >> >> > Thanks for the pointer! Posted here:
> > > > >> >>
>
http://lists.dwarfstd.org/pipermail/dwarf-discuss-dwarfstd.org/2020-July/004683.html
> > > > >> >> >
> > > > >> >> >>
> > > > >> >> >> --paulr
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> From: Eric Christopher
<echristo at gmail.com>
> > > > >> >> >> Sent: Saturday, July 25, 2020
12:56 AM
> > > > >> >> >> To: Fāng-ruì Sòng <maskray at
google.com>
> > > > >> >> >> Cc: Hans Wennborg <hans at
chromium.org>;
> llvm-dev at lists.llvm.org; Alexey
> > > > >> >> Lapshin <a.v.lapshin at mail.ru>;
George Rimar <
> grimar at accesssoftek.com>;
> > > > >> >> Robinson, Paul <paul.robinson at
sony.com>; Adrian Prantl <
> aprantl at apple.com>;
> > > > >> >> Jonas Devlieghere <jdevlieghere at
apple.com>; James Henderson <
> > > > >> >> jh7370.2008 at my.bristol.ac.uk>; Peter
Smith <
> Peter.Smith at arm.com>; David
> > > > >> >> Blaikie <dblaikie at gmail.com>;
Pavel Labath <pavel at labath.sk>
> > > > >> >> >> Subject: Re: [llvm-dev] Switch to
ld.bfd tombstone behavior
> by default
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> Hi Ray :)
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> While I understand the desire to
say "we've updated
> everything we've
> > > > >> >> found" the problem is that we
don't know that we've found even
> much of the
> > > > >> >> uses or can guarantee that people can
upgrade, say their gdb
> or breakpad,
> > > > >> >> as fast as their compiler. Even worse
it's a change in
> behavior without
> > > > >> >> much notice at all or even a particularly
friendly way to
> recognize what
> > > > >> >> they should do to fix it.
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> I still think that we do bfd locs
with a decent option to
> change for at
> > > > >> >> least the current release and sources and
then, once we're a
> little more
> > > > >> >> certain we have everything that might want
to parse dwarf (say
> by working
> > > > >> >> with dwarf-discuss), we can change the
default.
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> -eric
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> On Fri, Jul 24, 2020 at 7:33 PM
Fāng-ruì Sòng <
> maskray at google.com>
> > > > >> >> wrote:
> > > > >> >> >>
> > > > >> >> >> From my understanding the
breakpad bug was also only
> related to
> > > > >> >> .debug_line and has been fixed by
> > > > >> >>
> https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2317730
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> > a)
.debug_ranges&.debug_loc => -2, .debug_line => 0,
> other .debug_*
> > > > >> >> -> -1
> > > > >> >> >> > b)
.debug_ranges&.debug_loc => -2, other .debug_* => 0
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> I am still of the opinion that we
should just do a), not b).
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >>
> > > > >> >> >> On Fri, Jul 24, 2020 at 9:33 AM
Hans Wennborg <
> hans at chromium.org>
> > > > >> >> wrote:
> > > > >> >> >>
> > > > >> >> >> On Fri, Jul 24, 2020 at 5:12 PM
Fangrui Song <
> maskray at google.com>
> > > > >> >> wrote:
> > > > >> >> >> >
> > > > >> >> >> > On 2020-07-24, Hans Wennborg
via llvm-dev wrote:
> > > > >> >> >> > >Sounds good to me from a
release perspective.
> > > > >> >> >> >
> > > > >> >> >> > I think we need more input
from the triage of
> > > > >> >> >> >
> https://chromium-review.googlesource.com/c/chromium/src/+/2291352
> > > > >> >> >> > whether it is just
.debug_line or .debug_*
> > > > >> >> >>
> > > > >> >> >> The issue we hit in Chromium is
tracked here:
> > > > >> >> >>
> https://bugs.chromium.org/p/chromium/issues/detail?id=1105559 Doesn't
> > > > >> >> >> look like it has any more info at
the moment.
> > > > >> >> >>
> > > > >> >> >> >
> > > > >> >> >> > >On Fri, Jul 24, 2020 at
7:53 AM Eric Christopher via
> llvm-dev
> > > > >> >> >> > ><llvm-dev at
lists.llvm.org> wrote:
> > > > >> >> >> > >>
> > > > >> >> >> > >> Hi All,
> > > > >> >> >> > >>
> > > > >> >> >> > >> In general I think
we should adopt Dave's plan here.
> The number of
> > > > >> >> consumers that can (and have) been caught
off guard by this
> change is just
> > > > >> >> too high.
> > > > >> >> >> > >>
> > > > >> >> >> > >> At the very least I
think we should move this to opt
> in to the new
> > > > >> >> tombstoning behavior by default and at
most migrate to bfd's
> behavior for
> > > > >> >> both the current release and in the
current tree. If we want
> to make this
> > > > >> >> sort of change in the future by default I
think we're going to
> need to
> > > > >> >> provide release notes about this and do
aggressive outreach
> towards the
> > > > >> >> consumers we do know before making the
change. If anyone wants
> to drive
> > > > >> >> that effort I'll happily provide any
help or assistance in
> getting you in
> > > > >> >> touch with at least the consumers I know
about.
> > > > >> >> >> > >>
> > > > >> >> >> > >> This change and the
need for it is also probably worth
> a quick
> > > > >> >> message to dwarf-discuss at dwarfstd.org
as well. Most of the
> major
> > > > >> >> consumers and producers are on that list
and it's probably one
> of the
> > > > >> >> easiest ways to get this change out there.
> > > > >> >> >> > >>
> > > > >> >> >> > >> Any strong
objections to this path in the meantime?
> > > > >> >> >> > >>
> > > > >> >> >> > >> Thanks!
