[llvm-dev] [LLD] Support DWARF64, debug_info "sorting"

Wed Nov 11 16:52:22 PST 2020

Thanks for feedback.

I agree with patch and numbers this will be a more concrete discussion, but I wanted to judge overall receptiveness to this approach and see maybe there was a better way.

"Whilst the majority of objects will only have a single CU in them, there will be exceptions (LTO-generated objects, -r merged objects etc), so we do need to consider this approach."
David can you elaborate under which conditions LTO-generated objects will have a mix of DWARF32/64 in same .debug_info? Looking at how dwarf64 was implemented same flag will be used for the entirety of the dwarf output, even if multiple CUs are included.

I think if object does have a mix of CUs that are 32/64, linker can do a best effort ordering, and output a warning. My approach to this is from covering common cases while solving a problem with relocations overflow in large libraries/binaries.

@Fangrui Song<mailto:maskray at google.com>
That's a good point with relocations. Although is it always a guarantee a first one will be representative of entire relocation record?
For debug_info even with DWARF32 there can be 64bit relocations.
0000000000000c57  0000001800000001 R_X86_64_64            0000000000000000 .text._"some_mangeled_name" + 0

On one hand since this is only applicable for when DWARF64 is used, special option would be the way to go. Although the user will need to be aware of yet another LLD option. Maybe an error when relocations overflow occur can be modified to display this option along with -fdebug-types-section

Thank You
Alex
________________________________
From: Fangrui Song <maskray at google.com>
Sent: Wednesday, November 11, 2020 2:59 PM
To: Robinson, Paul <paul.robinson at sony.com>; James Henderson <jh7370.2008 at my.bristol.ac.uk>
Cc: David Blaikie <dblaikie at gmail.com>; Eric Christopher <echristo at gmail.com>; Alexander Yermolovich <ayermolo at fb.com>; Igor Kudrin <ikudrin at accesssoftek.com>; 'llvm-dev at lists.llvm.org' <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [LLD] Support DWARF64, debug_info "sorting"

(Adding back Cc: which got dropped)

> (Igor - I don't know what happened, but your email split the mail thread in gmail for me.)

The problem is that https://lists.llvm.org/pipermail/llvm-dev/2020-November/146528.html  does not have an In-Reply-To: header.
Added Igor to the Cc: list.

If we go down the route (sorting DWARF64 after DWARF32), compared with a
lightweight parse, I'd prefer the relocation based approach: if a .debug_* has
an 64-bit absolute relocation type (e.g. R_X86_64_64).

In LLD, for an input section, we don't know its associated SHT_REL[A] section.
So when adding an orphan section we would have another loop iterating
over inputSections. We can reuse the dependentSections to have this
piece of information (generalizing the existing special case for -r/--emit-relocs)

> This way as long as DWARF32 sections don't themselves go over 4gigs, the final binary can contain debug information that exceeds 4gig.
> Which I think will be the common case.

I would not expect the linker behaves differently when linking a few additional sections change the behavior so drastically
in a not-easily-explainable way. This deserves a dedicated linker option (see below, I have a concern about the inconsistency
with an input section description)

