[llvm-dev] [Debuginfo][DWARF][LLD] Remove obsolete debug info in lld.
David Blaikie via llvm-dev
llvm-dev at lists.llvm.org
Thu May 14 18:34:26 PDT 2020
On Wed, May 13, 2020 at 12:36 PM Alexey Lapshin <alapshin at accesssoftek.com>
wrote:
> Hi David, Excuse me for delayed answer. It took some time to prepare.
> Please, find the answers bellow...
>
>
> >Broad question: Do you have any specific motivation/users/etc in
> implementing this (if you can speak about it)?
>
> > - it might help motivate the work, understand what tradeoffs might be
> suitable for you/your users, etc.
>
> There are two general requirements:
> 1) Remove (or clean) invalid debug info.
>
Perhaps a simpler direct solution for your immediate needs might be a much
narrower, and more efficient linker-DWARF-awareness feature:
With DWARFv5, rnglists present an opportunity for a DWARF linker to rewrite
the ranges without parsing the rest of the DWARF. /technically/ this isn't
guaranteed - rnglist entries can be referenced either directly, or by
index. If all rnglists are referenced by index, then a linker could parse
only the debug_rnglists section and rewrite ranges to remove any address
ranges that refer to optimized-out code.
This would only be correct for rnglists that had no direct references to
them (that only were referenced via the indexes) - but we could either
implement it with that assumption, or could add an LLVM extension attribute
on the CU that would say "I promise I only referenced rnglists via rnglistx
forms/indexes). If this DWARF-aware linking would have to read the CU DIE
(not all the other DIEs) it /could/ also then rewrite high/low_pc if the CU
wasn't using ranges... but that wouldn't come up in the function-removal
case, because then you'd have ranges anyway, so no need for that.
Such a DWARF-aware rnglist linking could also simplify rnglists, in cases
where functions ended up being laid out next to each other, the linker
could coalesce their ranges together.
I imagine this could be implemented with very little overhead to linking,
especially compared to the overhead of full DWARF-aware linking.
Though none of this fixes Split DWARF, where the linker doesn't get a
chance to see the addresses being used - but if you only want/need the
CU-level ranges to be correct, this might be a viable fix, and quite
efficient.
> 2) Optimize the DWARF size.
>
Do your users care much about this? I imagine if they had significant DWARF
size issues, they'd have significant link time issues and the kind of cost
to link time this feature has would be prohibitive - but perhaps they're
sharing linked binaries much more often than they're actually performing
linking.
>
> The specifics which our users have:
> - embedded platform which uses 0 as start of .text section.
> - custom toolset which does not support all features yet(f.e. split
> dwarf).
> - tolerant of the link-time increase.
> - need a useful way to share debug builds.
>
Sharing two files (executable and dwp) is significantly less useful than
sharing one file?
>
> For the first point: we have a problem "Overlapping address ranges
> starting from 0"(D59553).
> We use custom solution, but the general solution like D74169 would be
> better here.
>
If CU ranges are the only ones that need fixing, then I think the above
solution might be as good/better - if more than CU ranges need fixing, then
I think we might want to start talking about how to fix DWARF itself (split
and non-split) to signal certain addresses point to dead code with a
specific blessed value that linkers would need to implement - because with
Split DWARF there's no way to solve the non-CU addresses at the linker.
> For the second point: split dwarf could be a good alternative to have
> debug info with minimal size.
> Still, it has drawbacks (not supported by tools currently, does not solve
> the "Overlapping address ranges"
> problem, not very convenient to share(even using .dwp)).
>
> Thus in long terms, the D74169 looks to be a good solution for us:
> resolves "Overlapping address ranges"
> problem, binary with minimal size, supported by current tools, easy to
> share debug build(single binary with
> minimal size).
>
> > In general, in the current state, I don't have strong feelings either
> way about this going in as-is with the intent to >improve it to make it
> more viable - or some of that work being done out-of-tree until it's a more
> viable >performance tradeoff. Mostly happy to leave that up to folks more
> involved with lld.
> >
> >A couple of minor points...
>
> >> C: --function-sections --gc-sections --fdebug-types-section
>
>> > ^ not sure of the point of testing/showing comparisons with a situation
> that's currently unsupported
>
>
> that situation is currently supported(--gc-debuginfo is not used in this
> measurement).
