[llvm-dev] [Proposal][Debuginfo] dsymutil-like tool for ELF.
Alexey Lapshin via llvm-dev
llvm-dev at lists.llvm.org
Wed Oct 28 06:01:16 PDT 2020
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?
> 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. The current solution - loading everything in
memory - makes it hard to use in a non-dsymutil scenario(llvm-dwarfutil).
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.
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/20201028/f8ef4a2d/attachment.html>
More information about the llvm-dev
mailing list