Re: BTF tag support in DWARF (notes for today's BPF Office Hours)

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

> On 1/5/23 10:30 AM, Jose E. Marchesi wrote:
>> We agreed in the meeting to implement Solution 2 below in both GCC
>> and
>> clang.
>> The DW_TAG_LLVM_annotation DIE number will be changed in order to
>> make
>> it possible for pahole to handle the current tags.  The number of the
>> new tag will be shared by both GCC and clang.
>
> w.r.t c2x attribute syntax discussion in 01/19 office hour discussion.
>
> I have checked clang c2x syntax w.r.t.
> btf_type_tag and btf_decl_tag. They are both supported
> with clang 15 and 16.
>
> See:
> https://clang.llvm.org/docs/AttributeReference.html
>
> The c2x btf_decl_tag attr syntax is [[clang::btf_decl_tag("")]].
> The c2x btf_type_tag attr syntax is [[clang::btf_type_tag("")]].
>
> $ cat t.c
> int [[clang::btf_type_tag("aa")]] * [[clang::btf_type_tag("bb")]] *f;
> [[clang::btf_decl_tag("cc")]] int foo() { return 5; }
> int bar() { return foo(); }
> $ clang -std=c2x -g -O2 -c t.c
> $ llvm-dwarfdump t.o | grep btf | grep tag
>                   DW_AT_name    ("btf_type_tag")
>                   DW_AT_name    ("btf_type_tag")
>                   DW_AT_name    ("btf_decl_tag")
>
> I double checked and the c2x syntax above generates the *same*
> type IR and dwarf compared to __attribute__ style attributes.
>
> [...]

Thanks for checking.

That matches our impression that C2X type attributes actually order the same
way than sparse type annotations, at least in the cases we are
interested on.