[llvm-dev] [Debuginfo][DWARF][LLD] Remove obsolete debug info in lld.
Alexey Lapshin via llvm-dev
llvm-dev at lists.llvm.org
Mon Aug 3 08:32:07 PDT 2020
Hi Eric, please
On 31.07.2020 22:02, Eric Christopher wrote:
> Hi Alexey,
>
> On Fri, Jul 31, 2020 at 4:02 AM Alexey Lapshin via llvm-dev
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>
> On 28.07.2020 19:28, David Blaikie wrote:
> > On Tue, Jul 28, 2020 at 8:55 AM Alexey Lapshin
> <avl.lapshin at gmail.com <mailto:avl.lapshin at gmail.com>> wrote:
> >>
> >> On 28.07.2020 10:29, David Blaikie via llvm-dev wrote:
> >>> On Fri, Jun 26, 2020 at 9:28 AM Alexey Lapshin
> >>> <alapshin at accesssoftek.com <mailto:alapshin at accesssoftek.com>>
> wrote:
> >>>>>>>>>>>> This idea goes in another direction than fragmenting
> dwarf
> >>>>>>>>>>>> using elf sections&tricks. It seems to me that the
> cost of fragmenting is too high.
> >>>>>>>>>>> I tend to agree - but I'm sort of leaning towards
> trying to use object
> >>>>>>>>>>> features as much as possible, then implementing just
> enough custom
> >>>>>>>>>>> handling in the linker to recoup overhead, etc. (eg:
> add some kind of
> >>>>>>>>>>> small header/brief description that makes it easy for
> the linker to
> >>>>>>>>>>> slice-and-dice - but hopefully a domain-specific such
> header can be a
> >>>>>>>>>>> bit more compact than the fully general ELF form)
> >>>>>>>>>> I think this indeed should be implemented and evaluated.
> >>>>>>>>>> So that various approaches could be compared.
> >>>>>>>>>>
> >>>>>>>>>>>> It is not only the sizes of structures describing
> fragments but also the complexity
> >>>>>>>>>>>> of tools that should be taught to work with
> fragmented DWARF.
> >>>>>>>>>>>> (f.e. llvm-dwarfdump applied to object file should be
> able to read fragmented DWARF,
> >>>>>>>>>>>> but applied to linked executable it should work with
> non-fragmented DWARF).
> >>>>>>>>>>>> That idea is for the tool which works the same way as
> dsymutil ODR.
> >>>>>>>>>>>>
> >>>>>>>>>>>> I will shortly describe the idea of making DWARF be
> easier processed by dsymutil/DWARFLinker:
> >>>>>>>>>>>>
> >>>>>>>>>>>> The idea is to have only one "type table" per object
> file(special section .debug_types_table).
> >>>>>>>>>>>> This "type table" would contain all types.
> >>>>>>>>>>>> There could be a special type of reference -
> type_offset - that offset points into the type table.
> >>>>>>>>>>>> Basic types could always be placed into the start of
> "type table" thus, offsets to basic types
> >>>>>>>>>>>> most often would be 1 byte. There also would be a
> special kind of reference - reference inside the type.
> >>>>>>>>>>>> Type units sig8 system - would not be used to
> reference types.
> >>>>>>>>>>>>
> >>>>>>>>>>>> Types deduplication is assumed to be done, not by
> linker mechanism for COMDAT,
> >>>>>>>>>>>> but by a tool like dsymutil. This tool would create
> resulting .debug_types_table by putting there
> >>>>>>>>>>>> types from source .debug_types_table-s. Only one copy
> of the type would be placed into the
> >>>>>>>>>>>> resulting table. All references pointing to the
> deleted copy would be corrected to point
> >>>>>>>>>>>> to the single copy inside "type table". (that is how
> dsymutil works currently)
> >>>>>>>>>>> ^ that's the step that's probably a bit expensive for
> a general-use
> >>>>>>>>>>> tool - it implies parsing all the DWARF to find those
> references and
> >>>>>>>>>>> rewrite them, I think. For a high-performance solution
> that could be
> >>>>>>>>>>> run by the linker I think it'd be necessary to have a
> solution that
> >>>>>>>>>>> doesn't involve parsing all the DIEs.
> >>>>>>>>>> According to the current dsymutil processing,
> >>>>>>>>>> exactly this process is not the most time-consuming.
> >>>>>>>>>> That could be done relatively fast.
> >>>>>>>>> Fair enough - though I'd still imagine any solution that
> involves
> >>>>>>>>> parsing all the DIEs still wouldn't be fast enough
> (maybe an order of
> >>>>>>>>> magnitude faster than the current solution even - but
> that's stuill,
> >>>>>>>>> what, 6 or 7x slower than linking without the feature?)
> for most users
> >>>>>>>>> to consider it a good trade-off.
> >>>>>>>> It seems to me that even the current 6x-7x slowdown could
> be useful.
> >>>>>>>> Users who already use dsymutil or llvm-dwp(assuming
> DWARFLinker
> >>>>>>>> would be taught to work with a split dwarf) tools spend
> this time and,
> >>>>>>>> in some scenarios, waste disk space by inter-mediate files.
> >>>>>>> FWIW, dwp (llvm-dwp hasn't really been optimized compared
> to binutils
> >>>>>>> dwp) is designed to be very quick - by not needing to do a
> lot of
> >>>>>>> parsing/fixups. Which, yes, means larger output files than
> would be
> >>>>>>> possible with more parsing/etc. It also doesn't take any
> input from
> >>>>>>> the linker (so it can run in parallel with the linker) -
> so it can't
> >>>>>>> remove dead subprograms. Given Google's the major (perhaps
> only
> >>>>>>> significant?) user of Split DWARF - I can say that the
> needs don't
> >>>>>>> necessarily overlap well with something that would take
> significantly
> >>>>>>> longer to run or use significantly more memory.
> Faster/cheaper/with
> >>>>>>> somewhat bigger output files is probably the right
> tradeoff for
> >>>>>>> Google's use case, at least.
> >>>>>>>
> >>>>>>> I imagine Apple's use for dsymutil is somewhat similar -
> it's not used
> >>>>>>> in the iterative development cycle, only in final releases
> - well,
> >>>>>>> maybe their situation is more "neutral" - not a major pain
> point in
> >>>>>>> any case I'd guess.
