[llvm-dev] [Proposal][Debuginfo] dsymutil-like tool for ELF.

Tue Oct 27 15:49:16 PDT 2020

On Tue, Oct 27, 2020 at 12:34 PM Alexey Lapshin <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>
> 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>
>> 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.

>
>
>> 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.

> 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.

> 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.

- Dave
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