[Dwarf-Discuss] Discrepancy Between Implementation and Spec in Template Types

[David Blaikie]

"quality of implementation" thing - but in general, even if a few bugs were
fixed/improvements were made to both Clang and GCC, it's going to be
hard/impossible to track certain things through templates in DWARF - for
similar reasons that it's hard to provide diagnostic messages that describe
types in the way the user wrote them (not impossible in a few cases - Clang
and GCC are getting better at saying "std::string" often instead of
"std::basic_string<...>" for instance)

For instance - if you had a member that was an instantiation of another
template:

template<typename T1> struct t1 { };
template<typename T2> struct t2 { t1<T2> m1; };
t1<int> v1;
t2<int> v2;

The DW_AT_type of v1 and the DW_AT_type of t2<int>::m1 would need to point
to the same DIE, otherwise there would be much confusion about these being
different types, but being the same type in the DWARF - so that type
description can't mention T2 (because T2 has nothing to do with t1<int>) so
there's no way to describe that use of T2, for instance.

Basically: Due to necessary canonicalization, this isn't doable in general,
so compilers don't bother doing it at all - roughly?

On Thu, Apr 9, 2020 at 10:37 AM Jay Kamat via Dwarf-Discuss <
dwarf-discuss at lists.dwarfstd.org> wrote:

> I wasn't on the list when I originally sent this message, and it didn't
> show
> up in the archive, so I'm sending it again. Sorry if there's duplication:
>
> Hi!
>
> I'm currently working on a debugger which consumes dwarf information
> and I noticed a possible discrepancy between output from popular
> compilers (gcc, clang) and the DWARF 5 and 4 spec.
>
> In section 'D.11 Template Example' for the given source:
>
> template<class T>
> struct wrapper {
>         T comp;
> };
> wrapper<int> obj;
>
> It says:
>
> The actual type of the component comp is int, but in the DWARF the
> type references the DW_TAG_template_type_parameter for T, which in
> turn references int. This implies that in the original template comp
> was of type T and that was replaced with int in the instance.
>
> And this is reflected in the example output:
>
> 11$: DW_TAG_structure_type
>        DW_AT_name("wrapper")
> 12$:    DW_TAG_template_type_parameter
>            DW_AT_name("T")
>            DW_AT_type(reference to "int")
> 13$:    DW_TAG_member
>            DW_AT_name("comp")
>            DW_AT_type(reference to 12$)
>
> Where the 'comp' DW_TAG_member has a type referencing to the
> DW_TAG_template_type_parameter. However, upon compiling the given
> example and dumping the dwarf (gcc 8.3.0, clang 7.0.1), I get:
>
> 0x0000085e:   DW_TAG_structure_type
>                DW_AT_name  ("wrapper<int>")
>                DW_AT_byte_size (0x04)
>                DW_AT_decl_file ("/home/jay/Code/tmp/test/test.cpp")
>                DW_AT_decl_line (6)
>                DW_AT_decl_column   (0x08)
>                DW_AT_sibling   (0x00000880)
>
> 0x0000086b:     DW_TAG_member
>                  DW_AT_name    ("comp")
>                  DW_AT_decl_file   ("/home/jay/Code/tmp/test/test.cpp")
>                  DW_AT_decl_line   (7)
>                  DW_AT_decl_column (0x04)
>                  DW_AT_type    (0x00000057 "int")
>                  DW_AT_data_member_location    (0x00)
>
> 0x00000878:     DW_TAG_template_type_parameter
>                  DW_AT_name    ("T")
>                  DW_AT_type    (0x00000057 "int")
>
> Where the 'comp' DW_TAG_member points directly to the "int" type, and
> nothing points to 0x878.
>
> The information provided by the indirection mentioned in the spec is
> useful, as it lets us determine which members are template parameters
> (and could let us reconstruct the original templated type). As is, we
> can only tell that there is a template parameter, and that it was
> filled with type "int" in this case.
>
> Is there a reason for this discrepancy, and is there anything that can
> be done to resolve it? If I'm overlooking something simple, sorry!
>
> Thanks in advance,
> -Jay
>
>
> PS: Sorry if this has already been discussed, but the search function
> on the archive seems to be throwing an error:
>
> Unable to read word database file
> '/dh/mailman/rank/archives/private/
> dwarf-discuss-dwarfstd.org/htdig/db.words.db'
> Did you run htdig?
> _______________________________________________
> Dwarf-Discuss mailing list
> Dwarf-Discuss at lists.dwarfstd.org
> http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.dwarfstd.org/pipermail/dwarf-discuss-dwarfstd.org/attachments/20200409/5c8a6cf8/attachment.html>