>
Ah, I was confused because it looks like/the description said it was...
> "--fdebug-types-section" is supported functionality.
> The purpose of these data is to compare results for
> "--fdebug-types-section" and "--gc-debuginfo".
>
OK
>
>
> >>2. Support of type units.
>
>
> >> That could be implemented further.
>
>> >Enabling type units increases object size to make it easier to
> deduplicate at link time by a DWARF-unaware
> >linker. With a DWARF aware linker it'd be generally desirable not to have
> to add that object size overhead to
> >get the linking improvements.
>
> But, DWARFLinker should adequately work with type units since they are
> already implemented.
>
Maybe - it'd be nice & all, but I don't think it's an outright necessity -
if someone knows they're using a DWARF-aware linker, they'd probably not
use type units in their object files. It's possible someone doesn't know
for sure & maybe they have pre-canned debug object files from someone else,
etc.
> If someone uses --fdebug-types-section, then it should adequately work
> when used together
> with --gc-debuginfo(if --gc-debuginfo would be accepted).
> Right?
>
> Another thing is that the idea behind type units has the potential to help
> Dwarf-aware linker to work faster.
> Currently, DWARFLinker analyzes context to understand whether types are
> the same or not.
>
When you say "analyzes context" what do you mean? Usually I'd take that to
mean "looks at things outside the type itself - like what namespace it's
in, etc" - which, yes, it should do that, but it doesn't seem very
expensive to do. But I guess you actually mean something about doing
structural equivalence in some way, looking at things inside the type?
> But the context is known when types are generated. So, no need to spent
> the time analyzing it.
> If types could be compared without analyzing context, then Dwarf-aware
> linker would work faster.
> That is just an idea(not for immediate implementation): If types would be
> stored in some "type table"
> (instead of COMDAT section group) and could be accessed through
> hash-id(like type units)
> - then it would be the solution requiring fewer bits to store but allowing
> to compare types
> by hash-id(not analysing context).
> In this case, size increasing would be small. And processing time could be
> done faster.
>
> this is just an idea and could be discussed separately from the problem of
> integrating of D74169.
>
>
> >>4. split DWARF support.
>
>
> >> This solution does not work with split DWARF currently. But it could
> be useful for the split dwarf in two ways:
> >> a) The generation of skeleton file could be changed in such a way
> that address ranges pointing to garbage
>
> >> collected code would be replaced with lowpc=0, highpc=0. That would
> solve the problem of overlapping
>
> >> address ranges(D59553).
>
>
> >This wouldn't/couldn't completely address the issue - because some
> address ranges would be in the .dwo files >the linker can't see - and
> they'd still end up with the interesting address ranges.
>
> I see, Thank you. Thus it would not be a complete solution.
>
>
> >> 6. -flto=thin
>
>
> >> That problem was described in this review
> https://reviews.llvm.org/D54747#1503720. It also exists in
>
> >> current DWARFLinker/dsymutil implementation. I think that problem
> should be discussed more: it could
>
> >> probably be fixed by avoiding generation of such incomplete declaration
> during thinlto,
>
>> >> That would be costly to produce extra/redundant debug info in ThinLTO
> - actually ThinLTO could be doing
> >> more to reduce that redundancy early on (actually removing definitions
> from some llvm Modules if the type
> >> definition is known to exist in another Module, etc)
>
> >I don't know if it's a problem since that patch was reverted.
>
> Yes. That patch was reverted, but this patch(D74169) has the same problem.
> if D74169 would be applied and --gc-debuginfo used then structure type
> definition would be removed.
>
> DWARFLinker could handle that case - "removing definitions from some llvm
> Modules if the type
> definition is known to exist in another Module".
> i.e. DWARFLinker could replace the declaration with the definition.
