On 5/10/22 22:05, Yonghong Song wrote:
On 5/10/22 8:43 PM, Yonghong Song wrote:
On 5/6/22 2:18 PM, David Faust wrote:
On 5/5/22 16:00, Yonghong Song wrote:
On 5/4/22 10:03 AM, David Faust wrote:
On 5/3/22 15:32, Joseph Myers wrote:
On Mon, 2 May 2022, David Faust via Gcc-patches wrote:
Consider the following example:
#define __typetag1 __attribute__((btf_type_tag("tag1")))
#define __typetag2 __attribute__((btf_type_tag("tag2")))
#define __typetag3 __attribute__((btf_type_tag("tag3")))
int __typetag1 * __typetag2 __typetag3 * g;
The expected behavior is that 'g' is "a pointer with tags 'tag2' and
'tag3',
to a pointer with tag 'tag1' to an int". i.e.:
That's not a correct expectation for either GNU __attribute__ or
C2x [[]]
attribute syntax. In either syntax, __typetag2 __typetag3 should
apply to
the type to which g points, not to g or its type, just as if you had a
type qualifier there. You'd need to put the attributes (or qualifier)
after the *, not before, to make them apply to the pointer type. See
"Attribute Syntax" in the GCC manual for how the syntax is defined for
GNU
attributes and deduce in turn, for each subsequence of the tokens
matching
the syntax for some kind of declarator, what the type for "T D1"
would be
as defined there and in the C standard, as deduced from the type for
"T D"
for a sub-declarator D.
>> But GCC's attribute parsing produces a variable 'g' which is "a
pointer with
tag 'tag1' to a pointer with tags 'tag2' and 'tag3' to an int", i.e.
In GNU syntax, __typetag1 applies to the declaration, whereas in C2x
syntax it applies to int. Again, if you wanted it to apply to the
pointer
type it would need to go after the * not before.
If you are concerned with the fine details of what construct an
attribute
appertains to, I recommend using C2x syntax not GNU syntax.
Joseph, thank you! This is very helpful. My understanding of the syntax
was not correct.
(Actually, I made a bad mistake in paraphrasing this example from the
discussion of it in the series cover letter. But, the reason why it is
incorrect is the same.)
Yonghong, is the specific ordering an expectation in BPF programs or
other users of the tags?
This is probably a language writing issue. We are saying tags only
apply to pointer. We probably should say it only apply to pointee.
$ cat t.c
int const *ptr;
the llvm ir debuginfo:
!5 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !6, size: 64)
!6 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !7)
!7 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
We could replace 'const' with a tag like below:
int __attribute__((btf_type_tag("tag"))) *ptr;
!5 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !6, size: 64,
annotations: !7)
!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
!7 = !{!8}
!8 = !{!"btf_type_tag", !"tag"}
In the above IR, we generate annotations to pointer_type because
we didn't invent a new DI type for encode btf_type_tag. But it is
totally okay to have IR looks like
!5 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !11, size: 64)
!11 = !DIBtfTypeTagType(..., baseType: !6, name: !"Tag")
!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
OK, thanks.
There is still the question of why the DWARF generated for this case
that I have been concerned about:
int __typetag1 * __typetag2 __typetag3 * g;
differs between GCC (with this series) and clang. After studying it,
GCC is doing with the attributes exactly as is described in the
Attribute Syntax portion of the GCC manual where the GNU syntax is
described. I do not think there is any problem here.
So the difference in DWARF suggests to me that clang is not handling
the GNU attribute syntax in this particular case correctly, since it
seems to be associating __typetag2 and __typetag3 to g's type rather
than the type to which it points.
I am not sure whether for the use purposes of the tags this difference
is very important, but it is worth noting.
As Joseph suggested, it may be better to encourage users of these tags
to use the C2x attribute syntax if they are concerned with precisely
which construct the tag applies.
This would also be a way around any issues in handling the attributes
due to the GNU syntax.
I tried a few test cases using C2x syntax BTF type tags with a
clang-15 build, but ran into some issues (in particular, some of the
tag attributes being ignored altogether). I couldn't find confirmation
whether C2x attribute syntax is fully supported in clang yet, so maybe
this isn't expected to work. Do you know whether the C2x syntax is
fully supported in clang yet?
Actually, I don't know either. But since the btf decl_tag and type_tag
are also used to compile linux kernel and the minimum compiler version
to compile kernel is gcc5.1 and clang11. I am not sure whether gcc5.1
supports c2x or not, I guess probably not. So I think we most likely
cannot use c2x syntax.
Okay, I think we can guard btf_tag's with newer compiler versions.
What kind of c2x syntax you intend to use? I can help compile kernel
with that syntax and llvm15 to see what is the issue and may help
fix it in clang if possible.
I am thinking to use the [[]] C2x standard attribute syntax. The syntax
makes it quite clear to which entity each attribute applies, and in my
opinion is a little more intuitive/less surprising too.
It's documented here (PDF):
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2731.pdf
See sections 6.7.11 for the syntax and 6.7.6 for declarations. Section
6.7.6.1 specifically describes using the attribute syntax with pointer
declarators.
