LinuxKPI: add __counted_by() and __nonstring

[bz]

Summary

Add checks for counted_by(_x) and nonstring as and depending
on compiler support enable the attribute/feature.

This is needed to make mtk76 and ath1xk drivers to compile.

Sposnored by: The FreeBSD Foundation
MFC after: 3 days

[emaste]

In general __has_attribute(foo) to check
I think this is fine to get the driver compiling but ought to have a TODO comment

[bz]

In D44587#1016767, @emaste wrote:

In general __has_attribute(foo) to check
I think this is fine to get the driver compiling but ought to have a TODO comment

And how to figure out the naming of "foo" in an implementation in these cases?

[emaste]

From a textcase in the LLVM review that introduced it (https://reviews.llvm.org/D148381):

#if !__has_attribute(counted_by)
#error "has attribute broken"
#endif

#ifdef COUNTED_BY
#define __counted_by(member)    __attribute__((__counted_by__(member)))
#else
#define __counted_by(member)
#endif

[bz]

In D44587#1018196, @emaste wrote:

From a textcase in the LLVM review that introduced it (https://reviews.llvm.org/D148381):

#if !__has_attribute(counted_by)
#error "has attribute broken"
#endif

#ifdef COUNTED_BY
#define __counted_by(member)    __attribute__((__counted_by__(member)))
#else
#define __counted_by(member)
#endif

Hmm. Thanks! That review is a year old. I wonder if I had mis-spelled it.
Today when searching again https://clang.llvm.org/docs/AttributeReference.html#counted-by came up. That was kind-of what I was looking for. I'll update the review once I updated my base system after the latest clang import. 16.0.6 clearly didn't have it yet.

[bz]

add logic for counted_by and also add __nonstring

[bz]

@emaste it seems most attributes in cdef.h have the __x__ instea dof just x. This seems to work with gcc as well for as much as I could test. It seems up-to gcc14-devel non of our gccs recognizes the counter_by in either spelling.

Is this fine now?

[emaste]

LGTM, CC @dim in case he has any extra insight on the two forms.

[dim]

LGTM. (The forms without underscores have a higher probability of clashing with user-defined macros, which is why the forms with underscores are normally preferred.)