[llvm-dev] [Proposal][Debuginfo] dsymutil-like tool for ELF.
Alexey Lapshin via llvm-dev
llvm-dev at lists.llvm.org
Mon Nov 2 02:26:53 PST 2020
On 02.11.2020 04:11, David Blaikie wrote:
> I think if we're in the realm of DWARF extensions a whole bunch of
> other considerations come into it (& indeed, your suggested proposal
> may be a good one - but I think it's a very wide problem space once
> we're considering DWARF extensions). Mostly I was making
> arguments/suggestions/thoughts on the basis of being compatible with
> all existing DWARF producers.
the described scenario does not assume DWARF extensions. global type
table is not new DWARF construction. This is an artificial CU keeping
all types. That solution would be compatible with existing DWARF
consumers/produces.
>
> On Sun, Nov 1, 2020 at 2:05 PM Alexey Lapshin <avl.lapshin at gmail.com
> <mailto:avl.lapshin at gmail.com>> wrote:
>
>
> On 28.10.2020 20:38, David Blaikie wrote:
>>
>>
>> On Wed, Oct 28, 2020 at 6:01 AM Alexey Lapshin
>> <avl.lapshin at gmail.com <mailto:avl.lapshin at gmail.com>> wrote:
>>
>>
>> On 28.10.2020 01:49, David Blaikie wrote:
>>>
>>>
>>> On Tue, Oct 27, 2020 at 12:34 PM Alexey Lapshin
>>> <avl.lapshin at gmail.com <mailto:avl.lapshin at gmail.com>> wrote:
>>>
>>>
>>> On 27.10.2020 20:32, David Blaikie wrote:
>>>>
>>>>
>>>> On Tue, Oct 27, 2020 at 1:23 AM Alexey Lapshin
>>>> <avl.lapshin at gmail.com <mailto:avl.lapshin at gmail.com>>
>>>> wrote:
>>>>
>>>>
>>>> On 26.10.2020 22:38, David Blaikie wrote:
>>>>>
>>>>>
>>>>> On Sun, Oct 25, 2020 at 9:31 AM Alexey Lapshin
>>>>> <avl.lapshin at gmail.com
>>>>> <mailto:avl.lapshin at gmail.com>> wrote:
>>>>>
>>>>>
>>>>> On 23.10.2020 19:43, David Blaikie wrote:
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Ah, yeah - that seems like a missed
>>>>>>> opportunity - duplicating the whole type
>>>>>>> DIE. LTO does this by making monolithic
>>>>>>> types - merging all the members from
>>>>>>> different definitions of the same type
>>>>>>> into one, but that's maybe too expensive
>>>>>>> for dsymutil (might still be interesting
>>>>>>> to know how much more expensive, etc).
>>>>>>> But I think the other way to go would be
>>>>>>> to produce a declaration of the type,
>>>>>>> with the relevant members - and let the
>>>>>>> DWARF consumer identify this declaration
>>>>>>> as matching up with the earlier
>>>>>>> definition. That's the sort of DWARF you
>>>>>>> get from the non-MachO default
>>>>>>> -fno-standalone-debug anyway, so it's
>>>>>>> already pretty well tested/supported
>>>>>>> (support in lldb's a bit younger/more
>>>>>>> work-in-progress, admittedly). I wonder
>>>>>>> how much dsym size there is that could
>>>>>>> be reduced by such an implementation.
>>>>>>
>>>>>> I see. Yes, that could be done and I
>>>>>> think it would result in noticeable size
>>>>>> reduction(I do not know exact numbers at
>>>>>> the moment).
>>>>>>
>>>>>> I work on multi-thread DWARFLinker now
>>>>>> and it`s first version will do exactly
>>>>>> the same type processing like current
>>>>>> dsymutil.
>>>>>>
>>>>>> Yeah, best to keep the behavior the same
>>>>>> through that
>>>>>>
>>>>>> Above scheme could be implemented as a
>>>>>> next step and it would result in better
>>>>>> size reduction(better than current state).
>>>>>>
>>>>>> But I think the better scheme could be
>>>>>> done also and it would result in even
>>>>>> bigger size reduction and in faster
>>>>>> execution. This scheme is something
>>>>>> similar to what you`ve described above:
>>>>>> "LTO does - making monolithic types -
>>>>>> merging all the members from different
>>>>>> definitions of the same type into one".