> > > > >> >> >> > >>
> > > > >> >> >> > >> -eric
> > > > >> >> >> > >>
> > > > >> >> >> > >> On Tue, Jul 21,
2020 at 12:18 PM David Blaikie via
> llvm-dev <
> > > > >> >> llvm-dev at lists.llvm.org> wrote:
> > > > >> >> >> > >>>
> > > > >> >> >> > >>> On Tue, Jul 21,
2020 at 5:34 AM Alexey Lapshin
> > > > >> >> >> > >>> <alapshin at
accesssoftek.com> wrote:
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > >On
Mon, Jul 20, 2020 at 10:32 AM Alexey Lapshin
> > > > >> >> >> > >>> >
><alapshin at accesssoftek.com> wrote:
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
>On Fri, Jul 17, 2020 at 1:55 PM Alexey Lapshin <
> > > > >> >> a.v.lapshin at mail.ru> wrote:
> > > > >> >> >> > >>> > >>
>>
> > > > >> >> >> > >>> > >>
>> >Пятница, 17 июля 2020, 19:42 +03:00 от David
> Blaikie <
> > > > >> >> dblaikie at gmail.com>:
> > > > >> >> >> > >>> > >>
>> >
> > > > >> >> >> > >>> > >>
>> >On Fri, Jul 17, 2020 at 12:03 AM Fangrui
> Song <
> > > > >> >> maskray at google.com> wrote:
> > > > >> >> >> > >>> > >>
>> >>
> > > > >> >> >> > >>> > >>
>> >> Thanks for the write-up!
> > > > >> >> >> > >>> > >>
>> >>
> > > > >> >> >> > >>> > >>
>> >> On 2020-07-16, David Blaikie wrote:
> > > > >> >> >> > >>> > >>
>> >> >In short: Perhaps we should switch lld to
> the
> > > > >> >> bfd-style tombstoning
> > > > >> >> >> > >>> > >>
>> >> >behavior for a release or two, letting
> users opt-in
> > > > >> >> to testing with the new
> > > > >> >> >> > >>> > >>
>> >> >-1/-2 tombstoning in the interim, before
> switching to
> > > > >> >> the new tombstone by
> > > > >> >> >> > >>> > >>
>> >> >default (while still having the flag to
> switch back
> > > > >> >> when users find
> > > > >> >> >> > >>> > >>
>> >> >surprise places that can't handle the new
> behavior).
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> >In long:
> > > > >> >> >> > >>> > >>
>> >> >https://reviews.llvm.org/D81784 and
> follow-on
> > > > >> >> patches modified the behavior
> > > > >> >> >> > >>> > >>
>> >> >of lld with regards to resolving
> relocations from
> > > > >> >> debug sections to dead
> > > > >> >> >> > >>> > >>
>> >> >code (either comdat deduplicated, or
> gc-sections use).
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> >A very quick summary of the situation:
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> >Original Behavior:
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> > - bfd: 1 for debug_ranges(0 would
> prematurely
> > > > >> >> terminate the list), 0
> > > > >> >> >> > >>> > >>
>> >> > elsewhere
> > > > >> >> >> > >>> > >>
>> >> > - gold/lld: 0+addend everywhere
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> >Limitations/bugs:
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> > - bfd/gold/lld
> > > > >> >> >> > >>> > >>
>> >> > - doesn't support 0 as a valid
> executable address
> > > > >> >> without ambiguities
> > > > >> >> >> > >>> > >>
>> >> > - gold/lld
> > > > >> >> >> > >>> > >>
>> >> > - ambiguities with large gc'd functions
> combined
> > > > >> >> with a .text mapping
> > > > >> >> >> > >>> > >>
>> >> > that starts in relative low addresses
> > > > >> >> >> > >>> > >>
>> >> > - premature debug_range termination with
> zero-length
> > > > >> >> functions (Clang
> > > > >> >> >> > >>> > >>
>> >> > produces these with
> __builtin_unreachable or
> > > > >> >> non-void return
> > > > >> >> >> > >>> > >>
>> >> >type functions
> > > > >> >> >> > >>> > >>
>> >> > without a return statement)
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> >New behavior:
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> > - -2 for DWARFv4 debug_loc, debug_ranges
> (-1 is a
> > > > >> >> base address specifier
> > > > >> >> >> > >>> > >>
>> >> > there)
> > > > >> >> >> > >>> > >>
>> >> > - -1 elsewhere
> > > > >> >> >> > >>> > >>
>> >> > - linker flag to customize to other
> values if desired
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> >Known issues:
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> > - lldb's line table parsing can't handle
> -1 well at
> > > > >> >> all (essentially
> > > > >> >> >> > >>> > >>
>> >> > unusable)
> > > > >> >> >> > >>> > >>
>> >>
> > > > >> >> >> > >>> > >>
>> >> Pavel Labath will fix this soon
> > > > >> >> https://reviews.llvm.org/D83957
> > > > >> >> >> > >>> > >>
>> >> This is an unhandled address-space
> wraparound problem.
> > > > >> >> >> > >>> > >>
>> >> This pattern is potentially common - and
> other
> > > > >> >> downstream DWARF
> > > > >> >> >> > >>> > >>
>> >> consumers might make similar line table
> handling
> > > > >> >> mistakes.
> > > > >> >> >> > >>> > >>
>> >
> > > > >> >> >> > >>> > >>
>> >That's the thing - I'm not sure we can
> really classify
> > > > >> >> them as
> > > > >> >> >> > >>> > >>
>> >"mistakes". I think bfd.ld's tombstoning
> behavior is
> > > > >> >> about the only
> > > > >> >> >> > >>> > >>
>> >thing we can reasonably say DWARF consumers
> should
> > > > >> >> /probably/ be
> > > > >> >> >> > >>> > >>
>> >expected to handle - and I'd imagine in many
> cases they
> > > > >> >> haven't been
> > > > >> >> >> > >>> > >>
>> >written intentionally to handle it, but
> whatever
> > > > >> >> behavior they have
> > > > >> >> >> > >>> > >>
>> >has accidentally been sufficient for their
> needs.
