On Fri, 2022-10-28 at 11:56 -0700, Yonghong Song wrote:
> > > [...]
> >
> > Ok, could we change the problem to detecting if some type is defined.
> > Would it be possible to have something like
> >
> > #if !__is_type_defined(struct abc)
> > struct abc {
> > };
> > #endif
> >
> > I think we talked about this and there were problems with this
> > approach, but I don't remember details and how insurmountable the
> > problem is. Having a way to check whether some type is defined would
> > be very useful even outside of -target bpf parlance, though, so maybe
> > it's the problem worth attacking?
>
> Yes, we discussed this before. This will need to add additional work
> in preprocessor. I just made a discussion topic in llvm discourse
>
> https://discourse.llvm.org/t/add-a-type-checking-macro-is-type-defined-type/66268
>
> Let us see whether we can get some upstream agreement or not.
I did a small investigation of this feature.
The main pre-requirement is construction of the symbol table during
source code pre-processing, which implies necessity to parse the
source code at the same time. It is technically possible in clang, as
lexing, pre-processing and AST construction happens at the same time
when in compilation mode.
The prototype is available here [1], it includes:
- Change in the pre-processor that adds an optional callback
"IsTypeDefinedFn" & necessary parsing of __is_type_defined
construct.
- Change in Sema module (responsible for parsing/AST & symbol table)
that installs the appropriate "IsTypeDefinedFn" in the pre-processor
instance.
However, this prototype builds a backward dependency between
pre-processor and semantic analysis. There are currently no such
dependencies in the clang code base.
This makes it impossible to do pre-processing and compilation
separately, e.g. consider the following example:
$ cat test.c
struct foo { int x; };
#if __is_type_defined(foo)
const int x = 1;
#else
const int x = 2;
#endif
$ clang -cc1 -ast-print test.c -o -
struct foo {
int x;
};
const int x = 1;
$ clang -E test.c -o -
# ... some line directives ...
struct foo { int x; };
const int x = 2;
Note that __is_type_defined is computed to different value in the
first and second calls. This is so because semantic analysis (AST,
symbol table) is not done for -E.
It also breaks that C11 standard which clearly separates
pre-processing and semantic analysis phases, see [2] 5.1.1.2.
So, my conclusion is as follows: this is technically possible in clang
but has no chance to reach llvm upstream.
Thanks,
Eduard
[1] https://github.com/llvm/llvm-project/compare/main...eddyz87:llvm-project:is-type-defined-experiment
[2] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf
>
> >
> > >
> > > >
> > > > BTW, I suggest splitting libbpf btf_dedup and btf_dump changes into a
> > > > separate series and sending them as non-RFC sooner. Those improvements
> > > > are independent of all the header guards stuff, let's get them landed
> > > > sooner.
> > > >
> > > > > After some discussion with Alexei and Yonghong I'd like to request
> > > > > your comments regarding a somewhat brittle and partial solution to
> > > > > this issue that relies on adding `#ifndef FOO_H ... #endif` guards in
> > > > > the generated `vmlinux.h`.
> > > > >
> > > >
> > > > [...]
> > > >
> > > > > Eduard Zingerman (12):
> > > > > libbpf: Deduplicate unambigous standalone forward declarations
> > > > > selftests/bpf: Tests for standalone forward BTF declarations
> > > > > deduplication
> > > > > libbpf: Support for BTF_DECL_TAG dump in C format
> > > > > selftests/bpf: Tests for BTF_DECL_TAG dump in C format
> > > > > libbpf: Header guards for selected data structures in vmlinux.h
> > > > > selftests/bpf: Tests for header guards printing in BTF dump
> > > > > bpftool: Enable header guards generation
> > > > > kbuild: Script to infer header guard values for uapi headers
> > > > > kbuild: Header guards for types from include/uapi/*.h in kernel BTF
> > > > > selftests/bpf: Script to verify uapi headers usage with vmlinux.h
> > > > > selftests/bpf: Known good uapi headers for test_uapi_headers.py
> > > > > selftests/bpf: script for infer_header_guards.pl testing
> > > > >
> > > > > scripts/infer_header_guards.pl | 191 +++++
> > > > > scripts/link-vmlinux.sh | 13 +-
> > > > > tools/bpf/bpftool/btf.c | 4 +-
> > > > > tools/lib/bpf/btf.c | 178 ++++-
> > > > > tools/lib/bpf/btf.h | 7 +-
> > > > > tools/lib/bpf/btf_dump.c | 232 +++++-
> > > > > .../selftests/bpf/good_uapi_headers.txt | 677 ++++++++++++++++++
> > > > > tools/testing/selftests/bpf/prog_tests/btf.c | 152 ++++
> > > > > .../selftests/bpf/prog_tests/btf_dump.c | 11 +-
> > > > > .../bpf/progs/btf_dump_test_case_decl_tag.c | 39 +
> > > > > .../progs/btf_dump_test_case_header_guards.c | 94 +++
> > > > > .../bpf/test_uapi_header_guards_infer.sh | 33 +
> > > > > .../selftests/bpf/test_uapi_headers.py | 197 +++++
> > > > > 13 files changed, 1816 insertions(+), 12 deletions(-)
> > > > > create mode 100755 scripts/infer_header_guards.pl
> > > > > create mode 100644 tools/testing/selftests/bpf/good_uapi_headers.txt
> > > > > create mode 100644 tools/testing/selftests/bpf/progs/btf_dump_test_case_decl_tag.c
> > > > > create mode 100644 tools/testing/selftests/bpf/progs/btf_dump_test_case_header_guards.c
> > > > > create mode 100755 tools/testing/selftests/bpf/test_uapi_header_guards_infer.sh
> > > > > create mode 100755 tools/testing/selftests/bpf/test_uapi_headers.py
> > > > >
> > > > > --
> > > > > 2.34.1
> > > > >