>>>>>>
>>>>>> I believe the reason that's probably not been
>>>>>> done is that it can't be streamed - it'd lead
>>>>>> to buffering more of the output
>>>>>
>>>>> yes. The fact that DWARF should be streamed
>>>>> into AsmPrinter complicates parallel dwarf
>>>>> generation. In my prototype, I generate
>>>>> several resulting files(each for one source
>>>>> compilation unit) and then sequentially glue
>>>>> them into the final resulting file.
>>>>>
>>>>> How does that help? Do you use relocations in
>>>>> those intermediate object files so the DWARF in
>>>>> them can refer across files?
>>>>
>>>> It does not help with referring across the file. It
>>>> helps to parallel the generation of CU bodies.
>>>> It is not possible to write two CUs in parallel
>>>> into AsmPrinter. To make possible parallel
>>>> generation I stream them into different
>>>> AsmPrinters(this comment is for "I believe the
>>>> reason that's probably not been done is that it
>>>> can't be streamed". which initially was about
>>>> referring across the file, but it seems I added
>>>> another direction).
>>>>
>>>> Oh, I see - thanks for explaining, essentially
>>>> buffering on-disk.
>>>>
>>>>>
>>>>>> (if two of these expandable types were in one
>>>>>> CU - the start of the second type couldn't be
>>>>>> known until the end because it might keep
>>>>>> getting pushed later due to expansion of the
>>>>>> first type) and/or having to revisit all the
>>>>>> type references (the offset to the second
>>>>>> type wouldn't be known until the end - so
>>>>>> writing the offsets to refer to the type
>>>>>> would have to be deferred until then).
>>>>>
>>>>> That is the second problem: offsets are not
>>>>> known until the end of file.
>>>>> dsymutil already has that situation for
>>>>> inter-CU references, so it has extra pass to
>>>>> fixup offsets.
>>>>>
>>>>> Oh, it does? I figured it was one-pass, and that
>>>>> it only ever refers back to types in previous CUs?
>>>>> So it doesn't have to go back and do a second
>>>>> pass. But I guess if sees a declaration of T1 in
>>>>> CU1, then later on sees a definition of T1 in CU2,
>>>>> does it somehow go back to CU1 and remove the
>>>>> declaration/make references refer to the
>>>>> definition in CU2? I figured it'd just leave the
>>>>> declaration and references to it as-is, then add
>>>>> the definition and use that from CU2 onwards?
>>>>
>>>> For the processing of the types, it do not go back.
>>>> This "I figured it was one-pass, and that it only
>>>> ever refers back to types in previous CUs"
>>>> and this "I figured it'd just leave the declaration
>>>> and references to it as-is, then add the definition
>>>> and use that from CU2 onwards" are correct.
>>>>
>>>> Great - thanks for explaining/confirming!
>>>>
>>>>
>>>>> With multi-thread implementation such
>>>>> situation would arise more often
>>>>> for type references and so more offsets should
>>>>> be fixed during additional pass.
>>>>>
>>>>>> DWARFLinker could create additional
>>>>>> artificial compile unit and put all
>>>>>> merged types there. Later patch all type
>>>>>> references to point into this additional
>>>>>> compilation unit. No any bits would be
>>>>>> duplicated in that case. The performance
>>>>>> improvement could be achieved due to less
>>>>>> amount of the copied DWARF and due to the
>>>>>> fact that type references could be
>>>>>> updated when DWARF is cloned(no need in
>>>>>> additional pass for that).
>>>>>>
>>>>>> "later patch all type references to point
>>>>>> into this additional compilation unit" -
>>>>>> that's the additional pass that people are
>>>>>> probably talking/concerned about. Rewalking
>>>>>> all the DWARF. The current dsymutil approach,
>>>>>> as far as I know, is single pass - it knows
>>>>>> the final, absolute offset to the type from
>>>>>> the moment it emits that type/needs to refer
>>>>>> to it.
>>>>>
>>>>> Right. Current dsymutil approach is single
>>>>> pass. And from that point of view, solution
>>>>> which you`ve described(to produce a
>>>>> declaration of the type, with the relevant
>>>>> members)
>>>>> allows to keep that single pass implementation.
>>>>>
>>>>> But there is a restriction for current
>>>>> dsymutil approach: To process inter-CU references
>>>>> it needs to load all DWARF into the
>>>>> memory(While it analyzes which part of DWARF
>>>>> is live,
>>>>> it needs to have all CUs loaded into the memory).
