llvm dev - Jun 2020 - [Debuginfo][DWARF][LLD] Remove obsolete debug info in lld.

>On Mon, Jun 22, 2020 at 7:21 AM Alexey Lapshin
><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.
2. DWARFLinker from lld = linking time for clang 72 sec, clang=760M.

>> 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.
>> >> 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. 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.
>
>> 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. 
>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.

Thank you, Alexey.
> >> >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> On
Behalf Of James Henderson via llvm-dev
> > > > Sent: Wednesday, June 3, 2020 3:48 AM
> > > > To: David Blaikie <dblaikie at gmail.com>
> > > > Cc: 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> wrote:
> > > >
> > > > On Tue, May 19, 2020 at 7:17 AM Alexey Lapshin
> > > > <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
> > > > >>>
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

On Thu, Jun 25, 2020 at 2:23 PM Alexey Lapshin
<alapshin at accesssoftek.com> wrote:
>
>
> >On Mon, Jun 22, 2020 at 7:21 AM Alexey Lapshin
> ><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?
> 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?
> >> 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.
> >> >> 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.
> >> 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.

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?
> >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!
>
> Thank you, Alexey.
>
> > >> >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> On Behalf Of James Henderson via llvm-dev
> > > > > Sent: Wednesday, June 3, 2020 3:48 AM
> > > > > To: David Blaikie <dblaikie at gmail.com>
> > > > > Cc: 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> wrote:
> > > > >
> > > > > On Tue, May 19, 2020 at 7:17 AM Alexey Lapshin
> > > > > <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
> > > > > >>>
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

>> >> >> >> 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> On Behalf Of James Henderson via llvm-dev
> > > > > Sent: Wednesday, June 3, 2020 3:48 AM
> > > > > To: David Blaikie <dblaikie at gmail.com>
> > > > > Cc: 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> wrote:
> > > > >
> > > > > On Tue, May 19, 2020 at 7:17 AM Alexey Lapshin
> > > > > <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
> > > > > >>>
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> > > > > _______________________________________________
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> > > > > llvm-dev at lists.llvm.org
> > > > >
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