> > > > >> >> >> > >>> > >>
>>
> > > > >> >> >> > >>> > >>
>> >> > - gdb's line table parsing ends up with
> different
> > > > >> >> handling when breaking
> > > > >> >> >> > >>> > >>
>> >> > on gc'd functions (minor functionality
> issue)
> > > > >> >> >> > >>> > >>
>> >>
> > > > >> >> >> > >>> > >>
>> >> This is just a behavior difference, not
> affecting
> > > > >> >> users.
> > > > >> >> >> > >>> > >>
>> >> It did break a test if linked with LLD (gdb
> > > > >> >> intrinsically has lots of
> > > > >> >> >> > >>> > >>
>> >> failing tests even if built with GCC+GNU
> ld).
> > > > >> >> >> > >>> > >>
>> >>
> > > > >> >> >> > >>> > >>
>> >> Previous behavior (when an address is
> zero): a
> > > > >> >> breakpoint on a
> > > > >> >> >> > >>> > >>
>> >> --gc-sections discarded function will be
> redirected to
> > > > >> >> a larger line
> > > > >> >> >> > >>> > >>
>> >> number with debug info, even if that line
> can be an
> > > > >> >> unrelated different
> > > > >> >> >> > >>> > >>
>> >> function.
> > > > >> >> >> > >>> > >>
>> >> New behavior is that the breakpoint is on a
> > > > >> >> wrapped-around small address.
> > > > >> >> >> > >>> > >>
>> >>
> > > > >> >> >> > >>> > >>
>> >> GDB 9.3 will restore the previous behavior
> > > > >> >> >> > >>> > >>
>> >> (
> > > > >> >>
>
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=a8caed5d7faa639a1e6769eba551d15d8ddd9510
> > > > >> >> )
> > > > >> >> >> > >>> > >>
>> >>
> > > > >> >> >> > >>> > >>
>> >> >
> > > > >> >> >> > >>> > >>
>> >> >I think there's enough risk in this work
> (even given
> > > > >> >> the small number of
> > > > >> >> >> > >>> > >>
>> >> >bugs found so far), given there's a
> pretty wide array
> > > > >> >> of debug info
> > > > >> >> >> > >>> > >>
>> >> >consumers out there, that we should
> change lld's
> > > > >> >> default to match the
> > > > >> >> >> > >>> > >>
>> >> >long-lived bfd strategy. This would
> address my
> > > > >> >> original motivation for
> > > > >> >> >> > >>> > >>
>> >> >raising all this (empty functions
> prematurely
> > > > >> >> terminating the list), while
> > > > >> >> >> > >>> > >>
>> >> >letting users who want to experiment with
> it, or need
> > > > >> >> it (like Alexey), can
> > > > >> >> >> > >>> > >>
>> >> >opt-in to the -1/-2 behavior.
> > > > >> >> >> > >>> > >>
>> >>
> > > > >> >> >> > >>> > >>
>> >> I think we can only confidently say that
> there is
> > > > >> >> enough risk in using
> > > > >> >> >> > >>> > >>
>> >> tombstone value -1 in .debug_line, but I'd
> not say
> > > > >> >> tombstone value -1 in
> > > > >> >> >> > >>> > >>
>> >> other .debug_* can cause problems.
> > > > >> >> >> > >>> > >>
>>
> > > > >> >> >> > >>> > >>
>> >Given how many DWARF parsers we've had to
> cleanup or
> > > > >> >> migrate off in
> > > > >> >> >> > >>> > >>
>> >transitioning to DWARFv5 inside Google, I
> think that's a
> > > > >> >> fair bit of
> > > > >> >> >> > >>> > >>
>> >evidence the set of parsers isn't as
> narrow/closed as
> > > > >> >> we'd like and
> > > > >> >> >> > >>> > >>
>> >thus the number of places that might have
> issues isn't
> > > > >> >> known/easily
> > > > >> >> >> > >>> > >>
>> >exhaustively tested. So I'm not so concerned
> about the
> > > > >> >> bugs we've
> > > > >> >> >> > >>> > >>
>> >seen, but what that might indicate about the
> things that
> > > > >> >> we don't know
> > > > >> >> >> > >>> > >>
>> >about and can't test (because we don't know
> about them).
> > > > >> >> (of course,
> > > > >> >> >> > >>> > >>
>> >the flipside of that is that if we don't
> know about
> > > > >> >> them, we can't
> > > > >> >> >> > >>> > >>
>> >tell people who own them to go check if they
> work with
> > > > >> >> this opt-in
> > > > >> >> >> > >>> > >>
>> >feature - so they'll break whenever we turn
> this on by
> > > > >> >> default - but
> > > > >> >> >> > >>> > >>
>> >perhaps in the interim we can get at least a
> few big LLD
> > > > >> >> customers to
> > > > >> >> >> > >>> > >>
>> >deploy the feature and flush out some of the
> issues -
> > > > >> >> happy for Google
> > > > >> >> >> > >>> > >>
>> >to use this internally, hopefully we can
> encourage Apple
> > > > >> >> folks, Sony
> > > > >> >> >> > >>> > >>
>> >of course already has this semantic so
> nothing to find
> > > > >> >> there most
> > > > >> >> >> > >>> > >>
>> >likely - Chromium, maybe Firefox, etc)
> > > > >> >> >> > >>> > >>
>>
> > > > >> >> >> > >>> > >>
>> From the other side — when we already
> switched new
> > > > >> >> behavior ON as default —
> > > > >> >> >> > >>> > >>
>> then it is easier to discover all these
> unknown cases. We
> > > > >> >> have an option restoring
> > > > >> >> >> > >>> > >>
>> old behavior(which should be fine for all of
> the current
> > > > >> >> users). Thus, everybody
> > > > >> >> >> > >>> > >>
>> who needs old behavior is able to continue
> using it.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
>The cost to those users figuring out that it's
> a problem
> > > > >> >> and finding
> > > > >> >> >> > >>> > >>
>the flags/adding them, etc, is non-zero.