>>>>>
>>>>> All DWARF for a single file (which for dsymutil is
>>>>> mostly a single CU, except with LTO I guess?), not
>>>>> all DWARF for all inputs in memory at once, yeah?
>>>>
>>>> right. In dsymutil case - all DWARF for a single
>>>> file(not all DWARF for all inputs in memory at once).
>>>> But in llvm-dwarfutil case single file contains
>>>> DWARF for all original input object files and it
>>>> all becomes
>>>> loaded into memory.
>>>>
>>>> Yeha, would be great to try to go CU-by-CU.
>>>>
>>>>> That leads to huge memory usage.
>>>>> It is less important when source is a set of
>>>>> object files(like in dsymutil case) and this
>>>>> become a real problem for llvm-dwarfutil
>>>>> utility when source is a single file(With current
>>>>> implementation it needs 30G of memory for
>>>>> compiling clang binary).
>>>>>
>>>>> Yeah, that's where I think you'd need a fixup pass
>>>>> one way or another - because cross-CU references
>>>>> can mean that when you figure out a new layout for
>>>>> CU5 (because it has a duplicate type definition of
>>>>> something in CU1) then you might have to touch CU4
>>>>> that had an absolute/cross-CU forward reference to
>>>>> CU5. Once you've got such a fixup pass (if
>>>>> dsymutil already has one? Which, like I said, I'm
>>>>> confused why it would have one/that doesn't match
>>>>> my very vague understanding) then I think you
>>>>> could make dsymutil work on a per-CU basis
>>>>> streaming things out, then fixing up a few offsets.
>>>>
>>>> When dsymutil deduplicates types it changes local
>>>> CU reference into inter-CU reference(so that
>>>> CU2(next) could reference type definition from
>>>> CU1(prev)). To do this change it does not need to
>>>> do any fixups currently.
>>>>
>>>> When dsymutil meets already existed(located in the
>>>> input object file) inter-CU reference pointing into
>>>> the CU which has not been processed yet(and then
>>>> its offset is unknown) it marks it as "forward
>>>> reference" and patches later during additional pass
>>>> "fixup forward references" at a time when offsets
>>>> are known.
>>>>
>>>> OK, so limited 2 pass system. (does it do that second
>>>> pass once at the end of the whole dsymutil run, or at
>>>> the end of each input file? (so if an input file has
>>>> two CUs and the first CU references a type in the
>>>> second CU - it could write the first CU with a "forward
>>>> reference", then write the second CU, then fixup the
>>>> forward reference - and then go on to the next file and
>>>> its CUs - this could improve performance by touching
>>>> recently used memory/disk pages only, rather than going
>>>> all the way back to the start later on when those pages
>>>> have become cold)
>>>
>>> yes, It does it in the end of each input file.
>>>
>>>
>>>>
>>>> If CUs would be processed in parallel their offsets
>>>> would not be known at the moment when local type
>>>> reference would be changed into inter-CU reference.
>>>> So we would need to do the same fix-up processing
>>>> for all references to the types like we already do
>>>> for other inter-CU references.
>>>>
>>>> Yeah - though the existence of this second "fixup
>>>> forward references" system - yeah, could just use it
>>>> much more generally as you say. Not an extra pass, just
>>>> the existing second pass but having way more fixups to
>>>> fixup in that pass.
>>> If we would be able to change the algorithm in such way :
>>>
>>> 1. analyse all CUs.
>>> 2. clone all CUs.
>>>
>>> Then we could create a merged type table(artificial CU
>>> containing types) during step1.
>>> If that type table would be written first, then all
>>> following CUs could use known offsets
>>> to the types and we would not need additional fix-up
>>> processing for type references.
>>> It would still be necessary to fix-up other inter-CU
>>> references. But it would not be necessary
>>> to fix-up type references (which constitute the vast
>>> majority).
>>>
>>>
>>> To me, that sounds more expensive than the fixup forward
>>> references pass.
>>
>> If we would speak about direct comparison then yes loading
>> DWARF one more time looks more expensive than fixup forward
>> references pass. But if we would speak about the general
>> picture then it could probably be beneficial:
>>
>> 1. merging types would lead to a smaller size of resulting
>> DWARF. This would speed up the process.
