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

[Yonghong Song <yhs@meta.com>]

On 1/23/23 10:43 AM, David Faust wrote:
> 
> On 1/23/23 07:50, Jose E. Marchesi wrote:
>>
>>> 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.
> 
> I have been experimenting with the C2x syntax in GCC and the results are
> similarly promising. It looks like with the C2x syntax, the 'type_tag's
> always associate in the same way as sparse.

Thanks for confirmation.

> 
> For GCC the syntax is (or will be)
>    [[gnu::btf_decl_tag("foo")]] and
>    [[gnu::btf_type_tag("bar")]]
> respectively.

Clang could add support for [[gnu::btf_decl_tag("foo")]] as well
once the syntax is agreed by the community and upstreamed.

> 
> I am not sure it is necessary to use the C2x syntax for decl_tag, iirc
> there are no issues with the __attribute__ syntax for decl_tag. Either
> one should be ok.

The same for me. Either is okay.

> 
> With C2x syntax, in the internal representation and in the generated
> DWARF, the type_tag attributes are attached to the same elements of
> the declaration as sparse attaches them to.
> 
> I checked all the examples we looked at and it seems they are
> all "fixed" with the C2x syntax, in that GCC agrees with sparse.
> For example,
> 
> $ cat ex2.c
> int __attribute__((btf_type_tag("tag1"))) * __attribute__((btf_type_tag("tag2"))) * g;
> 
> We saw that this example was problematic with the __attribute__ syntax
> in that GCC associates "tag1" with (int **) while sparse associates
> "tag1" with (int).
> 
> Using the c2x syntax, "tag1" is associated with (int) and "tag2" with
> (int *) the same as in sparse:
> $ cat ex2-c2x.c
> int [[gnu::btf_type_tag("tag1")]] * [[gnu::btf_type_tag("tag2")]] * g;
> $ bpf-unknown-none-gcc --std=c2x -c -gbtf -gdwarf ex2-c2x.c -o ex2-c2x.o
> $ bpftool btf dump file ex2-c2x.o
> [1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
> [2] TYPE_TAG 'tag1' type_id=1
> [3] PTR '(anon)' type_id=2
> [4] TYPE_TAG 'tag2' type_id=3
> [5] PTR '(anon)' type_id=4
> [6] VAR 'g' type_id=5, linkage=global
> 
> 
> I also spent some studying the C2x draft standard [1] to check whether
> this ordering is documented by the standard or up to the implementation.
>    [1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3054.pdf
>    (I think this is the most recent draft, dated 3 Sep 2022)
> 
> I believe the "sparse-like" ordering is in fact required by the
> standard, which is great for us.
> 
> The relevant section is 6.7: Declarations. Section 6.7.12 covers only
> syntax of attributes themselves. The ordering/association rules are
> documented by the sections for each component of a declaration. Section
> 6.7.6 is particularly relevant, 6.7.6.1 discusses pointer declarators
> specifically.
> 
>  From my understanding, the general rule is that an attribute modifies
> the element of a declaration immediately to the left of it, which is
> the same as the intuitive sparse ordering.
> 
> So it seems like using the C2x standard attribute syntax may be a
> very nice solution to our problem. But we should keep in mind that C2x
> is still a draft so this attribute syntax could potentially change.