Re: Follow up from the btf_type_tag discussion in the BPF office hours

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> Of the two problems discussed:
>
> 1. DW_TAG_LLVM_annotation not being able to denote annotations to
>    non-pointed based types.  clang currently ignores these instances.
>
>    We discussed two possible options to deal with this:
>    1.1 To continue ignoring these cases in the front-end, keep the dwarf
>        expressiveness limitation, and document it.
>    1.2 To change DW_TAG_LLVM_annotation so it behaves like a qualifier
>        DIE (like const, volatile, etc.) so it can apply to any type.

Note that the non-pointed based types don't have to be basic types.  The
limitation also impacts non-basic types that are not pointed to,
including pointer types themselves.

Therefore:

>     : int * __tag1 * __tag2 h;
>
>     - sparse
>       +  __tag1 applies to int*, __tag2 applies to int**
>       : got int *[noderef] __tag1 *[addressable] [noderef] [toplevel] __tag2 h
>     - clang
>       + According to DWARF __tag1 applies to int*, no __tag2 (??).
>       + According to BTF  __tag1 applies to int*, no __tag2 (??).
>       : DWARF
>       : 0x00000023:   DW_TAG_variable
>       :                 DW_AT_name	("h")
>       :                 DW_AT_type	(0x0000002e "int **")
>       :
>       : 0x0000002e:   DW_TAG_pointer_type
>       :                 DW_AT_type	(0x00000037 "int *")
>       :
>       : 0x00000033:     DW_TAG_LLVM_annotation
>       :                 DW_AT_name	("btf_type_tag")
>       :                 DW_AT_const_value	("tag1")
>       : BTF
>       : [1] TYPE_TAG 'tag1' type_id=3
>       : [2] PTR '(anon)' type_id=1
>       : [3] PTR '(anon)' type_id=4
>       : [4] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
>       : [5] VAR 'h' type_id=2, linkage=global
>       :
>       : 'h' -> ptr -> 'tag1' -> ptr -> int

In the example above, `tag2' doesn't appear in neither DWARF nor BTF
because the type int** isn't pointed itself, and as Yonghong mentioned
in the call, the implementation of btf_type_tag in clang ignores these
cases.