> > > > >> >> >> > >>> > >>
>But, yes, as I said - eventually some of that
> will happen,
> > > > >> >> sooner or
> > > > >> >> >> > >>> > >>
>later. But reducing how much of it happens is
> valuable too.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >> I
agree. Reducing the impact on customers is
> important.
> > > > >> >> >> > >>> > >>
Though in this case, as you said, it would be
> done
> > > > >> >> anyway(sooner or later).
> > > > >> >> >> > >>> > >>
Finally, I do not know what is more important:
> a) get
> > > > >> >> feedback faster with
> > > > >> >> >> > >>> > >>
the cost of involved users and requiring them to
> make efforts
> > > > >> >> >> > >>> > >>
to figure out the problem. or b) test with a
> smaller set of
> > > > >> >> users,
> > > > >> >> >> > >>> > >>
find more problematic cases, make new tombstone
> value to be
> > > > >> >> default later.
> > > > >> >> >> > >>> > >> I
am kind of preferring a). But, b) is more safe
> and is OK.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
>> That makes transition more understandable.
> All problems
> > > > >> >> would be explicitly seen. Then,
> > > > >> >> >> > >>> > >>
>> If we will know about new problems - we could
> adapt the
> > > > >> >> current solution. Similar to this
>suggestion:
> > > > >> >> >> > >>> > >>
>>
> > > > >> >> >> > >>> > >>
>> a) .debug_ranges&.debug_loc => -2,
> .debug_line => 0,
> > > > >> >> other .debug_* -> -1
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
>The issue I have with this is that it's
> reflective of the
> > > > >> >> known
> > > > >> >> >> > >>> > >>
>breakage in some tools that happen to have a
> quick
> > > > >> >> turnaround/feedback
> > > > >> >> >> > >>> > >>
>- and assumes all the other sections are
> significantly less
> > > > >> >> likely to
> > > > >> >> >> > >>> > >>
>have problems with this format - and I don't
> think we have
> > > > >> >> enough data
> > > > >> >> >> > >>> > >>
>to make that estimate.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
We could extend that rule when/if new problems
> become known.
> > > > >> >> >> > >>> > >>
Or, customers could fix their code or use option
> returning
> > > > >> >> old behavior
> > > > >> >> >> > >>> > >>
and it would probably not be necessary to extend
> the rule.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
>I think having a clear flag for bfd behavior or
> new
> > > > >> >> behavior would be
> > > > >> >> >> > >>> > >>
>good, keeping it bfd-by-default for now,
> evangelizing
> > > > >> >> (patch notes,
> > > > >> >> >> > >>> > >>
>outreach to other projects and tool developers)
> the new
> > > > >> >> functionality
> > > > >> >> >> > >>> > >>
>& then changing the default shortly after an
> LLVM release
> > > > >> >> (perhaps 12
> > > > >> >> >> > >>> > >>
>- but perhaps longer) to give it time to bake
> for
> > > > >> >> trunk-following
> > > > >> >> >> > >>> > >>
>users (ample time for them to pick it up on
> their schedule,
> > > > >> >> see what
> > > > >> >> >> > >>> > >>
>other tools might break, whether they're
> isolated enough to
> > > > >> >> expect a
> > > > >> >> >> > >>> > >>
>quick turnaround or whether we should get them
> updated and
> > > > >> >> then wait a
> > > > >> >> >> > >>> > >>
>while longer.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
yes, doing clear flag for bfd behavior or new
> behavior,
> > > > >> >> >> > >>> > >>
and keep it bfd-by-default for now would be OK
> and is more
> > > > >> >> safe.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
>> This would help while dwarf users are
> preparing their
> > > > >> >> code to the new solution.
> > > > >> >> >> > >>> > >>
>>
> > > > >> >> >> > >>> > >>
>> Is option restoring old behavior not enough
> to solve
> > > > >> >> problems caused by new tombstone
>value?