> >>>>>>>
> >>>>>>>
> >>>>>> I see. FWIW, Comparison splitdwarf+dwp and DWARFLinker from
> lld:
> >>>>>>
> >>>>>> 1. split-dwarf+llvm-dwp = linking time for clang 6 sec,
> >>>>>> generating time for .dwp 53 sec, clang=997M
> clang.dwp=1.1G.
> >>>>> FWIW, llvm-dwp is not very well optimized (which is to say:
> it is not
> >>>>> optimized), binutils dwp might be a better comparison (&
> even that
> >>>>> doesn't have the parallelism & some potential further memory
> savings
> >>>>> that lld has that we could take advantage of in a dwp-like tool)
> >>>>>
> >>>>> What build mode was the clang binary built in? Optimized or
> unoptimized?
> >>>> right, that is unoptimized build with -ffunction-sections.
> >>>>
> >>>>>> 2. DWARFLinker from lld = linking time for clang 72 sec,
> clang=760M.
> >>> And this is without Split DWARF? Without linker DWARF
> compression? -
> >>> that seems quite a bit surprising, that the deduplication of DWARF
> >>> could fit into less space than the wasted/reclaimed space in
> ranges (&
> >>> line)?
> >> that was without split dwarf, without linker compression.
> >>
> >>> Could you double check these numbers & provide a clearer summary?
> >> sure, I would re-check it.
> >>
> >>> Here's my attempt at numbers (all with
> function-sections+gc-sections)...
> >>>
> >>> Split DWARF tests didn't seem meaningful - gc-debuginfo +
> split DWARF
> >>> seemed to drop all the debug info (except gdb_index) so wasn't
> >>> working/comparison wasn't meaningful for Apples to Apples, but
> >>> included it for comparing gc'd non-split to non-gc'd split
> (disabled
> >>> gnu-pubnames/gdb-index (-gsplit-dwarf -gno-gnu-pubnames)
> (which turns
> >>> on by default with Split DWARF because gdb needs it - but a
> bit of an
> >>> unfair comparison without turning on gnu-pubnames/gdb-index in
> other
> >>> build modes too, since it... /shouldn't/ be necessary) which
> might've
> >>> been a factor in the data you were looking at)
> >> that might be the case. i.e. clang=997M for split dwarf(from my
> previous
> >> measurement) might include gnu-pubnames.
> >>
> >> would recheck it and if that is the case then it is a unfair
> comparison.
> >>
> >>
> >> My point was that "DWARFLinker from lld" takes less space than
> singleton
> >> split dwarf file+.dwp file.
> >>
> >> for -O0 uncompressed:
> >>
> >> - .dwp took 1.1G(if I built it correctly), singleton clang(from
> your
> >> measurements) 566 MB
> >>
> >> overall 1.6G.
> > Oh, yeah, even if there are some measurement issues, linked
> executable
> > + .dwp is going to be larger than a linked executable using
> non-split
> > DWARF (in v5), since v5 uses all the same representations as
> non-split
> > DWARF, and split DWARF adds the indirection overhead of a split
> file,
> > etc.
> >
> > Even without DWARF linking, it's true that split DWARF has overhead
> > (dwp+executable will be larger than executable non-split).
> >
> > But maybe we've ended up down a bit of a tangent in any case.
> >
> > Trying to bring this back to "should this be committed to lld" seems
> > valuable, and I'm not sure what the right criteria are for that.
> I think it would be useful to do "removing obsolete debug info"
> in the linker. First thing is that it would be the fastest way(no need
> to copy data/create temp files/built address map...) Second thing
> is that it would be a good separation of concepts. All debug info
> processing, currently done in the linker(gdb_index, upcoming
> debug_names), could be moved into separate library processing
> debug info. When gdb_index/debug_names should be built without
> "removing of obsolete debug info" it would have the same
> performance results as it currently has.
>
> We decided to give the idea of "removing of obsolete debug info"
> another try and are going to implement it as a separate utility
> working with built binary. Making it to be multi-thread would
> probably show better performance results and then it could
> probably be considered as acceptable to use from the linker.
>
>
> I'm quite interested in this direction. One thought I had was to
> incorporate such a library into dsymutil but with support for ELF. If
> you get a proposal written up I'd love to take a look and comment.
>
yes, I would share the proposal in a separate thread within a week or two.
Shortly: we decided to move in slightly other direction than adding this
functionality
into dsymutil. Though if there is a preference to implement it as part
of dsymutil
we are OK to do this way.
In its first version, this new utility supposed to receive built binary
with debug info
as input(with the new marking for references to removed code sections -1/-2
-https://reviews.llvm.org/D84825) and create a new binary with removed
obsolete
debug info according to the above marking. In the next versions, it
could be extended
with other debug info optimizations tasks. F.e. generation new index
tables, debug info
optimizing... etc...
We considered three options:
1. add new functionality into dsymutil. So that dsymutil behaves
differently
on a non-darwin platform and supports another set of command-line
options.
2. add new functionality into llvm-objcopy. llvm-objcopy already
supports various
binary objects formats(MachO,ELF,COFF,wasm). It also has several
options
to work with debug-info.
3. create new utility llvm-dwarfutil which would implement the above
functionality
and reuse DWARFLinker(extracted from dsymutil) library and new
library
ObjectCopy(extracted from llvm-objcopy).
So far our preference is number three. The reason for this is that separate
utility specifically working with debug info looks as good separation of
concepts.
Adding another behavior to dsymutil looks not very good. Extending the
already
rich interface of llvm-objcopy looks also not very good. Having in mind
that actual
implementation would be shared by libraries, the separate utility,
working specifically
with debug info, looks like the right choice. That is our current idea.
I would publish the proposal shortly to discuss it.
Thank you, Alexey.
> Thanks!
>
> -eric
>
> Alexey.
>
> >
> > Ray's the best person to weigh in on that. My 2c is that I think it
> > probably is worthwhile, even just as an experiment, assuming
> it's not
> > too intrusive to lld.
> >
> >> - The "DWARFLinker from lld" 820 MB(from your measurements).
> >>
> >>
> >> So "DWARFLinker from lld" looks two times better.
> >>
> >>
> >> Anyway, thank you for pointing me to possible mistake. I would
> recheck
> >> it and update results.
> >>
> >>
> >> Alexey.
