[lldb-dev] Resolving dynamic type based on RTTI fails in case of type names inequality in DWARF and mangled symbols

[xgsa via lldb-dev]

Thank you for clarification, Jim, you are right, I misunderstood a little bit what lldb actually does.

It is not that the compiler can't be fixed, it's about the fact that relying on correspondence of mangled and demangled forms are not reliable enough, so we are looking for more robust alternatives. Moreover, I am not sure that such fuzzy matching could be done just basing on class name, so it will require reading more DIEs. Taking into account that, for instance, in our project there are quite many such types, it could noticeable slow down the debugger.

Thus, I'd like to mention one more alternative and get your feedback, if possible. Actually, what is necessary is the correspondence of mangled and demangled vtable symbol. Possibly, it worth preparing a separate section during compilation (like e.g. apple_types), which would store this correspondence? It will work fast and be more reliable than the current approach, but certainly, will increase debug info size (however, cannot estimate which exact increase will be, e.g. in persent).

What do you think? Which solution is preferable?

Thanks,
Anton.

15.12.2017, 23:34, "Jim Ingham" <jingham at apple.com>:
> First off, just a technical point. lldb doesn't use RTTI to find dynamic types, and in fact works for projects like lldb & clang that turn off RTTI. It just uses the fact that the vtable symbol for an object demangles to:
>
> vtable for CLASSNAME
>
> That's not terribly important, but I just wanted to make sure people didn't think lldb was doing something fancy with RTTI... Note, gdb does (or at least used to do) dynamic detection the same way.
>
> If the compiler can't be fixed, then it seems like your solution [2] is what we'll have to try.
>
> As it works now, we get the CLASSNAME from the vtable symbol and look it up in the the list of types. That is pretty quick because the type names are indexed, so we can find it with a quick search in the index. Changing this over to a method where we do some additional string matching rather than just using the table's hashing is going to be a fair bit slower because you have to run over EVERY type name. But this might not be that bad. You would first look it up by exact CLASSNAME and only fall back on your fuzzy match if this fails, so most dynamic type lookups won't see any slowdown. And if you know the cases where you get into this problem you can probably further restrict when you need to do this work so you don't suffer this penalty for every lookup where we don't have debug info for the dynamic type. And you could keep a side-table of mangled-name -> DWARF name, and maybe a black-list for unfound names, so you only have to do this once.
>
> This estimation is based on the assumption that you can do your work just on the type names, without having to get more type information out of the DWARF for each candidate match. A solution that relies on realizing every class in lldb so you can get more information out of the type information to help with the match will defeat all our attempts at lazy DWARF reading. This can cause quite long delays in big programs. So I would be much more worried about a solution that requires this kind of work. Again, if you can reject most potential candidates by looking at the name, and only have to realize a few likely types, the approach might not be that slow.
>
> Jim
>
>>  On Dec 15, 2017, at 7:11 AM, xgsa via lldb-dev <lldb-dev at lists.llvm.org> wrote:
>>
>>  Sorry, I probably shouldn't have used HTML for that message. Converted to plain text.
>>
>>  -------- Original message --------
>>  15.12.2017, 18:01, "xgsa" <xgsa at yandex.ru>:
>>
>>  Hi,
>>
>>  I am working on issue that in C++ program for some complex cases with templates showing dynamic type based on RTTI in lldb doesn't work properly. Consider the following example:
>>  enum class TagType : bool
>>  {
>>     Tag1
>>  };
>>
>>  struct I
>>  {
>>     virtual ~I() = default;
>>  };
>>
>>  template <TagType Tag>
>>  struct Impl : public I
>>  {
>>  private:
>>     int v = 123;
>>  };
>>
>>  int main(int argc, const char * argv[]) {
>>     Impl<TagType::Tag1> impl;
>>     I& i = impl;
>>     return 0;
>>  }
>>
>>  For this example clang generates type name "Impl<TagType::Tag1>" in DWARF and "__ZTS4ImplIL7TagType0EE" when mangling symbols (which lldb demangles to Impl<(TagType)0>). Thus when in ItaniumABILanguageRuntime::GetTypeInfoFromVTableAddress() lldb tries to resolve the type, it is unable to find it. More cases and the detailed description why lldb fails here can be found in this clang review, which tries to fix this in clang [1].
>>
>>  However, during the discussion around this review [2], it was pointed out that DWARF names are expected to be close to sources, which clang does perfectly, whereas mangling algorithm is strictly defined. Thus matching them on equality could sometimes fail. The suggested idea in [2] was to implement more semantically aware matching. There is enough information in the DWARF to semantically match "Impl<(TagType)0>)" with "Impl<TagType::Tag1>", as enum TagType is in the DWARF, and the enumerator Tag1 is present with its value 0. I have some concerns about the performance of such solution, but I'd like to know your opinion about this idea in general. In case it is approved, I'm going to work on implementing it.
>>
>>  So what do you think about type names inequality and the suggested solution?
>
>>  [1] - https://reviews.llvm.org/D39622
>>  [2] - http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20171211/212859.html
>>
>>  Thank you,
>>  Anton.
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