>> f.e. If we would switch "odr types deduplication" off in
>> current implementation then it would increase execution time
>> two times. That is because more DWARF should be cloned and
>> written in the result. Implementation of "merging types"
>> would probably have a similar effect
>> - It would speed-up the overall process. So from one side
>> additional step for loading DWARF would
>> decrease performance but a smaller amount of resulting
>> data would increase performance.
>>
>> 2. When types would be put in the first CU then we would have
>> a simple strategy for our liveness analysis algorithm: just
>> always keep the first CU in memory. This allows us to speed
>> up our liveness analysis step.
>>
>> Anyway, all the above is just an idea for future work.
>> Currently, I am going to implement multithread processing for
>> CUs loaded into memory and having the same type of processing
>> as it currently is(Which assumes that "fixup forward
>> references pass" started to do more work by fixing types
>> references).
>>
>>
>>>
>>>>
>>>>> Without loading all CU into the memory it
>>>>> would require two passes solution. First to
>>>>> analyze
>>>>> which part of DWARF relates to live code and
>>>>> then second pass to generate the result.
>>>>>
>>>>> Not sure it'd require any more second pass than a
>>>>> "fixup" pass, which it sounds like you're saying
>>>>> it already has?
>>>>
>>>> It looks like it would need an additional pass to
>>>> process inter-CU references(existed in incoming
>>>> file) if we do not want to load all CUs into memory.
>>>>
>>>> Usually inter-CU references aren't used, except in LTO
>>>> - and in LTO all the DWARF deduplication and function
>>>> discarding is already done by the IR linker anyway.
>>>> (ThinLTO is a bit different, but really we'd be better
>>>> off teaching it the extra tricks anyway (some can't be
>>>> fixed in ThinLTO - like emitting a "Home" definition of
>>>> an inline function, only to find out other ThinLTO
>>>> backend/shards managed to optimize away all uses of the
>>>> function... so some cleanup may be useful there)). It
>>>> might be possible to do a more dynamic/rolling cache -
>>>> keep only the CUs with unresolved cross-CU references
>>>> alive and only keep them alive until their cross-CU
>>>> references are found/marked alive. This should make
>>>> things no worse than the traditional dsymutil case -
>>>> since cross-CU references are only effective/generally
>>>> used within a single object file (it's possible to
>>>> create relocations for them into other files - but I
>>>> know LLVM doesn't currently do this and I don't think
>>>> GCC does it) with multiple CUs anyway - so at most
>>>> you'd keep all the CUs from a single original input
>>>> file alive together.
>>> But, since it is a DWARF documented case the tool should
>>> be ready for such case(when inter-CU
>>> references are heavily used).
>>>
>>>
>>> Sure - but by implementing a CU liveness window like that
>>> (keeping CUs live only so long as they need to be rather
>>> than an all-or-nothing approach) only especially quirky
>>> inputs would hit the worst case while the more normal inputs
>>> could perform better.
>>
>> It is not clear what should be put in such CU liveness
>> window. If CU100 references CU1 - how could we know that we
>> need to put CU1 into CU liveness window before we processed
>> CU100?
>>
>> Fair point, not just forward references to worry about but
>> backward references too. I wonder how much savings there is in
>> the liveness analysis compared to "keep one copy of everything,
>> no matter whether it's live or not", then it can be a pure
>> forward progress situation. (with the quirk that you might emit a
>> declaration for an entity once, then a definition for it later -
>> alternatively if a declaration is seen it could be skipped under
>> the assumption that a definition will follow (& use a forward ref
>> fixup) - and if none is found, splat some stub declarations into
>> a trailing CU at the end)
>>
>>
>>> Moreover, llvm-dwarfutil would be the tool producing
>>> exactly such situation. The resulting file(produced by
>>> llvm-dwarfutil) would contain a lot of
>>> inter-CU references. Probably, there is no practical
>>> reasons to apply llvm-dwarfutil to the same
>>> file twice but it would be a good test for the tool.
>>>
>>>
>>> It'd be a good stress test, but not necessarily something
>>> that would need to perform the best because it wouldn't be a
>>> common use case.
>>
>> I agree that we should not slow down the DWARFLinker in
>> common cases only because we need to support the worst cases.
>> But we also need to implement a solution which works in some
>> acceptable manner for the worst case.
