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 _______________________________________________ lldb-dev mailing list lldb-dev@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-dev