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
>I don't personally think it is enough - given
> some of the
> > > > >> >> known
> > > > >> >> >> > >>> > >>
>breakage - I think that points to a fair bit of
> reasonably
> > > > >> >> possible
> > > > >> >> >> > >>> > >>
>unknown breakage. Having a few big (like Apple
> and Google)
> > > > >> >> users
> > > > >> >> >> > >>> > >>
>switch over by default & potentially flush out
> a few more
> > > > >> >> issues (or
> > > > >> >> >> > >>> > >>
>not -but build confidence that there aren't
> more issues) I
> > > > >> >> think is an
> > > > >> >> >> > >>> > >>
>appropriate way to roll out a change like this.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
>(minor doubt: I wonder how well the bfd
> tombstoning works
> > > > >> >> in DWARFv5
> > > > >> >> >> > >>> > >>
>(rnglists/loclists) or in debug_range/debug_loc
> that uses
> > > > >> >> base address
> > > > >> >> >> > >>> > >>
>specifiers, where the zero-without-addend
> doesn't have a
> > > > >> >> chance to
> > > > >> >> >> > >>> > >>
>make an empty range (because it's not a
> start/end pair,
> > > > >> >> it's a
> > > > >> >> >> > >>> > >>
>start+offset pair - so the offset remains
> non-zero)... if
> > > > >> >> bfd
> > > > >> >> >> > >>> > >>
>tombstoning breaks DWARFv5 parsing (or base
> address
> > > > >> >> specifiers in
> > > > >> >> >> > >>> > >>
>range/loc) in common consumers I might be more
> inclined to
> > > > >> >> support
> > > > >> >> >> > >>> > >>
>enabling new tombstoning by default sooner
> (though if we
> > > > >> >> could enable
> > > > >> >> >> > >>> > >>
>it just for DWARFv5 that might be nice - but
> not practical,
> > > > >> >> since the
> > > > >> >> >> > >>> > >>
>linker doesn't know what DWARF it's linking &
> could be
> > > > >> >> linking
> > > > >> >> >> > >>> > >>
>multiple versions))
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >> -
From the point of view overlapping address
> ranges,
> > > > >> >> >> > >>> > >>
both bfd-solution(using 1) and old lld
> solution(using 0) are
> > > > >> >> equal.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > >gold
and lld's solution was a bit more complicated
> than using
> > > > >> >> zero (&
> > > > >> >> >> > >>> >
>bfd's solution uses 1 in debug_ranges, but not in
> other debug
> > > > >> >> >> > >>> >
>sections). lld and gold use 0+addend in ranges.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > right. I
am talking about the same(though assumed
> that context
> > > > >> >> >> > >>> > instead of
detailed explanation). it should be
> > > > >> >> "bfd-solution(using 1
> > > > >> >> >> > >>> > for
debug_ranges) and lld/gold solution(using
> 0+addend)".
> > > > >> >> >> > >>> > Thank you
for pointing into that.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > >0
alone in ranges
> > > > >> >> >> > >>> > >would
be more problematic than any of the other
> deployed
> > > > >> >> solutions,
> > > > >> >> >> > >>> >
>because it would lead to the debug_ranges
> contributions being
> > > > >> >> >> > >>> >
>terminated early (0, 0 terminates the list - so
> debug_ranges
> > > > >> >> for a CU
> > > > >> >> >> > >>> > >with
one gc'd function mid-way through the range
> list would
> > > > >> >> drop the
> > > > >> >> >> > >>> > >rest
of the range lit) - that's why bfd uses 1 for
> > > > >> >> debug_ranges rather
> > > > >> >> >> > >>> > >than 0
which it uses elsewhere (though the same
> fix should,
> > > > >> >> >> > >>> >
>technically, be applied to debug_loc too for the
> same reasons
> > > > >> >> as
> > > > >> >> >> > >>> >
>debug_ranges). the 0+addend approach of gold/lld
> works OK
> > > > >> >> (except for
> > > > >> >> >> > >>> > >the
low address overlap) except when there's a
> zero-length
> > > > >> >> function,
> > > > >> >> >> > >>> > >then
0+addend == 0 for the end of the range, andy
> ou get a 0,0
> > > > >> >> entry
> > > > >> >> >> > >>> > >with
the premature range list termination issues.
> > > > >> >> >> > >>> > >
> > > > >> >> >> > >>> > >>
for the "start+offset pair" case they could
> result in
> > > > >> >> overlapping address ranges.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > >Not
quite - the way bfd does things, at least in
> debug_ranges
> > > > >> >> without
> > > > >> >> >> > >>> > >base
address specifiers (bit hard to isolate for a
> consumer,
> > > > >> >> really -
> > > > >> >> >> > >>> > >though
they could special case zero base
> addresses... sort of)
> > > > >> >> the
> > > > >> >> >> > >>> > >empty
ranges are (1, 1) - they're actually empty.
> Whereas lld's
> > > > >> >> >> > >>> >
>approach ends up with (0+addend, 0+addend) which
> is usually
> > > > >> >> (0, size)
> > > > >> >> >> > >>> > >which
has issues with overlap on systems that have
> a valid low
> > > > >> >> address
> > > > >> >> >> > >>> > >range
(or if you have sufficiently large
> functions).
> > > > >> >> >> > >>> > >
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > There is a
case when size of address range is set
> not by
> > > > >> >> relocated value but by constant value:
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > DWARF5:
> > > > >> >> >> > >>> >
DW_RLE_startx_length
> > > > >> >> >> > >>> >
DW_RLE_start_length
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > {address
of deleted code, length}
> > > > >> >> >> > >>> > {address
of existing code, length}
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > DWARF4:
> > > > >> >> >> > >>> > base
address selection entry: {-1, address of
> deleted code}
> > > > >> >> >> > >>> > following
range list entry: {0, length}
> > > > >> >> >> > >>> > base
address selection entry: {-1, address of
> existing code}
> > > > >> >> >> > >>> > following
range list entry: {0, length}
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > In these
cases linker do not see the length of
> address range
> > > > >> >> >> > >>> > and could
not change it(using 1, or 0, or 0+addend)
> to be zero
> > > > >> >> length
> > > > >> >> >> > >>> > address
range. Thus address ranges could overlap.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > So in both
DWARF5 and DWARF4 there are cases when
> address ranges
> > > > >> >> >> > >>> > could be
overlapped and it could not be solved
> without new
> > > > >> >> meaning for tombstone value.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > >> -
From the point of view correct parsing of
> DWARF 5 - it
> > > > >> >> looks like they are equally good.