>>
>> I think that depends on "acceptable" - correct, yes. Practical to
>> run in reasonable time/memory? Not necessarily, in my opinion.
>>
>> The current solution - loading everything in memory - makes
>> it hard to use in a non-dsymutil scenario(llvm-dwarfutil).
>>
>> I agree it's worth exploring the non-dsymutil scenario, as you
>> are - I'm just saying we don't necessarily need to support high
>> usability (fast/low memory usage/etc) llvm-dwarfutil on an
>> already dwarfutil'd binary (but as you've pointed out, the
>> "window" is unknowable because of backward references, so this
>> whole subthread is perhaps irrelevant).
>>
>> There could be several things which could be used to decide
>> whether we need to go on a light or heavy path:
>>
>> 1. If the input contains only a single CU we do not need to
>> unload it from memory. Thus - we would not need to do an
>> extra DWARF loading pass.
>> 2. If abbreviations from the whole input file do not contain
>> inter-CU references then while doing liveness analysis, we
>> do not need to wait until other CUs are processed.
>>
>> (2) Yeah, that /may/ be a good idea, cheap to test, etc. Though
>> I'd still wonder if a more general implementation strategy could
>> be found that would make it easier to get a sliding scale of
>> efficiency depending on how much inter-CU references where were,
>> not a "if there are none it's good, if there are any it's bad or
>> otherwise very different to implement".
>
> I think, there is a scenario which would make it possible to
> process CU once for not referenced CUs and handle inter-CU
> references in a scalable way(even for dwarfutil`d binary):
>
> 1. Implement a global type's table and types merging. This allows
> us to have all types in the memory.
> Then, all inter-CU type references would point into that memory
> type table.
> (we do not know which CU should be put into CU liveness window,
> we also could not put all CUs into the memory, but we could put
> all types into the memory).
>
> 2. If there are not other inter-CU references then all CUs would
> be handled by one pass.
>
> 3. If there are other inter-CU references, then after all CU
> processed by the first pass we would have a list of referenced
> CUs. Then, we could delete already cloned data(for referenced CU)
> and start the process again:
> load CU, mark liveness, clone data. This second pass would be
> done for only referenced CUs.
> For not-complex, not closely coupled cases it would work
> relatively fast.
>
> 4. put memory type table into artificial CU. Update all type`s
> references.
>
>
>>
>> Then that scheme would be used for worst cases:
>>
>> 1. for (CU : CU1...CU100) {
>> load CU.
>> analyse CU.
>> unload CU.
>> }
>> 2. for (CU : CU1...CU100) {
>> load CU.
>> clone CU.
>> unload CU.
>> }
>> 3. fixup forward references.
>>
>> and that scheme for light cases:
>>
>> 1. for (CU : CU1...CU100) {
>> load CU.
>> analyse CU.
>> clone CU.
>> unload CU.
>> }
>> 2. fixup forward references.
>>
>>> Generally, I think we should not assume that inter-CU
>>> references would be used in a limited way.
>>>
>>> Anyway, if this scheme:
>>>
>>> 1. analyse all CUs.
>>> 2. clone all CUs.
>>>
>>> would work slow then we would need to continue with
>>> one-pass solution and not support complex closely
>>> coupled inputs.
>>>
>>>
>>> yeah, certainly seeing the data/experiments will be
>>> interesting, if you end up implementing some different
>>> strategies, etc.
>>>
>>> I guess one possibility for parallel generation could be
>>> something more like Microsoft's approach with a central
>>> debug info server that compilers communicate with - not that
>>> exact model, I mean, but if you've got parallel threads
>>> generating reduced DWARF into separate object files - they
>>> could communicate with a single thread responsible for type
>>> emission - the type emitter would be given types from the
>>> separate threads and compute their size, queue them up to be
>>> streamed out to the type CU (& keep the source CU alive
>>> until that work was done) - such a central type emitter
>>> could quickly determine the size of the type to be emitted
>>> and compute future type offsets (eg: if 5 types were in the
>>> queue, it could've figured out the offset of those types
>>> already) to answer type offset queries quickly and unblock
>>> the parallel threads to continue emitting their CUs
>>> containing type references.
>>
>> yes. Thank you. Would think about it.
>>
>> Alexey.
>>
>>>
>>> - Dave
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20201102/7ab208c3/attachment-0001.html>
More information about the llvm-dev
mailing list