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

[Yonghong Song <yhs@meta.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.

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