The attribute syntax itself for BTF tags is:
[[clang::btf_type_tag("tag1")]]
or
[[gnu::btf_type_tag("tag1")]]
I am also looking into whether, with the C2x syntax, we really need two
separate attributes (type_tag and decl_tag) at the language level. It
might be possible with C2x syntax to use just one language attribute
(e.g. just btf_tag).
A simple declaration for a tagged pointer to an int:
int * [[gnu::btf_type_tag("tag1")]] x;
And for the example from this thread:
#define __typetag1 [[gnu::btf_type_tag("type-tag-1")]]
#define __typetag2 [[gnu::btf_type_tag("type-tag-2")]]
#define __typetag3 [[gnu::btf_type_tag("type-tag-3")]]
int * __typetag1 * __typetag2 __typetag3 g;
Here each tag applies to the preceding pointer, so the result is
unsurprising.
Actually, this is where I found something that looks like an issue with
the C2x attribute syntax in clang. The tags 2 and 3 go missing, but with
no warning nor other indication.
Compiling this example with gcc:
$ ~/toolchains/bpf/bin/bpf-unknown-none-gcc -c -gbtf -gdwarf c2x.c -o
c2x.o --std=c2x
$ ~/toolchains/llvm/bin/llvm-dwarfdump c2x.o
0x0000000c: DW_TAG_compile_unit
DW_AT_producer ("GNU C2X 12.0.1 20220401 (experimental)
-gbtf -gdwarf -std=c2x")
DW_AT_language (DW_LANG_C11)
DW_AT_name ("c2x.c")
DW_AT_comp_dir ("/home/dfaust/playpen/btf/tags")
DW_AT_stmt_list (0x00000000)
0x0000001e: DW_TAG_variable
DW_AT_name ("g")
DW_AT_decl_file ("/home/dfaust/playpen/btf/tags/c2x.c")
DW_AT_decl_line (16)
DW_AT_decl_column (0x2a)
DW_AT_type (0x00000032 "int **")
DW_AT_external (true)
DW_AT_location (DW_OP_addr 0x0)
0x00000032: DW_TAG_pointer_type
DW_AT_byte_size (8)
DW_AT_type (0x0000004e "int *")
DW_AT_sibling (0x0000004e)
0x0000003b: DW_TAG_LLVM_annotation
DW_AT_name ("btf_type_tag")
DW_AT_const_value ("type-tag-3")
0x00000044: DW_TAG_LLVM_annotation
DW_AT_name ("btf_type_tag")
DW_AT_const_value ("type-tag-2")
0x0000004d: NULL
0x0000004e: DW_TAG_pointer_type
DW_AT_byte_size (8)
DW_AT_type (0x00000061 "int")
DW_AT_sibling (0x00000061)
0x00000057: DW_TAG_LLVM_annotation
DW_AT_name ("btf_type_tag")
DW_AT_const_value ("type-tag-1")
0x00000060: NULL
0x00000061: DW_TAG_base_type
DW_AT_byte_size (0x04)
DW_AT_encoding (DW_ATE_signed)
DW_AT_name ("int")
0x00000068: NULL
and with clang (changing the attribute prefix to clang:: appropriately):
$ ~/toolchains/llvm/bin/clang -target bpf -g -c c2x.c -o c2x.o.ll --std=c2x
$ ~/toolchains/llvm/bin/llvm-dwarfdump c2x.o.ll
0x0000000c: DW_TAG_compile_unit
DW_AT_producer ("clang version 15.0.0
(https://github.com/llvm/llvm-project.git
f80e369f61ebd33dd9377bb42fcab64d17072b18)")
DW_AT_language (DW_LANG_C99)
DW_AT_name ("c2x.c")
DW_AT_str_offsets_base (0x00000008)
DW_AT_stmt_list (0x00000000)
DW_AT_comp_dir ("/home/dfaust/playpen/btf/tags")
DW_AT_addr_base (0x00000008)
0x0000001e: DW_TAG_variable
DW_AT_name ("g")
DW_AT_type (0x00000029 "int **")
DW_AT_external (true)
DW_AT_decl_file ("/home/dfaust/playpen/btf/tags/c2x.c")
DW_AT_decl_line (12)
DW_AT_location (DW_OP_addrx 0x0)
0x00000029: DW_TAG_pointer_type
DW_AT_type (0x00000032 "int *")
0x0000002e: DW_TAG_LLVM_annotation
DW_AT_name ("btf_type_tag")
DW_AT_const_value ("type-tag-1")
0x00000031: NULL
0x00000032: DW_TAG_pointer_type
DW_AT_type (0x00000037 "int")
0x00000037: DW_TAG_base_type
DW_AT_name ("int")
DW_AT_encoding (DW_ATE_signed)
DW_AT_byte_size (0x04)
0x0000003b: NULL
This example comes from my testing against clang to check that the BTF
generated by both toolchains is compatible. In this case we get
different results when using the GNU attribute syntax.
[...]