> >>
> >>
> >>> * -O0: (baseline, just using strip -g: 356 MB)
> >>> * compressed: 25% smaller with gc-debuginfo (481 MB / 641
> MB) (407
> >>> MB split/non-gc)
> >>> * uncompressed: 30% smaller (820 MB / 1.2 GB) (566 MB
> split/non-gc)
> >>> * -O3: (baseline: 116 MB)
> >>> * compressed: 16% smaller (361 MB / 462 MB) (283 MB
> split/non-gc)
> >>> * uncompressed: 22% smaller (1022 MB / 1.2 GB) (156 MB
> split/non-gc)
> >>>
> >>>
> >>>
> >>>
> >>> On Fri, Jun 26, 2020 at 9:28 AM Alexey Lapshin
> >>> <alapshin at accesssoftek.com <mailto:alapshin at accesssoftek.com>>
> wrote:
> >>>>>>>>>>>> This idea goes in another direction than fragmenting
> dwarf
> >>>>>>>>>>>> using elf sections&tricks. It seems to me that the
> cost of fragmenting is too high.
> >>>>>>>>>>> I tend to agree - but I'm sort of leaning towards
> trying to use object
> >>>>>>>>>>> features as much as possible, then implementing just
> enough custom
> >>>>>>>>>>> handling in the linker to recoup overhead, etc. (eg:
> add some kind of
> >>>>>>>>>>> small header/brief description that makes it easy for
> the linker to
> >>>>>>>>>>> slice-and-dice - but hopefully a domain-specific such
> header can be a
> >>>>>>>>>>> bit more compact than the fully general ELF form)
> >>>>>>>>>> I think this indeed should be implemented and evaluated.
> >>>>>>>>>> So that various approaches could be compared.
> >>>>>>>>>>
> >>>>>>>>>>>> It is not only the sizes of structures describing
> fragments but also the complexity
> >>>>>>>>>>>> of tools that should be taught to work with
> fragmented DWARF.
> >>>>>>>>>>>> (f.e. llvm-dwarfdump applied to object file should be
> able to read fragmented DWARF,
> >>>>>>>>>>>> but applied to linked executable it should work with
> non-fragmented DWARF).
> >>>>>>>>>>>> That idea is for the tool which works the same way as
> dsymutil ODR.
> >>>>>>>>>>>>
> >>>>>>>>>>>> I will shortly describe the idea of making DWARF be
> easier processed by dsymutil/DWARFLinker:
> >>>>>>>>>>>>
> >>>>>>>>>>>> The idea is to have only one "type table" per object
> file(special section .debug_types_table).
> >>>>>>>>>>>> This "type table" would contain all types.
> >>>>>>>>>>>> There could be a special type of reference -
> type_offset - that offset points into the type table.
> >>>>>>>>>>>> Basic types could always be placed into the start of
> "type table" thus, offsets to basic types
> >>>>>>>>>>>> most often would be 1 byte. There also would be a
> special kind of reference - reference inside the type.
> >>>>>>>>>>>> Type units sig8 system - would not be used to
> reference types.
> >>>>>>>>>>>>
> >>>>>>>>>>>> Types deduplication is assumed to be done, not by
> linker mechanism for COMDAT,
> >>>>>>>>>>>> but by a tool like dsymutil. This tool would create
> resulting .debug_types_table by putting there
> >>>>>>>>>>>> types from source .debug_types_table-s. Only one copy
> of the type would be placed into the
> >>>>>>>>>>>> resulting table. All references pointing to the
> deleted copy would be corrected to point
> >>>>>>>>>>>> to the single copy inside "type table". (that is how
> dsymutil works currently)
> >>>>>>>>>>> ^ that's the step that's probably a bit expensive for
> a general-use
> >>>>>>>>>>> tool - it implies parsing all the DWARF to find those
> references and
> >>>>>>>>>>> rewrite them, I think. For a high-performance solution
> that could be
> >>>>>>>>>>> run by the linker I think it'd be necessary to have a
> solution that
> >>>>>>>>>>> doesn't involve parsing all the DIEs.
> >>>>>>>>>> According to the current dsymutil processing,
> >>>>>>>>>> exactly this process is not the most time-consuming.
> >>>>>>>>>> That could be done relatively fast.
> >>>>>>>>> Fair enough - though I'd still imagine any solution that
> involves
> >>>>>>>>> parsing all the DIEs still wouldn't be fast enough
> (maybe an order of
> >>>>>>>>> magnitude faster than the current solution even - but
> that's stuill,
> >>>>>>>>> what, 6 or 7x slower than linking without the feature?)
> for most users
> >>>>>>>>> to consider it a good trade-off.
> >>>>>>>> It seems to me that even the current 6x-7x slowdown could
> be useful.
> >>>>>>>> Users who already use dsymutil or llvm-dwp(assuming
> DWARFLinker
> >>>>>>>> would be taught to work with a split dwarf) tools spend
> this time and,
> >>>>>>>> in some scenarios, waste disk space by inter-mediate files.
> >>>>>>> FWIW, dwp (llvm-dwp hasn't really been optimized compared
> to binutils
> >>>>>>> dwp) is designed to be very quick - by not needing to do a
> lot of
> >>>>>>> parsing/fixups. Which, yes, means larger output files than
> would be
> >>>>>>> possible with more parsing/etc. It also doesn't take any
> input from
> >>>>>>> the linker (so it can run in parallel with the linker) -
> so it can't
> >>>>>>> remove dead subprograms. Given Google's the major (perhaps
> only
> >>>>>>> significant?) user of Split DWARF - I can say that the
> needs don't
> >>>>>>> necessarily overlap well with something that would take
> significantly
> >>>>>>> longer to run or use significantly more memory.
> Faster/cheaper/with
> >>>>>>> somewhat bigger output files is probably the right
> tradeoff for
> >>>>>>> Google's use case, at least.
> >>>>>>>
> >>>>>>> I imagine Apple's use for dsymutil is somewhat similar -
> it's not used
> >>>>>>> in the iterative development cycle, only in final releases
> - well,
> >>>>>>> maybe their situation is more "neutral" - not a major pain
> point in
> >>>>>>> any case I'd guess.
> >>>>>>>
> >>>>>>>
> >>>>>> I see. FWIW, Comparison splitdwarf+dwp and DWARFLinker from
> lld:
> >>>>>>
> >>>>>> 1. split-dwarf+llvm-dwp = linking time for clang 6 sec,
> >>>>>> generating time for .dwp 53 sec, clang=997M
> clang.dwp=1.1G.
