On 6/8/26 4:21 PM, Dave Blaikie via Dwarf-discuss wrote:
On Thu, Jun 4, 2026 at 6:00 PM Cary Coutant<[email protected]> wrote:
template<typename T> struct t1 {
T n1;
};
template<typename T> struct t2 {
T m1;
t1<T> m2;
};
int main()
{
t2<int> v1;
return 0;
}
Based on the current specification, the type of m1 should reference
a template parameter entry. This is a bug in gcc which has already
been described [1]. llvm/clang attempted to fix this in PR [2].
Also for m2 we can see that there is no information that its type was
originally described by a template type parameter of t2. However,
there is no specification addressing this specific case in the DWARF
Standard.
Like Ben, I'm not sure I see why this isn't possible given the current spec. The type
attribute for v1 should point to a type DIE for t2<int>, which will have a
template type parameter and two data members, m1 and m2. Briefly:
L1: DW_TAG_structure_type (name: t2<int>)
L2: DW_TAG_template_type_parameter (name: T, type: ref to type int)
L3: DW_TAG_member (name: m1, type: ref to L2) [NOTE 1]
L4: DW_TAG_member (name: m2, type: ref to type t1<T>) [NOTE 2]
NOTE 1: This is the bug you point out where gcc and clang don't do what the
spec says, and put a ref to type int instead of to the template type parameter
entry.
NOTE 2: This is the part that may not be so obvious. We want a ref to a DIE that represents the
type t1<T> rather than t1<int>. I think we can have that, but that new type entry
will need to be a child of the t2<int> type. Otherwise, it won't be able to reference the
template type parameter entry (technically, it *could*, but pointing from one type DIE into the
interior of another is ugly, and could conceivably cause problems down the road).
L1: DW_TAG_structure_type (name: t2<int>)
L2: DW_TAG_template_type_parameter (name: T, type: ref to type int)
L3: DW_TAG_member (name: m1, type: ref to L2)
L4: DW_TAG_member (name: m2, type: ref to L5)
L5: DW_TAG_structure_type (name: t1<T>)
L6: DW_TAG_template_type_parameter (name: T, type: ref to type int)
L7: DW_TAG_member (name: n1, type: ref to L6)
Besides missing consumer support (e.g. GDB or LLDB), missing support for
nested templates was one further reason why the llvm PR [2] was rejected
by David Blaikie. I discussed this topic with him since I am working on
the enablement in GDB and submitted a patch series [3] to support template
type resolution based on the current DWARF specification. David Blaikie
asked me to submit a DWARF issue for this, since we could not come up with
a useful specification for the nested case (type of m2 in the example
above).
In the PR, David noted:
That's basically my point, sorry, that v1 must be represented as "trait::type"
(because it has to be canonical) and so if many uses, like this one, of a type in a
template can't reference the DW_TAG_template_type_parameter, because it's non-canonical -
I'm not sure there's a lot of value in making it work for the subset of cases where it is
workable.
I guess I don't understand the part about "it has to be canonical". David, can
you explain why the approach I used above wouldn't work for the PR's example with
trait::type?
Consider some further use of t1<int> elsewhere (in some t3<int> that's
similar to t2<int>, or independently, etc) - the consumer needs to
know that t1<int> is the same type as t1<T> inside t1<int>.
Also from a debug info size perspective, we wouldn't want to duplicate
the type description of t2<int> with all its members, etc.
That is specifically what I was thinking about in my two further scenarios.
I'm going to argue that:
t1<int> is not the same as the t1<T> declared inside of t2 even when t2
is instantiated with T being an int. They are certainly compatible types
and the language rules probably allow something like:
t1<int> foo;
t2<int> bar;
bar.m2=foo;
but I would argue that they are different types from the perspective of
DWARF.
foo is a t1 instantiated with a T parameter referencing an int.
bar.m2 is a t1 instantiated with a T (t1's T) parameter referencing t2's
T parameter that then references an int.
I believe that we NEED to convey that to consumers. C++ template
metaprogramming is unbelievably hard to unscramble and trying to flatten
everything down to its fundamental types is just going to make it harder.
I think what bothers you is the combinatoric expansion that this can
lead to within the type system and maybe the fact that DWARF types and
the rules around them may need to differ slightly from the source
language. However, the language and the DWARF actually are telling
different stories. The compiler (with some help from the linker) wants
to make sure that any program text may result from template expansion is
not duplicated. This of course inclines producers toward flattening the
template expansion. On the other hand, DWARF is there to connect the
programmer's source view of the code they have written in the source
language to the resulting binary after the producer has done its work.