On 2020-11-11, Robinson, Paul via llvm-dev wrote:
>
>
>> -----Original Message-----
>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of David
>> Blaikie via llvm-dev
>> Sent: Wednesday, November 11, 2020 12:46 PM
>> To: James Henderson <jh7370.2008 at my.bristol.ac.uk>
>> Cc: llvm-dev at lists.llvm.org
>> Subject: Re: [llvm-dev] [LLD] Support DWARF64, debug_info "sorting"
>>
>> On Wed, Nov 11, 2020 at 12:55 AM James Henderson
>> <jh7370.2008 at my.bristol.ac.uk> wrote:
>> >
>> >
>> >
>> > On Wed, 11 Nov 2020 at 05:41, David Blaikie <dblaikie at gmail.com> wrote:
>> >>
>> >> +James for context too (always good to include the folks from the
>> >> original threads for continuity)
>> >>
>> >> Yeah, my general attitude there was just twofold, one that the
>> >> discussion had strayed fairly far from the review (so interested
>> >> parties might not see it, both because it's a targeted review thread
>> >> on the noisy llvm-commits, and because fo the title not having much
>> >> connection to the discussion) and it seemed to be somewhat
>> >> abstract/general - and there's a balance there. "We should do this
>> >> because I need it" (we shouldn't be implementing features for
>> >> especially niche use cases/if they don't generalize) isn't always a
>> >> compelling motivation but "we should do this because someone might
>> >> need it" isn't either (we shouldn't be implementing features that have
>> >> no users).
>> >>
>> >> The major drawback in sorting, is the need to parse DWARF, even a
>> >> little bit of it (only the first 4 bytes of a section to tell which
>> >> version it is - first 12 if you want to be able to jump over
>> >> contributions and check /all/ contributions coming from a given input
>> >> object file (it might contain a combination of DWARFv4 and DWARFv5)
>> >> and then the hairy uncertainty of which sections to check (do you
>> >> check them all? well, all the ones with length prefixes that
>> >> communicate DWARF32/64 - some sections don't
>> >> (debug_ranges/loc/str/macro for instance, if I recall correctly)...
>> >> and if something has some 4 and 5, does it get sorted to the start? I
>> >> guess so.
>> >>
>> > I assume this comment is meant to say DWARF32/DWARF64, not DWARFv4 and
>> DWARFv5, as the DWARF version (as opposed to the 32/64 bit style) is
>> irrelevant to this, I believe, at least for the current known DWARF
>> standards.
>>
>> Yep! thanks for the correction - had a lot of DWARFv4/v5 on my mind
>> due to other work, so got the terms jumbled up.
>>
>> >  Whilst the majority of objects will only have a single CU in them,
>> there will be exceptions (LTO-generated objects, -r merged objects etc),
>> so we do need to consider this approach. Mixtures would certainly be
>> possible, and there's no guarantee the CUs would be in a nice order with
>> 32-bit blocks before 64-bit blocks. If I follow this to its full
>> conclusion, you could potentially end up with a single .debug_info
>> (.debug_line, .debug_rnglists etc) input section with a mixture of
>> DWARF32/DWARF64 sub-sections, which, if following the reordering approach,
>> the linker might have to split up internally in order to rearrange (aside
>> - there's some interesting crossover with ideas I've been considering
>> regarding the Fragmented DWARF topic discussed elsewhere).

I'm still learning the internals but would expect that mixed DWARF32/DWARF64 is
a problem for LTO. A reloctable link (-r) can combine DWARF32/DWARF64 object
files and potentially nullify the aforementioned relocation based approach
(we probably just want to check the first relocation to save time;
if we link DWARF64 before DWARF32 we may create a .debug_info
which looks like DWARF64 but is actually restricted by DWARF32 relocations)

>> I think given this is a pragmatic feature I'd be inclined to say "eh,
>> sort any input object containing at least one DWARFv4 contribution
>> before input objects not containing any v4 contribution" - if that
>> doesn't solve some real world issues/situations, I'd be willing to
>> revisit this direction/consider more invasive/expensive solutions.
>
>I was under the impression that *object* order meant a lot to people,
>and changing that would have all sorts of unpleasant fallout.  If I'm
>remember that correctly, sorting DWARF sections really should be its
>own thing, separate from object order.  Shoving DWARF-64 sections to
>the end of the line seems like it would be less problematic than
>reordering entire objects, if the linker can handle that in some
>reasonably efficient way.
>--paulr

This behavior does add some inconsistency to the system:

For an output section description .debug_info 0 : { *(.debug_info) } ,
should the linker sort DWARF32 and DWARF64 components? It it does, the behavior
will be inconsistent with other input section descriptions *(foo)

If there is a magic keyword, say, SORT_BY_MAGIC_DEBUG, and the internal
linker script does something similar to

   *(SORT_BY_MAGIC_DEBUG(.debug_info))

then the system is still consistent.

>>
>> Though, as Eric said - some of this conversation might be better had
>> in terms of concrete patches with concrete performance measurements.
>>
>> > Maybe the solution here would be to change producers to produce separate
>> .debug_info sections containing DWARF32 and DWARF64.
>>
>> That'd involve changing how certain objects were generated - if that's
>> possible, then I assume it'd be possible to change that generation to
>> use DWARF64 anyway - in the limit: one might have precompiled binaries
>> with debug info that one cannot recompile, so any new format options I
>> doubt are able to address the original/likely use case for this
>> functionality.
>>
>> > I used the -u option more as an example that it might be possible to get
>> things to work the way we want without needing to have the linker do the
>> work. The linker currently has a --symbol-ordering-file option which can
>> be used to request an order for the specified list of symbols. The linker
>> does this by rearranging the input sections to get as close as it can to
>> the requested order. We could maybe implement the same on a file/section
>> basis. It would avoid needing to read the sections themselves, but doesn't
>> solve the "what to do about mixed single input" case directly (though
>> might allow the user to dodge the decision at least).