> >>>>> FWIW, llvm-dwp is not very well optimized (which is to say:
> it is not
> >>>>> optimized), binutils dwp might be a better comparison (&
> even that
> >>>>> doesn't have the parallelism & some potential further memory
> savings
> >>>>> that lld has that we could take advantage of in a dwp-like tool)
> >>>>>
> >>>>> What build mode was the clang binary built in? Optimized or
> unoptimized?
> >>>> right, that is unoptimized build with -ffunction-sections.
> >>>>
> >>>>>> 2. DWARFLinker from lld = linking time for clang 72 sec,
> clang=760M.
> >>>>> It does seem a tad strange that the clang binary would be
> smaller
> >>>>> non-split with DWARF linking than it was split. Though I
> could imagine
> >>>>> this might be possible in an optimized build (wehre debug_ranges
> >>>>> become quite relatively expensive in the .o file
> contribution with
> >>>>> Split DWARF)
> >>>>> Could you compare the section sizes between these two clang
> binaries, perhaps?
> >>>> .debug_ranges is three times bigger and .debug_line is twice
> bigger.
> >>>>
> >>>>>>>> Thus if they would use this LLD feature in its current state
> >>>>>>>> - they would still receive benefits.
> >>>>>>>>
> >>>>>>>> Speaking of performance results - LLD is a multi-thread
> linker;
> >>>>>>>> it handles sections in parallel. DWARFLinker generates
> DWARF using
> >>>>>>>> AsmPrinter which is a stream - so it could make resulting
> DWARF only
> >>>>>>>> continuously. It is not surprising that the parallel
> solution works faster.
> >>>>>>>> Making DWARFLinker truly multi-threaded would probably
> allow us
> >>>>>>>> to make slowdown to be at 2x-4x range.
> >>>>>>> *nod* that's still a really expensive link - but I
> understand that's a
> >>>>>>> suitable tradeoff for your users
> >>>>>>>
> >>>>>> Btw, 2x or 7x is for pure linking time. Overall compilation
> slowdown
> >>>>>> is not so significant. Building LLVM codebase has only 20%
> slowdown.
> >>>>> Understood - that's still quite significant to most users,
> I'd imagine.
> >>>> I see.
> >>>>
> >>>>>>>>>> Anyway, I think the dsymutil approach is still
> valuable, and it
> >>>>>>>>>> would be useful to optimize it.
> >>>>>>>>>> Do you think it would be useful to make
> dsymutil/DWARFLinker truly multi-thread?
> >>>>>>>>>> (To make dsymutil/DWARFLinker able to process each
> object file in a separate thread)
> >>>>>>>>> Perhaps - that I'd probably leave up to the folks who
> are more
> >>>>>>>>> invested in dsymutil (Adrian Prantl et al). Maybe one
> day we'll get it
> >>>>>>>>> integrated into llvm-dwp and then I'll be interested in
> getting as
> >>>>>>>>> much performance out of it as lld - so multithreading
> and things would
> >>>>>>>>> be on the books.
> >>>>>>>> I think improving dsymutil is a valuable thing.
> >>>>>>>> Though there are several directions which might be considered
> >>>>>>>> to make it more robust:
> >>>>>>>>
> >>>>>>>> 1. support of latest DWARF - DWARF5/DWARF64...
> >>>>>>> I expect/though some of the Apple folks had already worked
> on DWARF5 support?
> >>>>>>> DWARF64 - that's been around for a while, and just hasn't
> been needed
> >>>>>>> by LLVM users thus far, it seems (until recently - where some
> >>>>>>> developers have started working on that)
> >>>>>> There already implemented debug_names table, but
> debug_rnglists,
> >>>>>> debug_loclists, type units - are not implemented yet.
> >>>>> Superficially, type units wouldn't be on the list of
> features (like
> >>>>> DWARF64 - it's optional) I'd try to support in dsymutil -
> since their
> >>>>> size overhead is more justified for a DWARF-agnostic linker
> that's
> >>>>> using comdat groups. With a DWARF-aware linker I'd be
> specifically
> >>>>> hoping to avoid using type units to help
> >>>>>> The thing which
> >>>>>> should probably be changed is that dsymutil should not have
> its version
> >>>>>> of code generating DWARF tables. It should call already existed
> >>>>>> DWARF5/DWARF64 implementations. Then dsymutil would always
> >>>>>> use last DWARF generators.
> >>>>> Possibly - I don't know what the architectural tradeoffs for
> that look
> >>>>> like - I'd imagine DWARFLinker has sufficiently different
> >>>>> needs/tradeoffs than LLVM's DWARF generation code (rewriting
> existing
> >>>>> DIEs compared to building new ones from scratch, etc) that
> it might be
> >>>>> hard for them to share a lot of their implementation.
> >>>> It is not easy, and would require some additions, but it
> would benefit
> >>>> in that all format implementation is in one place. Thus
> changing that place
> >>>> would reflect in other places. There are at least three
> implementations for
> >>>> .debug_ranges, .debug_aranges currently...
> >>>>
> >>>>
> >>>>>>>> 2. implement multi-threaded execution.
> >>>>>>>> 3. support of split DWARF.
> >>>>>>> Maybe, though I'm still not sure it'd be the right tradeoff -
> >>>>>>> especially if it involved having to wait to run the .dwo
> merger (call
> >>>>>>> it DWARF-aware dwp, or dsymutil with dwp support) until
> after the
> >>>>>>> linker ran.
> >>>>>>>
> >>>>>>>> 4. implement dsymutil for non-darwin platform.
> >>>>>>> That's probably, essentially (3), more-or-less. Split DWARF is
> >>>>>>> somewhat of a formalization of Apple's/MachO DWARF
> distribution model
> >>>>>>> (leave DWARF it in files that aren't linked/use them from
> a debugger,
> >>>>>>> but also be able to merge them into some final file (dsym
> or dwp) for
> >>>>>>> archival purposes)
> >>>>>>>
> >>>>>>>> All of this is a massive piece of work.
> >>>>>>>> Our original investment was to solve two problems:
> >>>>>>>>
> >>>>>>>> 1. Overlapped address ranges, which is currently close to
> being solved. Thank you for helping with that!
> >>>>>>> Yeah, again, sorry that's taken quite so long/somewhat
> circuitous route.
> >>>>>>>
> >>>>>>>> 2. Size of debug info. That still becomes an issue, but
> we are unsure whether we are ready to
> >>>>>>>> invest in solving all the above 1-4 problems and how
> much community interested in it.
