[lldb-dev] LLDB Demangling

Pavel Labath via lldb-dev lldb-dev at lists.llvm.org
Wed Jul 25 04:11:22 PDT 2018

Hello Stefan,

thank you for working on this. I believe this work will bring a huge
improvement to LLDB.

Adding something like DemangleWithRichNameIndexInfo to the Mangled
class makes sense to me. However, I don't think that function should
be taking an ItaniumPartialDemangler as an argument. The reason for
that is that ItaniumPartialDemangler is, unsurprisingly, specific to
the itanium mangling scheme, and the Mangled class tries to hide the
specifics of different manglings.

I would propose to create a new class (RichMangingInfo?), which would
wrap both ItaniumPartialDemangler and the existing string-chopping
implementation, and provide a unified interface on top of them.
Internally it would use IPD for the itanium scheme and the legacy
implementation for the MSVC mangling, but the user would not have to
care about that, as it can just ask questions like "is this a
constructor?" and get the correct answer either way. If ever we get
something similar to IPD for the MSVC mangling scheme, we can just
replace the legacy implementation with that one, and nobody should
know the difference.

What do you think?

BTW, right now the Symtab class builds a table containing all
demangled names. This is mostly unnecessary, and in fact we have
recently removed a similar table for the demangled debug_info names.
The only present use for it that I am aware of is papering over a
clang "bug"*. If we are able to get rid of this, then we might be able
to get further speedup by having DemangleWithRichNameIndexInfo *not*
fill out the demangled string (as, I understand, building the actual
string is the most expensive operation), and just fetching the bits of
info we need from the IPD.

(*) The "bug" is that sometimes clang will not generate the C1 (full
object) flavour of an constructor if it is identical to the C2 version
(even though the CU in question definitely constructs full objects of
the given class). This means that we are not able able to construct
some objects during expression evaluation as we cannot find the C1
version anywhere. The demangling makes this work accidentally, as were
are able to match up the constructors by demangled names, as they both
demangle to the same string. I have recently fixed clang to emit C1
always (at -O0), but the question remains on what to do with older

On Tue, 24 Jul 2018 at 21:55, Stefan Gränitz <stefan.graenitz at gmail.com> wrote:
> Hello everyone
> I am relatively new to the LLDB sources and just submitted my first own
> patch for review in Phabricator. Based on this patch I would like to
> discuss a few details for further improvements on LLDB's demangling.
> First a short recap on the current state:
> * Name demangling used to be a lazy process, exclusively accessible via
> Mangled::GetDemangledName() - this is a valuable mechanism:
> https://github.com/llvm-mirror/lldb/blob/8ba903256fd92a2d8644b108a7c8a1a15efd90ad/source/Core/Mangled.cpp#L252
> * My patch wants to replace the existing combination of FastDemangle &
> itaniumDemangle() with LLVM's new ItaniumPartialDemangler (IPD)
> implementation and no fallbacks anymore. It slightly reduces complexity
> and slightly improves performance, but doesn't introduce conceptual
> changes: https://reviews.llvm.org/D49612
> * IPD provides rich information on names, e.g. isFuntion() or
> isCtorOrDtor(), but stores that in its own state rather than returning a
> queriable object:
> https://github.com/llvm-mirror/llvm/blob/a3de0cbb8f4d886a968d20a8c6a6e8aa01d28c2a/include/llvm/Demangle/Demangle.h#L36
> * IPD's rich info could help LLDB, where it currently parses mangled
> names on its own, on-top of demangling. Symtab::InitNameIndexes() seems
> to be the most prominent such place. LLDB builds an index with various
> categories from all its symbols here. This is performance-critical and
> it does not benefit from the laziness in GetDemangledName():
> https://github.com/llvm-mirror/lldb/blob/8ba903256fd92a2d8644b108a7c8a1a15efd90ad/source/Symbol/Symtab.cpp#L218
> My simple switch doesn't exploit IPD's rich demangling info yet and it
> uses a new IPD instance for each demangling request, which is considered
> quite costly as it uses a bump allocator internally. Over-all
> performance still didn't drop, but even seems to benefit.
> In order to fully exploit the remaining potential, I am thinking about
> the following changes:
> (1) In the Mangled class, add a separate entry-point for batch
> demangling, that allows to pass in an existing IPD:
> bool Mangled::DemangleWithRichNameIndexInfo(ItaniumPartialDemangler &IPD);
> (2) DemangleWithRichNameIndexInfo() will demangle explicitly, which is
> required to make sure we gather IPD's rich info. It's not lazy as
> GetDemangledName(), but it will store the demangled name and set the
> "MangledCounterpart" so that subsequent lazy requests will be fast.
> (3) DemangleWithRichNameIndexInfo() will be used by
> Symtab::InitNameIndexes(), which will have a single IPD instance that is
> reused for all symbols. Symtab::InitNameIndexes() is usually called
> before anything else, so it is basically "warming the cache" here.
> (4) Finally, with IPD's rich info, we can get rid of the additional
> string parsing in Symtab::InitNameIndexes(). I expect a considerable
> speedup here too.
> What do you think about the plan?
> Do you think it's a good idea to add DemangleWithRichNameIndexInfo()
> like this?
> Are you aware of more batch-processing places like
> Symtab::InitNameIndexes(), that I should consider as clients for
> DemangleWithRichNameIndexInfo()?
> Do you know potential side-effects I must be aware of?
> Would you consider the evidence on the performance benefits convincing,
> or do you think it needs bulletproof benchmarking numbers?
> When it comes to MSVC-mangled names:
> * It is certainly necessary to keep a legacy version of the current
> categorization mechanism for these. But in general, what do you think
> about their importance for LLDB? (Personally I would like to see LLDB on
> Windows, but I tried it only once and gave up quickly.)
> * I saw there is a new MicrosoftDemangler now in LLVM. Does anyone know
> more about it? Especially: Are there plans to provide rich demangling
> information similar to the IPD?
> So far I started writing a unit test for Symtab::InitNameIndexes(), so I
> won't accidentally break its indexing. I also experimented with a
> potential DemangleWithRichNameIndexInfo() and had a look on the numbers
> of the internal LLDB timers. This was, however, not exhaustive and real
> benchmarking is always hard.
> Thanks for all kinds of feedback.
> Best,
> Stefan

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