[Dwarf-Discuss] Using DWARF for C++ runtime reflection

[Michael Eager]

On 03/06/2018 10:37 AM, Roman Popov wrote:
> Hi everyone,
> I'm working on a dynamic analysis tool that needs runtime reflection in 
> C++. Since C++ has no standardized runtime reflection, I'm using DWARF 
> as a source of reflection metadata.
> 
> Is it a legitimate use-case from DWARF standard point of view?

It is certainly an interesting use case.  I haven't see this done
before, but since DWARF gives descriptions of classes, it seems
reasonable.

> It has been working great for me until I've upgraded to latest g++ and 
> clang compilers. Those compilers produce ambiguous names for some 
> template instantiations, so my tools no longer work. So I start to 
> wonder: whether it me misusing DWARF, or it is compilers that are buggy.
> 
> In particular, what is the DWARF answer for these questions:
> 
> 1.? Does DWARF guarantees that:
> 
>  ? ? ?typeid(T1) == typeid(T2)? ? ==> DW_AT_name(T1)? == DW_AT_name(T2)
>  ? ? ?typeid(T1) != typeid(T2)? ? ?==> DW_AT_name(T1)? != DW_AT_name(T2)
> 
> i.e. Is DW_AT_name an unique identifier for type?

The AT_name is the declared name of the type in the source, not the name
of an underlying type.  For example, if your source is

   typedef int int_name_1;
   typedef int int_name_2;

   int_name_1 name_1;
   int_name_2 name_2;

The (edited) DWARF for this

  <1d>: (DW_TAG_typedef)
     <1e>   DW_AT_name        : int_name_1
     <24>   DW_AT_type        : <0x28>
  <28>: (DW_TAG_base_type)
     <2b>   DW_AT_name        : int
  <2f>: (DW_TAG_typedef)
     <30>   DW_AT_name        : int_name_2
     <36>   DW_AT_type        : <0x28>
  <3a>: (DW_TAG_variable)
     <3b>   DW_AT_name        : name_1
     <41>   DW_AT_type        : <0x1d>
  <4f>: (DW_TAG_variable)
     <50>   DW_AT_name        : name_2
     <56>   DW_AT_type        : <0x2f>

typeid(name_1) == typeid(name_2) == "int" which is different from 
AT_name(name_1) or AT_name(name_2).


> 2.? Does DWARF guarantees that DW_AT_name(T)? is a valid source language 
> name?

Yes, assuming that the compiler generates a valid source type.  In some
cases, particularly with template classes, this may not be the case.

> i.e. If I copy string from DW_AT_name to source code, and compile it 
> with the same compiler that produced DWARF, will it produce the same type?
> 
>  ? ? T? ?var1;
>  ? ? DW_AT_name(T)? ?var2;
> 
>  ? typeid(var1)? ==? typeid (var2) ?

I expect that it would.

-- 
Michael Eager    eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306