> > > > >> >> >> > >>> > >
> > > > >> >> >> > >>> > >Yep,
DWARFv5 is more complicated - bfd's approach
> would be
> > > > >> >> marginally
> > > > >> >> >> > >>> > >better
even for addr+offset encodings in
> debug_rnglists -
> > > > >> >> since it'd
> > > > >> >> >> > >>> > >always
produce a 0 for the addr, whereas
> gold/lld's behavior
> > > > >> >> can in
> > > > >> >> >> > >>> > >some
cases (eg: "void f1() { }
> __attribute__((nodebug)) void
> > > > >> >> f2() { }
> > > > >> >> >> > >>> > >void
f3() { }" with -fno-function-sections -
> you'll end up
> > > > >> >> with the
> > > > >> >> >> > >>> > >range
for f3 starting at a non-zero addend - this
> debug info
> > > > >> >> will be
> > > > >> >> >> > >>> > >worse
for bfd/ld (if the DWARF for this object was
> included,
> > > > >> >> but the
> > > > >> >> >> > >>> > >whole
.text section was discarded (eg: maybe
> there's a global
> > > > >> >> variable
> > > > >> >> >> > >>> > >in
this object which is linked in, but -gc-section
> is still
> > > > >> >> able to
> > > > >> >> >> > >>> >
>discard the whole .text section as unreferenced)
> then
> > > > >> >> gold/lld's
> > > > >> >> >> > >>> >
>approach would make f3 unidentifiable as dead code
> (because it
> > > > >> >> doesn't
> > > > >> >> >> > >>> > >have a
tombstone start) but bfd's approach would
> work
> > > > >> >> (assuming zero
> > > > >> >> >> > >>> >
>isn't a valid address))
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > I see.
thanks.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > >
> > > > >> >> >> > >>> > >> -
From the point of view correct parsing of
> DWARF 4 - it
> > > > >> >> looks bfd-solution(using 1)
> > > > >> >> >> > >>> > >>
is better than the old lld solution(using 0).
> When range
> > > > >> >> list entry contains
> > > > >> >> >> > >>> > >>
addresses(start+end) which should be relocated
> and for the
> > > > >> >> zero-length functions,
> > > > >> >> >> > >>> > >>
bfd-solution would result in range list entry:
> {1, 1}, while
> > > > >> >> old lld solution
> > > > >> >> >> > >>> > >>
would result in {0, 0}, and match with the end
> of list entry.
> > > > >> >> >> > >>> > >>
That is the original problem that started this
> thread.
> > > > >> >> >> > >>> > >
> > > > >> >> >> > >>> > >Only
comes up for zero-length functions, because
> gold/lld's
> > > > >> >> approach
> > > > >> >> >> > >>> > >was
0+addend, not straight 0.
> > > > >> >> >> > >>> > >
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > right. for
zero-length functions.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > >>
Though it looks like there still exist case when
> range list
> > > > >> >> could be terminated earlier:
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
base address selection entry: {-1, address of
> deleted code}
> > > > >> >> >> > >>> > >>
following range list entry: {0, 0} << points to
> the same
> > > > >> >> address as set by base address selection
entry and has zero
> size.
> > > > >> >> >> > >>> > >
> > > > >> >> >> > >>> >
>That's a bug in the producer (though a good point
> - I've
> > > > >> >> probably made
> > > > >> >> >> > >>> > >that
bug in LLVM) - the linker can't solve that
> problem, since
> > > > >> >> the
> > > > >> >> >> > >>> > >linker
can't touch the literal unrelocated 0, 0.
> > > > >> >> >> > >>> > >
> > > > >> >> >> > >>> > >>
after linker resolved relocations it would look
> like this,
> > > > >> >> for bfd case:
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
base address selection entry: {-1, 1}
> > > > >> >> >> > >>> > >>
following range list entry: {0, 0} <<<<<<<<<<<
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
So there still exists {0,0} entry which could be
> considered
> > > > >> >> as the end of list entry.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >>
But old lld solution has the same problem, thus
> it would not
> > > > >> >> be new.
> > > > >> >> >> > >>> > >>
> > > > >> >> >> > >>> > >> -
Additionally, AFAIK gdb has special processing
> for
> > > > >> >> overlapped address
> > > > >> >> >> > >>> > >>
ranges starting from 0. Using bfd tombstone
> value could
> > > > >> >> break that processing - I would check it.
> > > > >> >> >> > >>> > >
> > > > >> >> >> > >>> > >Not
sure I understand - presumably gdb's special
> processing is
> > > > >> >> >> > >>> >
>intended to work with bfd's tombstoning, since
> it's been the
> > > > >> >> most
> > > > >> >> >> > >>> >
>common/prolific unix linker, the one intended to
> work with gdb
> > > > >> >> (they
> > > > >> >> >> > >>> > >exist
in the same repository) for decades, right?
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > I think I
misunderstood this: "I wonder how well
> the bfd
> > > > >> >> tombstoning works in DWARFv5
> > > > >> >> >> > >>> >
(rnglists/loclists)". I read it as we would like to
> use bfd
> > > > >> >> tombstoning(1 for ranges)
> > > > >> >> >> > >>> > for
rnglists/loclists also.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > So, right.
bfd's tombstoning works correctly with
> gdb until 1
> > > > >> >> is not used
> > > > >> >> >> > >>> > for
rnglists/loclists as tombstone value.
> > > > >> >> >> > >>>
> > > > >> >> >> > >>> Not quite
parsing this, but I think we're on the same
> page - that
> > > > >> >> >> > >>> bfd's
tombstoning "1 for debug_ranges/debug_loc, 0
> (not 0+addend,
> > > > >> >> but
> > > > >> >> >> > >>> absolute 0) for
everything else (including
> debug_loclists and
> > > > >> >> >> > >>>
debug_rnglists)" is probably the most likely to work
> for gdb,
> > > > >> >> since
> > > > >> >> >> > >>> it's been
deployed for a long time.
> > > > >> >> >> > >>>
> > > > >> >> >> > >>> - Dave
> > > > >> >> >> > >>>
> > > > >> >> >> > >>> PS: Fair point
about base address specifiers being
> able to
> > > > >> >> produce 0,0
> > > > >> >> >> > >>> entries -
wouldn't mind fixing that in LLVM if I knew
> of an easy
> > > > >> >> way
> > > > >> >> >> > >>> to test at
compile-time whether the difference
> between two labels
> > > > >> >> was
> > > > >> >> >> > >>> zero, then skip
that entry in the lists entirely.