> >>>>>>> Fair, for sure - I don't think you'd need to sign up to
> solve all of
> >>>>>>> them (don't think they necessarily need solving).
> Potentially moving
> >>>>>>> the logic out into a separate tool as Fangrui's
> considering - a
> >>>>>>> post-link DWARF optimizer, rather than in-linker DWARF
> optimization.
> >>>>>>>
> >>>>>>> I really don't want to give you the runaround like this -
> but multiple
> >>>>>>> times slower links is something that seems pretty
> problematic for most
> >>>>>>> users, to the point of weighing the maintainability of lld
> against the
> >>>>>>> convenience of having this functionality in-linker rather
> than in a
> >>>>>>> post-link optimizer.
> >>>>>>>
> >>>>>>> (I know you've spoken a bit before about your users needs
> - but if
> >>>>>>> it's possible, could you explain (again :/) why they have
> such a
> >>>>>>> strong need for smaller DWARF? While DWARF size is an
> ongoing concern
> >>>>>>> for many users (Google certainly - hence the invention of
> Split DWARF,
> >>>>>>> use of type units and compressed DWARF, etc) - usually
> it's in rather
> >>>>>>> large programs, but it sounds like you're dealing with
> relatively
> >>>>>>> small ones (otherwise the increase in link time, I'd
> imagine, would be
> >>>>>>> prohibitive for your users?)?
> >>>>>> We have many large programs and keep Dayly/Nightly debug
> builds,
> >>>>>> which takes a lot of disk space. Compilation time for these
> programs is big.
> >>>>>> The scenario is "compile once".(not
> compile-debug-compile-debug).
> >>>>>> So we think that solution(like dsymutil/DWARFLinker) would
> not slowdown
> >>>>>> the compilation time of overall build significantly(see
> above numbers for
> >>>>>> llvm codebase) and would allow us to reduce disk space
> required to keep
> >>>>>> all of these builds.
> >>>>> Ah, OK - for archival purposes. So the interactive
> developers wouldn't
> >>>>> necessarily be using this feature. Makes sense - similar to
> dsymutil
> >>>>> and dwp, mostly used for archival purposes & you can debug
> straight
> >>>> >from .o/.dwos for interactive/iterative development.
> >>>>
> >>>>> In that case, it seems more likely that a separate tool
> might suffice.
> >>>> agreed: if to continue the work on this then it makes sense to
> >>>> do it as separate tool. Make it fast enough. And if there
> would be interest
> >>>> in it - then it would probably be possible to return to idea
> calling it from linker.
> >>>>
> >>>>> Also, out of curiosity - have you tried just compressing the
> output
> >>>>> (-gz (I think that does the right thing for the linker level
> >>>>> compression too, otherwise -Wl,-compress-debug-sections
> might do it))
> >>>>> or are you already doing that in addition?
> >>>> sure. we use -Wl,-compress-debug-sections.
> >>>>
> >>>> Thank you, Alexey.
> >>>>
> >>>>>>> You mentioned that the usability cost of
> >>>>>>> Split DWARF for your users was too high (or high enough to
> justify
> >>>>>>> this alternative work of DWARF-aware linking)? That all
> seems a bit
> >>>>>>> surprising to me - though I understand the deployment
> issues of Split
> >>>>>>> DWARF do present some challenges to users in more heterogenous
> >>>>>>> environments than Google's... still, I'd have thought
> there was some
> >>>>>>> hope there)
> >>>>>> Our tools does not support split dwarf yet. Though we plan
> to implement it.
> >>>>>> When we would have support of split dwarf then it would be
> >>>>>> convenient to have easy way to share built debug binaries.
> llvm-dwp is the
> >>>>>> answer to this. DWARFLinker could probably be another answer.
> >>>>> Ah, fair enough - thanks for the context!
> >>>>>>>>> One way to do that would be to have a CU-local type
> indirection table.
> >>>>>>>>> DIEs reference local type numbers (like local
> address/string numbers -
> >>>>>>>>> addrx/strx/rnglistx) and that table contains either sig8
> (no linker
> >>>>>>>>> fixups required) or the local type offsets you describe
> - the linker
> >>>>>>>>> would then only need to read this type number
> indirection table and
> >>>>>>>>> rewrite them to the final type numbers.
> >>>>>>>> Yes, that could be additionally done if this process
> would be time-consuming.
> >>>>>>>>
> >>>>>>>> David, thank you for all your comments and explanations.
> They are extremely helpful.
> >>>>>>> Sure thing - really appreciate your patience with all this
> - it's... a
> >>>>>>> lot of moving parts.
> >>>>>>> - Dave
> >>>>>>> Thank you, Alexey.
> >>>>>>>
> >>>>>>>> sig8 hash-id would be used to compare types and to
> deduplicate them.
> >>>>>>>> It would speed up the current dsymutil context analysis.
> >>>>>>>> Types having the same hash-id could be deduplicated.
> >>>>>>>> This would allow deduplicating a more number of types
> than current dsymutil.
> >>>>>>>> Incomplete type definitions having a similar set of
> members are not deduplicated by dsymutil currently.
> >>>>>>>> In this case they would have the same hash-id.
> >>>>>>>>
> >>>>>>>> This "type table" would take less space than current
> "type units" and current ODR solution.
> >>>>>>>>
> >>>>>>>> Above is just an idea on how to help DWARF-aware
> linker(based on idea removing obsolete debug info)
> >>>>>>>> to work faster(if that is interesting).
> >>>>>>>>
> >>>>>>>> Alexey.
> >>>>>>>>
> >>>>>>>>> From: llvm-dev <llvm-dev-bounces at lists.llvm.org
> <mailto:llvm-dev-bounces at lists.llvm.org>> On Behalf Of James
> Henderson via llvm-dev
> >>>>>>>>> Sent: Wednesday, June 3, 2020 3:48 AM
> >>>>>>>>> To: David Blaikie <dblaikie at gmail.com
> <mailto:dblaikie at gmail.com>>
> >>>>>>>>> Cc: llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> >>>>>>>>> Subject: Re: [llvm-dev] [Debuginfo][DWARF][LLD] Remove
> obsolete debug info in lld.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> It makes me sad that the linker (via a library or
> otherwise) has to be "DWARF-aware" to be able to effectively
> handle --gc-sections, COMDATs, --icf etc for debug info, without
> leaving large blocks of data kicking around.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> The patching to -1 (or equivalent) is probably a good
> lightweight solution (though I'd love it if it could be done based
> on section type in the future rather than section name, but that's
> probably outside the realm of DWARF), as it requires only minimal
> understanding in the linker, but anything beyond that seems to be
> complicated logic that is mostly due to the structure of DWARF.
