[llvm-dev] [Proposal][Debuginfo] dsymutil-like tool for ELF.
Alexey Lapshin via llvm-dev
llvm-dev at lists.llvm.org
Sun Nov 1 14:05:35 PST 2020
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
>
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