Yeah, --symbol-ordering-file applies on both global and local symbols.
Unfortunately no symbols are defined relative to .debug_* sections
(if we don't consider the STT_SECTION symbols, which cannot be used
anyway because .debug_* do not have unique names).

(The usage of -u still requires the user to add archives (they want to
change order) before other object files. In LLD this requires https://reviews.llvm.org/D81052  )

>> > Other ideas I had involved changing the section header properties.
>> Currently DWARF sections are all SHT_PROGBITS, but we could change that to
>> e.g. SHT_DWARF_32 or similar, and/or use the sh_info field to contain a
>> value that would indicate the 32/64 bit nature. I'm not convinced by these
>> ideas though, as a) I don't know if it translates well to other non-ELF
>> formats, and b) we can't really control the producers of DWARF at this
>> stage to conform.

Inventing a new section type is not bad at a first glance. Leveraging it
can remove the inconsistency in the system as well.
Unfortunately linker scripts (as implemented by GNU ld and emulated by LLD)
don't provide a way to match input sections by section type.

If we are going to have many thoughts on the linker side design, might
be worth asking on https://groups.google.com/g/generic-abi  as well.
That would have to a separate discussion because the list is moderated
and users who haven't joined the group cannot reply there. If there are
opinions, we can share them with llvm-dev.

>> > It would be nice if there was a solution that could be consistently
>> applied across all build systems, linkers and DWARF producers. I don't
>> have one as yet though.
>> >
>> >>
>> >> On Tue, Nov 10, 2020 at 9:30 PM Eric Christopher via llvm-dev
>> >> <llvm-dev at lists.llvm.org> wrote:
>> >> >
>> >> >
>> >> >
>> >> > On Wed, Nov 11, 2020 at 12:19 AM Alexander Yermolovich via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >> >>
>> >> >> This year Igor Kudrin put in a lot of work in enabling DWARF64
>> support in LLVM. At Facebook we are looking into it as one of the options
>> for handling debug information over 4gigs in production environment. One
>> concern is that due to mix of third party libraries and llvm compiled code
>> the final library/binary will have a mix of CU that are DWARF32/64. This
>> is supported by DWARF format. With this mix it is possible that even with
>> DWARF64 enabled one can still encounter relocation overflows errors in LLD
>> if DWARF32 sections happen to be processed towards the end.
>> >> >>
>> >> >> One proposal that was discussed in
>> https://reviews.llvm.org/D87011
>> RvKRSx!pnSYzjQly_yuEU-ng7OYd8nr3h3tSYOjeCwnH7cr3hA73rA8aVlNzOfQPo9Ue7WdoQ$
>> , is to modify LLD linker to arrange debug_info sections so that DWARF32
>> comes first, and DWARF64 after them. This way as long as DWARF32 sections
>> don't themselves go over 4gigs, the final binary can contain debug
>> information that exceeds 4gig. Which I think will be the common case.
>> >> >>
>> >> >> An alternative approach that was proposed by James Henderson is for
>> build system to take care of it, and to use -u to enforce order.
>> >> >
>> >> >
>> >> > +Fangrui Song here for thread visibility
>> >> >
>> >> > Of these two approaches I think that the linker sorting is probably
>> the one I'd go with for the reasons you list below - I'm particularly
>> sympathetic to not wanting the unintended consequences of using -u here :)
>> >> >
>> >> > I do worry about slowing down general debug links so a "debug info
>> sorting" option may make sense, or it may not be worth it after measuring
>> the speed difference.
>> >> >
>> >> > Thanks for bringing this up on the list! :)
>> >> >
>> >> > -eric
>> >> >
>> >> >>
>> >> >>
>> >> >> As, I would imagine, most projects of scale are using configurable
>> build system that pulls in all the various dependencies automatically in a
>> multi-language environment. I think the alternative approach will be more
>> fragile than modifying LLD as it relies on a more complex system, and each
>> customer of LLD will have to implement this "sorting" in their own build
>> systems. The use of -u also kind of abuses this flag, and might have
>> unintended consequences. As was pointed out by Wen Lei.
>> >> >> From overhead perspective we only need to access few bytes of DWARF
>> to determine if it's 32 or 64 bits. Customers who need DWARF64, already
>> accept the overhead that it entails.
>> >> >>
>> >> >> Any thoughts?
>> >> >>
>> >> >> Thank You
>> >> >> Alex
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