> Patching to -1 does feel a bit like a sticking plaster/band aid to
> patch over the issue rather than properly solving it too - there
> will still be debug data (potentially significant amounts in
> COMDAT-heavy objects) that the linker has to write and the
> debugger has to somehow know how to skip (even if it knows that -1
> is special-case due to the standard being updated, it needs to get
> as far as the -1), which is all wasted effort.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> We've already seen from Alexey's prototyping, and from
> our own experiences with the Sony proprietary linker (which tried
> to rewrite .debug_line only) that deconstructing the DWARF so that
> it can be more optimally reassembled at link time is slow going,
> and will probably inevitably be however much effort is put into
> optimising it. For a start, given the current standards, it's
> impossible to know how to deconstruct it without having to parse
> vast amounts of DWARF, which is typically going to mean a lot more
> parsing work than the linker would normally have to deal with.
> Additionally, much of this parsing work is wasted effort, since it
> seems unlikely in many links that large amounts of the DWARF will
> be redundant. Having an option to opt-in doesn't help much there,
> since it just means the logic exists without most people using it,
> due to it not being good enough, or potentially they don't even
> know it exists.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> I don't have particularly concrete suggestions as to how
> to solve the structural problems with DWARF at this point. The
> only thing that seems obvious to me is a more "blessed" approach
> to fragmentation of sections, similar to what I tried with my
> prototype mentioned earlier in the thread, although we'd need to
> figure out the previously stated performance issues. Other ideas
> might tie into this, like somehow sharing the various table
> headers a bit like CIEs in .eh_frame that could be merged by the
> linker - each object could have separate table header sections,
> which are referenced by the individual .debug_* blocks, which in
> turn are one per function/data piece and easily discardable/merged
> by the linker.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> Just some thoughts.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> James
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> On Tue, 2 Jun 2020 at 19:24, David Blaikie via llvm-dev
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> >>>>>>>>>
> >>>>>>>>> On Tue, May 19, 2020 at 7:17 AM Alexey Lapshin
> >>>>>>>>> <alapshin at accesssoftek.com
> <mailto:alapshin at accesssoftek.com>> wrote:
> >>>>>>>>>> Hi David, please find my comments inside:
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>>>>> Broad question: Do you have any specific
> motivation/users/etc in implementing this (if you can speak about it)?
> >>>>>>>>>>>>> - it might help motivate the work, understand what
> tradeoffs might be suitable for you/your users, etc.
> >>>>>>>>>>>> There are two general requirements:
> >>>>>>>>>>>> 1) Remove (or clean) invalid debug info.
> >>>>>>>>>>> Perhaps a simpler direct solution for your immediate
> needs might be a much narrower,
> >>>>>>>>>>> and more efficient linker-DWARF-awareness feature:
> >>>>>>>>>>>
> >>>>>>>>>>> With DWARFv5, rnglists present an opportunity for a
> DWARF linker to rewrite the ranges
> >>>>>>>>>>> without parsing the rest of the DWARF. /technically/
> this isn't guaranteed - rnglist entries
> >>>>>>>>>>> can be referenced either directly, or by index. If all
> rnglists are referenced by index, then
> >>>>>>>>>>> a linker could parse only the debug_rnglists section
> and rewrite ranges to remove any
> >>>>>>>>>>> address ranges that refer to optimized-out code.
> >>>>>>>>>>>
> >>>>>>>>>>> This would only be correct for rnglists that had no
> direct references to them (that only were
> >>>>>>>>>>> referenced via the indexes) - but we could either
> implement it with that assumption, or could
> >>>>>>>>>>> add an LLVM extension attribute on the CU that would
> say "I promise I only referenced rnglists
> >>>>>>>>>>> via rnglistx forms/indexes). If this DWARF-aware
> linking would have to read the CU DIE (not
> >>>>>>>>>>> all the other DIEs) it /could/ also then rewrite
> high/low_pc if the CU wasn't using ranges...
> >>>>>>>>>>> but that wouldn't come up in the function-removal
> case, because then you'd have ranges anyway,
> >>>>>>>>>>> so no need for that.
> >>>>>>>>>>>
> >>>>>>>>>>> Such a DWARF-aware rnglist linking could also simplify
> rnglists, in cases where functions
> >>>>>>>>>>> ended up being laid out next to each other, the linker
> could coalesce their ranges together.
> >>>>>>>>>>>
> >>>>>>>>>>> I imagine this could be implemented with very little
> overhead to linking, especially compared
> >>>>>>>>>>> to the overhead of full DWARF-aware linking.
> >>>>>>>>>>>
> >>>>>>>>>>> Though none of this fixes Split DWARF, where the
> linker doesn't get a chance to see the
> >>>>>>>>>>> addresses being used - but if you only want/need the
> CU-level ranges to be correct, this
> >>>>>>>>>>> might be a viable fix, and quite efficient.
> >>>>>>>>>> Yes, we think about that alternative. This would
> resolve our problem of invalid debug info
> >>>>>>>>>> and would work much faster. Thus, if we would not have
> good results for D74169 then we
> >>>>>>>>>> will implement it. Do you think it could be useful to
> have this solution in upstream?
> >>>>>>>>> A pure rnglist rewriting - I think it'd be OK to have in
> upstream -
> >>>>>>>>> again, cost/benefit/etc would have to be weighed. I'm
> not sure it
> >>>>>>>>> would save enough space to be particularly valuable
> beyond the
> >>>>>>>>> correctness issue - and it doesn't completely solve the
> correctness
> >>>>>>>>> issue for zero-address usage or low-address usage
> (because you could
> >>>>>>>>> still have overlapping subprograms inside a CU - so if
> you were
> >>>>>>>>> symbolizing you could use the correct rnglist to filter,
> but then go
> >>>>>>>>> look inside the CU only to find two subprograms that had
> that address
> >>>>>>>>> & not know which one was the correct one an which one
> was the
> >>>>>>>>> discarded one).
> >>>>>>>>>
> >>>>>>>>> rnglist rewriting might be easy enough to prototype -
> but depends what
> >>>>>>>>> you want to spend your time on, I know this whole issue
> has been a
> >>>>>>>>> huge investment of your time already - but maybe this recent
> >>>>>>>>> revitalization of the conversation around having an
> explicit value in
> >>>>>>>>> the linker might be sufficient to address everyone's
> needs... *fingers
> >>>>>>>>> crossed*)
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>>>> 2) Optimize the DWARF size.