> Would save
> > > > >> >> space and
> > > > >> >> >> > >>> address the
original issue I had with debug_ranges
> terminating
> > > > >> >> early.
> > > > >> >> >> > >>> (should've
thought about that much earlier than my
> more alarmist
> > > > >> >> "oh
> > > > >> >> >> > >>> deer, we need
to fundamentally change how linkers
> resolve
> > > > >> >> relocations
> > > > >> >> >> > >>> because
everything's been broken and we just didn't
> realize it" -
> > > > >> >> >> > >>> fixing the
compiler not to produce zero-length ranges
> would've
> > > > >> >> been
> > > > >> >> >> > >>> less risky
& probably still worth doing - though
> addressing the
> > > > >> >> >> > >>> broader issue
to help with your situation of 0 as a
> valid address
> > > > >> >> I
> > > > >> >> >> > >>> think is still
good too)
> > > > >> >> >> > >>>
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > Alexey.
> > > > >> >> >> > >>> >
> > > > >> >> >> > >>> > > >
>> With consideration for satefy for the
> upcoming
> > > > >> >> release/11.x, we can make
> > > > >> >> >> > >>> > > >
>> two choices:
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> a) .debug_ranges&.debug_loc => -2,
> .debug_line => 0,
> > > > >> >> other .debug_* -> -1
> > > > >> >> >> > >>> > > >
>> b) .debug_ranges&.debug_loc => -2, other
> .debug_* => 0
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> Delaying .debug_line => -1 for one or two
> release sounds
> > > > >> >> good to me.
> > > > >> >> >> > >>> > > >
>> So LLD 11 or 12 linked binaries can be
> debugged by LLDB
> > > > >> >> 10. This is a
> > > > >> >> >> > >>> > > >
>> nice property.
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> This write-up proposes b), but I'd say a) is
> likely
> > > > >> >> sufficient. With the
> > > > >> >> >> > >>> > > >
>> available information, I cannot yet say that
> a) will
> > > > >> >> have more risk.
> > > > >> >> >> > >>> > > >
>
> > > > >> >> >> > >>> > > >
>Risk is about the unknowns - and it still
> seems like a lot
> > > > >> >> of
> > > > >> >> >> > >>> > > >
>unknowns. While there are probably many more
> consumers
> > > > >> >> that read
> > > > >> >> >> > >>> > > >
>.debug_line than other sections, reading
> debug_info (for
> > > > >> >> instance) is
> > > > >> >> >> > >>> > > >
>necessary for inline frames in symbolizing -
> still
> > > > >> >> probably one of the
> > > > >> >> >> > >>> > > >
>most common uses of DWARF I'd guess. (what
> about stack
> > > > >> >> unwinding using
> > > > >> >> >> > >>> > > >
>debug_frame? that'd worry me a bit if anyone
> got /that/
> > > > >> >> wrong because
> > > > >> >> >> > >>> > > >
>of this change)
> > > > >> >> >> > >>> > > >
>
> > > > >> >> >> > >>> > > >
>> > - chromium/firefox have some tools that
> were broken:
> > > > >> >> >> > >>> > > >
>> >
> > > > >> >>
> https://bugs.chromium.org/p/chromium/issues/detail?id=1102223#c5
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> This is potentially related to other
> .debug_* (not
> > > > >> >> .debug_line)
> > > > >> >> >> > >>> > > >
>> I hope Chromium developers can chime in
> here:) The
> > > > >> >> breakage was
> > > > >> >> >> > >>> > > >
>> unfortunate but I don't know how we could
> have avoided
> > > > >> >> that. IMHO this
> > > > >> >> >> > >>> > > >
>> is no different from "clang started to emit
> a new
> > > > >> >> DW_FORM_* and a
> > > > >> >> >> > >>> > > >
>> postprocessing tool of .debug chokes on
> that" Whether we
> > > > >> >> want to
> > > > >> >> >> > >>> > > >
>> suppress that particular DW_FORM_*
> definitely should
> > > > >> >> depend on how
> > > > >> >> >> > >>> > > >
>> likely it can cause problems, but we can't
> yet say we
> > > > >> >> have to hold off
> > > > >> >> >> > >>> > > >
>> on a feature for a solved (precisely,
> mitigated) problem.
> > > > >> >> >> > >>> > > >
>
> > > > >> >> >> > >>> > > >
>LLVM has no custom forms and I'd be super
> cautious about
> > > > >> >> adding any
> > > > >> >> >> > >>> > > >
>that were on by default because of how bad
> that breakage
> > > > >> >> would be.
> > > > >> >> >> > >>> > > >
>
> > > > >> >> >> > >>> > > >
>I'm not so concerned about the problems we
> know - but what
> > > > >> >> they tell
> > > > >> >> >> > >>> > > >
>us about the problems that might arise from
> use cases we
> > > > >> >> don't know.
> > > > >> >> >> > >>> > > >
>All the other projects out there that might
> have custom
> > > > >> >> DWARF parsers
> > > > >> >> >> > >>> > > >
>to do some ad-hoc things.
> > > > >> >> >> > >>> > > >
>
> > > > >> >> >> > >>> > > >
>(also, ultimately - given how far-reaching
> this is, I
> > > > >> >> think we'll want
> > > > >> >> >> > >>> > > >
>some tidier flags that are more user-focussed.
