[Bug debug/125888] btf_type_tag for types not supported for defined struct

[vineetg at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125888

--- Comment #1 from Vineet Gupta <vineetg at gcc dot gnu.org> ---

Clang recently started supporting this

commit 5754a48780f516cbc06eeb2a31b1e445ddd9c935
Author: yonghong-song <yhs@fb.com>
Date:   Mon Jun 15 10:51:02 2026 -0700

    [Clang][BPF] Support btf_type_tag on typedef underlying types (#203089)

    Emil Tsalapatis suggested to add type tag for typedef like below:
    ```
      $ cat tag.c
      #define __type_tag(x) __attribute__((btf_type_tag(x)))

      struct bar { int c; int d; };
      typedef struct bar __type_tag("a") bar_t;
      int use(bar_t *v)
      {
        return v->c + v->d;
      }
    ```
    This makes the code simpler -- using `bar_t *v` instead of the longer
    form `struct bar __type_tag("a") *v`.

    So the goal is to allow type tag for typedef underlying types. The
    following describes the main changes:

    - CGDebugInfo: factor the btf_type_tag walk out of CreatePointerLikeType
    into a new CollectBTFTypeTagAnnotations() helper, and use it when
    emitting a typedef's DIDerivedType so the typedef carries both its
    btf_decl_tag annotations (on the declaration) and the btf_type_tag
    annotations (on the underlying type).

    - BPF BTFDebug: emit type tags for DW_TAG_typedef the same way they are
    emitted for DW_TAG_pointer_type, building the intermediate
    BTF_KIND_TYPE_TAG chain. The BTFTypeDerived constructor now selects
    BTF_KIND_PTR or BTF_KIND_TYPEDEF based on the DWARF tag.