> >>>>>>>>>>> Do your users care much about this? I imagine if they
> had significant DWARF size issues,
> >>>>>>>>>>> they'd have significant link time issues and the kind
> of cost to link time this feature has would
> >>>>>>>>>>> be prohibitive - but perhaps they're sharing linked
> binaries much more often than they're
> >>>>>>>>>>> actually performing linking.
> >>>>>>>>>> Yes, they do. They also have significant link-time issues.
> >>>>>>>>>> So current performance results of D74169 are not very
> acceptable.
> >>>>>>>>>> We hope to improve it.
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>>>> The specifics which our users have:
> >>>>>>>>>>>> - embedded platform which uses 0 as start of .text
> section.
> >>>>>>>>>>>> - custom toolset which does not support all
> features yet(f.e. split dwarf).
> >>>>>>>>>>>> - tolerant of the link-time increase.
> >>>>>>>>>>>> - need a useful way to share debug builds.
> >>>>>>>>>>> Sharing two files (executable and dwp) is
> significantly less useful than sharing one file?
> >>>>>>>>>> Probably not significantly, but yes, it looks less
> useful comparing to D74169.
> >>>>>>>>>> Having only two files (executable and .dwp) looks
> significantly better than having executable and multiple .dwo files.
> >>>>>>>>>> Having only one file(executable) with minimal size
> looks better than the two files with a bigger size.
> >>>>>>>>>>
> >>>>>>>>>> clang compiled with -gsplitdwarf takes 0.9G for
> executable and 0.9G for .dwp.
> >>>>>>>>>> clang compiled with -gc-debuginfo takes only 0.76G for
> single executable.
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>>>> For the first point: we have a problem "Overlapping
> address ranges starting from 0"(D59553).
> >>>>>>>>>>>> We use custom solution, but the general solution like
> D74169 would be better here.
> >>>>>>>>>>> If CU ranges are the only ones that need fixing, then
> I think the above solution might be as
> >>>>>>>>>>> good/better - if more than CU ranges need fixing, then
> I think we might want to start talking about
> >>>>>>>>>>> how to fix DWARF itself (split and non-split) to
> signal certain addresses point to dead code with a
> >>>>>>>>>>> specific blessed value that linkers would need to
> implement - because with Split DWARF there's
> >>>>>>>>>>> no way to solve the non-CU addresses at the linker.
> >>>>>>>>>> I think the worthful solution for that signal value
> would be LowPC > HighPC.
> >>>>>>>>>> That does not require additional bits in DWARF.
> >>>>>>>>>> It would be natural to skip such address ranges since
> they explicitly marked as invalid.
> >>>>>>>>>> It could be implemented in a linker very easily.
> Probably, it would make sense to describe that
> >>>>>>>>>> usage in DWARF standard.
> >>>>>>>>>>
> >>>>>>>>>> As to the addresses which are not seen by the
> linker(since they are in .dwo files) - yes,
> >>>>>>>>>> they need to have another solution. Could you show an
> example of such a case, please?
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>>>>> 2. Support of type units.
> >>>>>>>>>>>>>> That could be implemented further.
> >>>>>>>>>>>>> Enabling type units increases object size to make it
> easier to deduplicate at link time by a DWARF-unaware
> >>>>>>>>>>>>> linker. With a DWARF aware linker it'd be generally
> desirable not to have to add that object size overhead to
> >>>>>>>>>>>>> get the linking improvements.
> >>>>>>>>>>>> But, DWARFLinker should adequately work with type
> units since they are already implemented.
> >>>>>>>>>>> Maybe - it'd be nice & all, but I don't think it's an
> outright necessity - if someone knows they're using
> >>>>>>>>>>> a DWARF-aware linker, they'd probably not use type
> units in their object files. It's possible someone
> >>>>>>>>>>> doesn't know for sure & maybe they have pre-canned
> debug object files from someone else, etc.
> >>>>>>>>>> I see.
> >>>>>>>>>>
> >>>>>>>>>>>> Another thing is that the idea behind type units has
> the potential to help Dwarf-aware linker to work faster.
> >>>>>>>>>>>> Currently, DWARFLinker analyzes context to understand
> whether types are the same or not.
> >>>>>>>>>>> When you say "analyzes context" what do you mean?
> Usually I'd take that to mean
> >>>>>>>>>>> "looks at things outside the type itself - like what
> namespace it's in, etc" - which, yes,
> >>>>>>>>>>> it should do that, but it doesn't seem very expensive
> to do. But I guess you actually
> >>>>>>>>>>> mean something about doing structural equivalence in
> some way, looking at things inside the type?
> >>>>>>>>>> I think it could be useful for both cases. Currently,
> dsymutil does only first thing
> >>>>>>>>>> (look at type name, namespace name, etc..) and does not
> do the second thing
> >>>>>>>>>> (doing structural equivalence). Analyzing type names is
> currently quite expensive
> >>>>>>>>>> (the only search in string pool takes ~10 sec from 70
> sec of overall time).
> >>>>>>>>>> That is expensive because of many things should be done
> to work with strings:
> >>>>>>>>>> parse DWARF, search and resolve relocations, compute a
> hash for strings,
> >>>>>>>>>> put data into a string pool, create a fully qualified
> name(like namespace::function::name).
> >>>>>>>>>> It looks like it could be optimized and finally require
> less time, but it still would be a noticeable
> >>>>>>>>>> part of the overall time.
> >>>>>>>>>>
> >>>>>>>>>> If dsymutil starts to check for the structural
> equivalence, then the process would be even more slowly.
> >>>>>>>>>> So, If instead of comparing types structure, there
> would be checked single hash-id - then this process
> >>>>>>>>>> would also be faster.
> >>>>>>>>>>
> >>>>>>>>>> Thus I think using hash-id to compare types would allow
> to make current implementation faster and would
> >>>>>>>>>> allow handling incomplete types by DWARFLinker without
> massive performance degradation also.
> >>>>>>>>>>
> >>>>>>>>>>>> But the context is known when types are generated.
> So, no need to spent the time analyzing it.
> >>>>>>>>>>>> If types could be compared without analyzing context,
> then Dwarf-aware linker would work faster.