> I'd hope
> > > > >> >> for a flag
> > > > >> >> >> > >>> > > >
>that gives BFD-like semantics (though I'd be
> OK with
> > > > >> >> fixing debug_loc
> > > > >> >> >> > >>> > > >
>(using 1 instead of 0) to work the same as
> debug_ranges
> > > > >> >> while we're
> > > > >> >> >> > >>> > > >
>there - a minor divergence from BFD, but
> highly likely to
> > > > >> >> not cause
> > > > >> >> >> > >>> > > >
>problems/fall out naturally from a simple
> implementation
> > > > >> >> of parsing
> > > > >> >> >> > >>> > > >
>that section) - something that's been in-use
> and tested by
> > > > >> >> basically
> > > > >> >> >> > >>> > > >
>everyone for decades. And another flag for the
> new
> > > > >> >> semantics (-2 for
> > > > >> >> >> > >>> > > >
>debug_loc/debug_ranges, -1 everywhere else).
> Customizable
> > > > >> >> per-section
> > > > >> >> >> > >>> > > >
>expression-based support I think is a recipe
> for platform
> > > > >> >> divergence &
> > > > >> >> >> > >>> > > >
>I'd rather it not be available/supported at
> all, but if
> > > > >> >> you really
> > > > >> >> >> > >>> > > >
>want to keep it in, I'd at least rather it not
> be the
> > > > >> >> feature we
> > > > >> >> >> > >>> > > >
>promote to users about how they can test/opt
> in/out of the
> > > > >> >> behavior
> > > > >> >> >> > >>> > > >
>when they're seeing breakages or want to test
> the future
> > > > >> >> semantics)
> > > > >> >> >> > >>> > > >
>
> > > > >> >> >> > >>> > > >
>> >I'm not sure how to get the word out to
> DWARF consumers
> > > > >> >> that they should
> > > > >> >> >> > >>> > > >
>> >consider this new experimental behavior.
> Ray's done a
> > > > >> >> good job
> > > > >> >> >> > >>> > > >
>> >evangelizing/discussing this with gdb and
> lldb at least
> > > > >> >> - and of course
> > > > >> >> >> > >>> > > >
>> >having turned it on by default briefly has
> found some
> > > > >> >> users (like Chromium)
> > > > >> >> >> > >>> > > >
>> >that we probably wouldn't have found no
> matter how long
> > > > >> >> we left this as an
> > > > >> >> >> > >>> > > >
>> >experimental option... so some things are
> going to
> > > > >> >> break when we switch no
> > > > >> >> >> > >>> > > >
>> >matter what.
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> Thank you for following up with some GNU
> folks on their
> > > > >> >> lists!
> > > > >> >> >> > >>> > > >
>> If folks want to follow along the thread:
> > > > >> >> >> > >>> > > >
>>
> > > > >> >>
> https://sourceware.org/pipermail/binutils/2020-June/111376.html
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> We have informed binutils, elfutils-devel
> (elfutils has
> > > > >> >> a few debug
> > > > >> >> >> > >>> > > >
>> tools) and gdb. I don't recall that anyone
> has thought
> > > > >> >> about problems
> > > > >> >> >> > >>> > > >
>> with a tombstone value.
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> >
> > > > >> >> >> > >>> > > >
>> >P.S: Sony's already been using the -1
> technique with
> > > > >> >> their debugger and
> > > > >> >> >> > >>> > > >
>> >linker for a while, so they may want to
> keep this on by
> > > > >> >> default for SCE -
> > > > >> >> >> > >>> > > >
>> >but I'm not sure how to do that in-tree.
> > > > >> >> >> > >>> > > >
>> >
> > > > >> >> >> > >>> > > >
>> >
> > > > >> >> >> > >>> > > >
>> >Clang doesn't know which lld
> > > > >> >> >> > >>> > > >
>> >version it's running, so whether the flag
> can be
> > > > >> >> specified, I would think?
> > > > >> >> >> > >>> > > >
>> >(so it'd be hard to have Clang go "if SCE
> and LLD, pass
> > > > >> >> the flag to use
> > > > >> >> >> > >>> > > >
>> >-1", I think) - if there is a way to make
> that decision
> > > > >> >> in the compiler
> > > > >> >> >> > >>> > > >
>.> >driver+linker, then we'd have a question
> of "default
> > > > >> >> new behavior except
> > > > >> >> >> > >>> > > >
>> >when tuning for LLDB and GDB" or "default
> bfd behavior
> > > > >> >> except when tuning
> > > > >> >> >> > >>> > > >
>> >for SCE".
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> I've been involed in another thread on
> SHF_LINK_ORDER (
> > > > >> >>
https://sourceware.org/pipermail/binutils/2020-
> > > > >> >> >> > >>> > > >
>July/112415.html ).
> > > > >> >> >> > >>> > > >
>> We may need a way to tell codegen about the
> used linker.
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> pcc proposed -mbinutils-version= - This is
> nice in that
> > > > >> >> some MC
> > > > >> >> >> > >>> > > >
>> decisions related to -fno-integrated-as can
> use this
> > > > >> >> option as well.
> > > > >> >> >> > >>> > > >
>> jyknight proposed -mlinker-version= and
> syntax like
> > > > >> >> -fuse-ld=bfd:2.34
> > > > >> >> >> > >>> > > >
>>
> > > > >> >> >> > >>> > > >
>> This may get more complex if the generated
> object file
> > > > >> >> want to be linked
> > > > >> >> >> > >>> > > >
>> with more than one linker. This discussion
> probably
> > > > >> >> deserves its own
> > > > >> >> >> > >>> > > >
>> thread.
> > > > >> >> >> > >>> > > >
> > > > >> _______________________________________________
> > > > >> LLVM Developers mailing list
> > > > >> llvm-dev at lists.llvm.org
> > > > >>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > > > >
> > > > > _______________________________________________
> > > > > LLVM Developers mailing list
> > > > > llvm-dev at lists.llvm.org
> > > > >
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
> >
> > --
> > 宋方睿
>
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