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