In this context, flattening the types in the template expansion
misrepresents the programmers intent by presenting an intermediate step
that the producer passed through when doing the template expansion.
Remember if t1 had any methods, both the one from foo and the one from
bar.m2 would point to the same program text. This isn't just about the
size of the binary. It does not impact code generation. This is just
debuginfo. It is about clearly showing the consumer the way that bar.m2
arrived at the instantiation t1<int> (where int is the
unreferenced template type parameter) was by having t2 take a template
parameter which was then passed to t1 as its template parameter.
If we need something canonical (and if I understand what that means), could we also have a
canonical instantiation of the type for t1<int> at the outermost scope, and add an
attribute to the entry for t1<T> at L5 that points to that canonical instantiation —
maybe call it DW_AT_canonical_type?
Possibly, though it gets verbose (we would want tnhe t1<T> at L5 to be
only a declaration, not a definition, for instance - so it doesn't
contain a duplicate description of all of t1<int>'s members). In the
above case, we could emit t1<T> as a typedef, or at least handle it
similarly to a typedef.
And there are more complicated situations, like this:
template<typename T1>
struct t1 {
typedef U1 = T1;
};
template<typename T2>
struct t2 {
typedef U2 = t1<T2>::U1;
};
int main() {
t2<int>::U2 x;
}
Following the above direction you're suggesting Cary, I suppose we
could still emit a declaration for t1<T2> inside t2<int>, and have
that declaration contain the U1 typedef referring to the T1 parameter
(with a value of T2) - though it does feel a bit awkward/verbose to
me. In template heavy code this may produce a lot of new declarations
to describe these template parameter mappings.
See my arguments above.
I think there's probably some worse cases of template parameter use
I'm not thinking of... but haven't come up with them yet.
If you want to see truly evil example of template metaprogramming, the
likes of which will make your eyes bug and leave you with nightmares
about being lost in a maze of twisty little passages which all look
about the same, may I suggest Raja https://github.com/llnl/raja or its
slightly better mannered cousin Kokkos https://github.com/kokkos/kokkos
There's also the concern that the compilers currently don't generate what the
DWARF spec says they should, and if they did, the debuggers wouldn't be able to
handle it. And if David's point above is right that we can represent the simple
case, but not the more complex ones, is it worth doing it even for the simple
cases? If the spec is calling for something that can't be done in the general
case, should we remove it from the spec?
That'd be the direction I'd lean. I'd at least be inclined to leave it
out/not say one way or the other - if a producer and consumer can
agree to do this sort of thing in some cases and they're OK with it
not being a complete solution, I wouldn't be so firm as to say they
shouldn't be allowed to - just that in my capacity as an LLVM/clang
debug info maintainer, I don't think it's the right direction
there/anywhere I'd have ownership over, at my current understanding of
the situation.
I worry about timeline here. I think with DWARF6 we at least have
something that at least _could_ work. There will be a big push in both
producers and consumers to support DWARF6. I think that these things
(which are arguably bugs in current toolchain's DWARF5 support) should
be part of that enablement. I'll file the bugs and track them, My
involvement with DWARF6 probably is going to continue through to the
implementation in the toolchains.
I believe that what you are really hoping for is a new design for how to
support template instantiation in DWARF. With the rise of STL and
generic programming and now with template metaprogramming, template
instantiation is a much bigger part of C++ than it was when the current
template instantiation support was added to DWARF and we are starting to
see some of the problems with that original design. This problem being
one of the problems. Another one is we have a huge amount of highly
similar almost duplicated debuginfo. However, I feel like taking on that
design challenge of how to more compactly represent the vastly increased
use of templates in C++ is more of A DWARF7 project than a DWARF6 project.
Unlike the DWARF for GPUs project which we are refining and hope to
fully land in DWARF6, where we had a fairly complete design and
implementation based on DWARF5, I have not even heard of the beginnings
of discussions about a new model design for template instantiation yet.
This is why I think it is best to push that off to DWARF7. I think we
should add an example or two to D.11 about how this should work based
upon what Cary and I have suggested for DWARF6, Then as part of the
overall DWARF6 enablement in the toolchains, we get both the producers
and consumers to fall in line with that standard.
Then if possible, we come up with a better design for DWARF7. I believe
that such a design may have at its core, a reconsideration of the policy
of DWARF toward templates: "/DWARF does not represent the generic
template definition, but does represent each instantiation./" (ref
section 2.22) However, starting with a reconsideration of that premise
and marching all the way through to a workable design which handles all
the scenarios that come up in modern C++ and in the even more modern
current C++ standards where template meta programming are a much bigger
factor is going to be quite a journey.
-ben
- Dave
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