>
> But that problem could be more easily resolved when debug info is
> generated(probably without
> significant increase of debug info size):
>
> Let`s check the example:
>
> 0x0000000b: DW_TAG_compile_unit
> DW_AT_low_pc (0x0000000000201700)
> DW_AT_high_pc (0x0000000000201719)
>
> 0x0000002a: DW_TAG_subprogram
> 0x00000043: DW_TAG_inlined_subroutine
> DW_AT_abstract_origin (0x0000000000000086 "_Z1fv")
> DW_AT_low_pc (0x0000000000201700)
> DW_AT_high_pc (0x0000000000201718)
>
> 0x00000057: DW_TAG_variable
> DW_AT_abstract_origin (0x0000000000000096 "var")
> 0x00000065: NULL
>
> 0x00000073: DW_TAG_compile_unit
> DW_AT_stmt_list (0x00000080)
>
> 0x00000086: DW_TAG_subprogram
> DW_AT_name ("f")
> DW_AT_inline (DW_INL_inlined)
>
> 0x00000096: DW_TAG_variable
> DW_AT_name ("var")
> DW_AT_type (0x000000a9 "volatile Foo")
> 0x000000a1: NULL
>
> 0x000000a9: DW_TAG_volatile_type
> DW_AT_type (0x000000ae "Foo")
>
> 0x000000ae: DW_TAG_structure_type
> DW_AT_name ("Foo")
> DW_AT_declaration (true)
>
> 0x000000c1: DW_TAG_compile_unit
> DW_AT_low_pc (0x0000000000000000)
> DW_AT_high_pc (0x0000000000000019)
>
> 0x000000e0: DW_TAG_subprogram
> DW_AT_low_pc (0x0000000000000000)
> DW_AT_high_pc (0x0000000000000019)
> DW_AT_name ("f")
>
> 0x000000fd: DW_TAG_variable
> DW_AT_name ("var")
> DW_AT_type (0x00000119 "volatile Foo")
>
> 0x00000119: DW_TAG_volatile_type
> DW_AT_type (0x0000011e "Foo")
>
> 0x0000011e: DW_TAG_structure_type
> DW_AT_name ("Foo")
> DW_AT_decl_line (1)
>
> Here we have:
>
> DW_TAG_compile_unit(0x0000000b) - compile unit containing concrete
> instance for function "f".
> DW_TAG_compile_unit(0x00000073) - compile unit containing abstract
> instance root for function "f".
> DW_TAG_compile_unit(0x000000c1) - compile unit containing function "f"
> definition.
>
> Code for function "f" was deleted. gc-debuginfo deletes compile unit
> DW_TAG_compile_unit(0x000000c1)
> containing "f" definition (since there is no corresponding code). But it
> has structure "Foo" definition
> DW_TAG_structure_type(0x0000011e) referenced from
> DW_TAG_compile_unit(0x00000073)
> by declaration DW_TAG_structure_type(0x000000ae). That declaration is
> exactly the case when definition
> was removed by thinlto and replaced with declaration.
>
> Would it cost too much if type definition would not be replaced with
> declaration for "abstract instance root"?
> The number of concrete instances is bigger than number of abstract
> instance roots.
> Probably, it would not be too costly to leave definition in abstract
> instance root?
>
> Alternatively, Would it cost too much if type definition would not be
> replaced with declaration when declaration references type from not used
> function? (lto could understand that concrete function is not used).
>
I don't follow this example - could you provide a small concrete test case
I could reproduce?
Oh, I guess this is happening perhaps because ThinLTO can't know for sure
that a standalone definition of 'f' won't be needed - so it produces one in
case one of the inlining opportunities doesn't end up inlining. Then it
turns out all calls got inlined, so the external definition wasn't needed.
Oh, you're suggesting that these 3 CUs got emitted into one object file
during LTO, but that DWARFLinker drops a CU without any code in it - even
though... So far as I know, in LTO, LLVM directly references types across
units if the CUs are all emitted in the same object file. (and if they
weren't in the same object file - then the abstract_origin couldn't be
pointing cross-CU).
I guess some basic things to say:
With ThinLTO, the concrete/standalone function definition is emitted in
case some call sites don't end up being inlined. So we know it'll be
emitted (but might not be needed by the actual linker)
ANy number of inline calls might exist - but we shouldn't put the type
information into those, because they aren't guaranteed to emit it (if the
inline function gets optimized away, there would be nothing to enforce the
type being emitted) - and even if we forced the type information to be
emitted into one object file that has an inline copy of the function -
there's no guarantee that object file will get linked in either.
So, no, I don't think there's much we can do to keep the size of object
files down, while guaranteeing the type information will be emitted with
the usual linker semantics.
>
>
> Thank you, Alexey.
>
>
>>
>>
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>
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