[lldb-dev] RTTI does not work stable in LLDB.

[Greg Clayton via lldb-dev]

Yeah, when doing dynamic type resolution, we look at the first pointer inside the pointer and see if it resolves to a virtual table symbol. If it does, we extract the class name from the demangled symbol name and try to look up. GDB does the same thing. All debuggers do AFAIK.

If the DWARF specified the vtable address in the DWARF on the class definition this would help, but without that the only thing we can really do is to try and figure out the class and look it up by name. Also, even if this is added to future DWARF, it doesn’t fix the problem that we have many compilers that don’t have the info so we would still need to do what we do. 

If anyone has any better ideas I am all ears?

Greg

> On Feb 6, 2017, at 11:48 AM, Robinson, Paul <paul.robinson at sony.com> wrote:
> 
> So is LLDB expecting the name in the DWARF info to match the demangled name of the vtable pointer?  The DWARF spec does not really specify what the name of a template instantiation should be, and in particular does not *want* to specify whether it matches any given demangler's opinion of the name.
> --paulr
>   <>
> From: lldb-dev [mailto:lldb-dev-bounces at lists.llvm.org] On Behalf Of Greg Clayton via lldb-dev
> Sent: Monday, February 06, 2017 11:08 AM
> To: Greg Clayton
> Cc: lldb-dev at lists.llvm.org
> Subject: Re: [lldb-dev] RTTI does not work stable in LLDB.
>  
> So I found the problem. This is a compiler bug. The DWARF for this type looks like:
>  
> 0x000065da:     TAG_structure_type [112] *
>                  AT_containing_type( {0x0000000000006628} )
>                  AT_name( "derived0<int, int, 1024>" )
>                  AT_byte_size( 0x08 )
>                  AT_decl_file( "/private/tmp/main.cpp" )
>                  AT_decl_line( 9 )
> 
>  
> But all of the type info in the symbol table has has the type named as "derived0<int, int, 1024u>”. Note the extra “u” that follows 1024. This stops LLDB from being able to lookup the type correctly so we can show the dynamic type. In LLDB we check the first pointer inside of a class to see if it is a symbol whose name is “vtable for TYPENAME”. If it is, we lookup the type “TYPENAME” to find it. In this case we try to lookup "derived0<int, int, 1024u>” and we fail since the DWARF has it as "derived0<int, int, 1024>”.
>  
> I have filed a radar on the compiler here at Apple for the fix.
>  
> Greg
>  
> 
> 
> On Feb 6, 2017, at 10:22 AM, Greg Clayton via lldb-dev <lldb-dev at lists.llvm.org <mailto:lldb-dev at lists.llvm.org>> wrote:
>  
> I am looking at this now. I will let you know what I find.
>  
> Greg
>  
> On Feb 6, 2017, at 10:00 AM, Roman Popov <ripopov at gmail.com <mailto:ripopov at gmail.com>> wrote:
>  
> Yes, that was my thought. 
>  
> FYI, checked in GDB: it's working correctly on this testcase showing correct dynamic type in both cases.
>  
> 2017-02-06 9:48 GMT-08:00 Greg Clayton <gclayton at apple.com <mailto:gclayton at apple.com>>:
> You have found a bug. It should be reporting this correctly but it isn’t. I verified it fails on MacOSX.
>  
> Greg Clayton
>  
> On Feb 5, 2017, at 1:19 PM, Roman Popov via lldb-dev <lldb-dev at lists.llvm.org <mailto:lldb-dev at lists.llvm.org>> wrote:
>  
> Hello,
> I'm observing very strange LLDB behavior: it does not always shows a correct dynamic type when I ask for.
>  
> Originally I was working with LLDB 3.9, but it looks like trunk version behaves the same strange way.  
>  
> I was able to capture this behavior in a small code snippet: 
>  
> #include <iostream>
> #include <typeinfo>
>  
> using namespace std;
>  
> struct base_type {  virtual ~base_type(){} };
>  
> template <class T1, class T2, unsigned SIZE>
> struct derived0 : base_type {};
>  
> template <class T1, class T2>
> struct derived1 : base_type {};
>  
> int main(int argc, char ** argv) {
>  
>     base_type * bptr0 = new derived0<int, int, 1024>();
>     base_type * bptr1 = new derived1<int, int >();
>  
>     cout << typeid(*bptr0).name() << endl;
>     cout << typeid(*bptr1).name() << endl;
>  
>     return 0;
> }
>  
>  
> lldb --version
> lldb version 5.0.0 (http://llvm.org/svn/llvm-project/lldb/trunk <http://llvm.org/svn/llvm-project/lldb/trunk> revision 293398)
>   clang revision 293398
>   llvm revision 293398
>  
>  
> Testing in LLDB:
> (lldb) break set --file main.cpp --line 22
>  
> (lldb) expression -d no-run --  bptr1
> (derived1<int, int> *) $2 = 0x0000000000614c40
>  
> (lldb) expression -d no-run --  bptr0
> (base_type *) $3 = 0x0000000000614c20
>  
>  
> Can someone explain me why for bptr0 I dont get a  derived0<int, int, 1024> * as I expected?
>  
>  
> Thanks,
> Roman
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