> >>>>>>>>>>>> That is just an idea(not for immediate
> implementation): If types would be stored in some "type table"
> >>>>>>>>>>>> (instead of COMDAT section group) and could be
> accessed through hash-id(like type units
> >>>>>>>>>>>> - then it would be the solution requiring fewer bits
> to store but allowing to compare types
> >>>>>>>>>>>> by hash-id(not analysing context).
> >>>>>>>>>>>> In this case, size increasing would be small. And
> processing time could be done faster.
> >>>>>>>>>>>>
> >>>>>>>>>>>> this is just an idea and could be discussed
> separately from the problem of integrating of D74169.
> >>>>>>>>>>>>>> 6. -flto=thin
> >>>>>>>>>>>>>> That problem was described in this review
> https://reviews.llvm.org/D54747#1503720. It also exists in
> >>>>>>>>>>>>>> current DWARFLinker/dsymutil implementation. I
> think that problem should be discussed more: it could
> >>>>>>>>>>>>>> probably be fixed by avoiding generation of such
> incomplete declaration during thinlto,
> >>>>>>>>>>>>>> That would be costly to produce extra/redundant
> debug info in ThinLTO - actually ThinLTO could be doing
> >>>>>>>>>>>>>> more to reduce that redundancy early on (actually
> removing definitions from some llvm Modules if the type
> >>>>>>>>>>>>>> definition is known to exist in another Module, etc)
> >>>>>>>>>>>>> I don't know if it's a problem since that patch was
> reverted.
> >>>>>>>>>>>> Yes. That patch was reverted, but this patch(D74169)
> has the same problem.
> >>>>>>>>>>>> if D74169 would be applied and --gc-debuginfo used
> then structure type
> >>>>>>>>>>>> definition would be removed.
> >>>>>>>>>>>> DWARFLinker could handle that case - "removing
> definitions from some llvm Modules if the type
> >>>>>>>>>>>> definition is known to exist in another Module".
> >>>>>>>>>>>> i.e. DWARFLinker could replace the declaration with
> the definition.
> >>>>>>>>>>>> But that problem could be more easily resolved when
> debug info is generated(probably without
> >>>>>>>>>>>> significant increase of debug info size):
> >>>>>>>>>>>> Here we have:
> >>>>>>>>>>>> DW_TAG_compile_unit(0x0000000b) - compile unit
> containing concrete instance for function "f".
> >>>>>>>>>>>> DW_TAG_compile_unit(0x00000073) - compile unit
> containing abstract instance root for function "f".
> >>>>>>>>>>>> DW_TAG_compile_unit(0x000000c1) - compile unit
> containing function "f" definition.
> >>>>>>>>>>>> Code for function "f" was deleted. gc-debuginfo
> deletes compile unit DW_TAG_compile_unit(0x000000c1)
> >>>>>>>>>>>> containing "f" definition (since there is no
> corresponding code). But it has structure "Foo" definition
> >>>>>>>>>>>> DW_TAG_structure_type(0x0000011e) referenced from
> DW_TAG_compile_unit(0x00000073)
> >>>>>>>>>>>> by declaration DW_TAG_structure_type(0x000000ae).
> That declaration is exactly the case when definition
> >>>>>>>>>>>> was removed by thinlto and replaced with declaration.
> >>>>>>>>>>>> Would it cost too much if type definition would not
> be replaced with declaration for "abstract instance root"?
> >>>>>>>>>>>> The number of concrete instances is bigger than
> number of abstract instance roots.
> >>>>>>>>>>>> Probably, it would not be too costly to leave
> definition in abstract instance root?
> >>>>>>>>>>
> >>>>>>>>>>>> Alternatively, Would it cost too much if type
> definition would not be replaced with declaration when
> >>>>>>>>>>>> declaration references type from not used function?
> (lto could understand that concrete function is not used).
> >>>>>>>>>>> I don't follow this example - could you provide a
> small concrete test case I could reproduce?
> >>>>>>>>>> I would provide a test case if necessary. But it looks
> like this issue is finally clear, and you already commented on that.
> >>>>>>>>>>
> >>>>>>>>>>> Oh, I guess this is happening perhaps because ThinLTO
> can't know for sure that a standalone
> >>>>>>>>>>> definition of 'f' won't be needed - so it produces one
> in case one of the inlining opportunities
> >>>>>>>>>>> doesn't end up inlining. Then it turns out all calls
> got inlined, so the external definition wasn't needed.
> >>>>>>>>>>> Oh, you're suggesting that these 3 CUs got emitted
> into one object file during LTO, but that DWARFLinker
> >>>>>>>>>>> drops a CU without any code in it - even though... So
> far as I know, in LTO, LLVM directly references
> >>>>>>>>>>> types across units if the CUs are all emitted in the
> same object file. (and if they weren't in the same
> >>>>>>>>>>> object file - then the abstract_origin couldn't be
> pointing cross-CU).
> >>>>>>>>>>> I guess some basic things to say:
> >>>>>>>>>>> With ThinLTO, the concrete/standalone function
> definition is emitted in case some call sites don't end up
> >>>>>>>>>>> being inlined. So we know it'll be emitted (but might
> not be needed by the actual linker)
> >>>>>>>>>>> ANy number of inline calls might exist - but we
> shouldn't put the type information into those, because
> >>>>>>>>>>> they aren't guaranteed to emit it (if the inline
> function gets optimized away, there would be nothing to
> >>>>>>>>>>> enforce the type being emitted) - and even if we
> forced the type information to be emitted into one
> >>>>>>>>>>> object file that has an inline copy of the function -
> there's no guarantee that object file will get linked in either.
> >>>>>>>>>>> So, no, I don't think there's much we can do to keep
> the size of object files down, while guaranteeing
> >>>>>>>>>>> the type information will be emitted with the usual
> linker semantics.
> >>>>>>>>>> Then dsymutil/DWARFLinker could be changed to handle
> that(though it would probably be not very efficient).
> >>>>>>>>>> If thinlto would understand that function is not used
> finally(and then must not contain referenced type definition),
> >>>>>>>>>> then this situation could be handled more effectively.
> >>>>>>>>>>
> >>>>>>>>>> Thank you, Alexey.
> >>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>> _______________________________________________
> >>>>>>>>>>>> LLVM Developers mailing list
> >>>>>>>>>>>> llvm-dev at lists.llvm.org <mailto: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 <mailto: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 <mailto:llvm-dev at lists.llvm.org>
> >>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
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