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

Alexey Lapshin via llvm-dev
Tue, 27 Oct 2020 08:23:00 -0000

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).

>
>>     (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.

>     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.

>     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.

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.

>     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.
When the input file contains inter-CU references, DWARFLinker needs to
follow them while doing liveness marking. i.e. if the original CU has a
live part which references another CU we need to follow this new CU and
mark the referenced part as life. At the current moment, while doing
liveness analysis, we have all CUs in memory. That allows us to load all
CUs once and analyze them all. In case llvm-dwarfutil(which loads all
DWARF for input file) it leads to huge memory usage.

Let's say CU1 references CU100. And CU100 references CU1. We could not
start generation for CU1 until we analyzed CU100 and marked the
corresponding part of CU1 as life. At the same time, we could not load
DWARF for all CUs. Then processing(in simplified form) could look like this:

1: for (CU : CU1...CU100)
   load CU, do liveness analysis, remember references, unload CU

2: for (all references)
   load CU, do liveness analysis, unload CU

3: for (CU : CU1...CU100)
   load CU, clone CU

That is a simplified scheme, but I think it is enough to show the idea.
In this scheme we have 1 and 2 which should be done before 3.


Alexey.

>     If we would have a two passes solution then we could create a
>     compilation unit with all
>     types at first pass and at the second pass we could generate
>     result with correct offsets(no
>     need to fix up them as it is currently required by dsymutil for
>     forward inter-CU references).
>     The open question currently: how expensive this two passes
>     approach is.
>
>     Thank you, Alexey.
>
>>         Anyway, that might be the next step after multi-thread
>>         DWARFLinker would be ready.
>>
>>     Yep, be interesting to see how it all goes!
>>
>>>
>>>             Do you suggest that 0x0000011b should be transformed
>>>             into something like that:
>>>
>>>             0x000000fc: DW_TAG_compile_unit
>>>                           DW_AT_language (DW_LANG_C_plus_plus)
>>>                           DW_AT_name ("templ.cpp")
>>>                           DW_AT_stmt_list (0x00000090)
>>>                           DW_AT_low_pc (0x0000000100000fa0)
>>>                           DW_AT_high_pc (0x0000000100000fab)
>>>
>>>             0x0000011b:   DW_TAG_structure_type
>>>                             DW_AT_specification (0x0000002a
"x")
>>>
>>>             0x00000124:     DW_TAG_subprogram
>>>             DW_AT_linkage_name ("_ZN1x2f3IiEEiv")
>>>                               DW_AT_name
("f3<int>")
>>>                               DW_AT_type (0x000000000000005e
"int")
>>>             DW_AT_declaration     (true)
>>>             DW_AT_external        (true)
>>>             DW_AT_APPLE_optimized (true)
>>>             0x00000138:       NULL
>>>             0x00000139:     NULL
>>>
>>>             0x00000140:   DW_TAG_subprogram
>>>                             DW_AT_low_pc (0x0000000100000fa0)
>>>                             DW_AT_high_pc (0x0000000100000fab)
>>>             DW_AT_specification (0x0000000000000124
"_ZN1x2f3IiEEiv")
>>>             0x00000155:     NULL
>>>
>>>             Did I correctly get the idea?
>>>
>>>
>>>         Yep, more or less. It'd be "safer" if 11b
didn't use
>>>         DW_AT_specification to refer to 2a, but instead was only a
>>>         completely independent declaration of "x" - that
path is
>>>         already well supported/tested (well, it's the
>>>         work-in-progress stuff for lldb to support
>>>         -fno-standalone-debug, but gdb's been consuming DWARF
like
>>>         this for years, Clang and GCC both produce DWARF like this
>>>         (if the type is "homed" in another file, then
Clang/GCC
>>>         produce DWARF that emits a declaration with just the
members
>>>         needed to define any member functions
>>>         defined/inlined/referenced in this CU)) for years.
>>>
>>>         But using DW_AT_specification, or maybe some other
extension
>>>         attribute might make the consumers task a bit easier (could
>>>         do both - use an extension attribute to tie them up, leave
>>>         DW_AT_declaration/DW_AT_name here for consumers that
don't
>>>         understand the extension attribute) in finding that
they're
>>>         all the same type/pieces of teh same type.
>>
>>         yes. would try this solution.
>>
>>         Thank you, Alexey.
>>
>>
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