Hi Martin,
Martin Sebor via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
On 11/10/20 12:38 PM, Iain Sandoe wrote:
—— commit message.
If an interface is marked 'deprecated' then, presumably, at some point it
will be withdrawn and no longer available. The 'unavailable' attribute
makes it possible to mark up interfaces to indicate this status.
Making an interface unavailable isn't the intent of deprecation in
standards like C, C++, or POSIX. Rather, the intended next stage
after deprecation is to make the interface available for other uses,
either by the standards themselves, or by implementations, or by
programs (if its name is not in the reserved namespace). So unless
you have other kinds of deprecation in mind this doesn't seem like
a fitting use case.
coming at things from the standards perspective .. perhaps.
.. however, in the first set of cases above, one never needs to indicate
unavailability since the entity never becomes unavailable, it changes
meaning.
In practice, we don’t have markup of keywords etc. to indicate this (such
deprecation has been handled specifically in the FEs).
The practical (and actual) use of these attributes is in describing the
lifecycle
of system APIs.
In that context, one could see it being used to describe APIs withdrawn in,
say,
a Posix standard (as implemented by a given system or according to specific
compile-time flags).
It is used
quite extensively in some codebases where a single set of headers can be
used
to permit code generation for multiple system versions.
This sounds like a different use case than the next stage after
deprecation. I haven't come across it but I imagine its purpose
is to foster portability between variants or flavors (rather
than versions) of APSs? Say one set of APIs for a feature-rich
desktop variant of an OS versus a subset of the same APIs for
an embedded, more limited variant of the same OS.
In the case of Darwin, the compilers are capable of targeting multiple
versions
of the system (one doesn’t need a separate GCC or clang to target each
version,
there is a -mmacosx-version-min= flag that allows the target version to be
specified
on the command line).
Rather than install many versions of headers (and libraries) for all the
system
versions, the designers elected to have one copy with markup that describes
the
availability of APIs.
the lifecycle is typically:
introduced from version P.Q (ergo, unavailable before that)
perhaps deprecated at version R.S (ergo the user should be warned)
withdrawn at version X.Y (and unavailable thereafter).
The headers contain macros that are expanded according to the version
for which the code is being compiled - to produce deprecation warnings or
unavailability errors if such APIs are *used* in the code.
From a configuration perspective, it also allows a compile test to
determine
that an interface is missing - rather than requiring a link test.
The implementation follows the pattern of attribute deprecated, but
produces
an error (where deprecation produces a warning).
This attribute has been implemented in clang for some years.
The Clang manual says the attribute is useful in conjunction with
the enable_if and overloadble attributes in C, to remove overloads
of C functions from the overload set. I'm trying to think how
the GCC implementation of the attribute might be useful in
the subset of cases that don't depend on the other two attributes
but I'm coming up empty (exceot for the different variants of
an API use case that seems rather esoteric). It seems to me
the use case is close to #pragma GCC poison except more nunanced
(i.e., it doesn't poison a name but its uses in the given namespace,
as in functions, types, members, etc.)
Hopefully, the description above clarifies the intent.
It is possible, even likely, that the implementations in clang have
additional
capabilities above those implemented in this first cut - and perhaps we might
adopt some of those in follow-on work.
however the immediate use-case is the hundreds of instances where APIs
are marked up in the manner I’ve outlined above.
+@cindex @code{unavailable} function attribute
+The @code{deprecated} attribute results in an error if the function
^^^^^^^^^^
This should presumably read unavailable.
yes, good catch
+It is expected that items marked as @code{deprecated} will eventually be
+withdrawn from interfaces, and then become unavailable. This attribute
+allows for marking them appropriately.
In Clang, declaring a member unavailable doesn't have the same effect
as withdrawing it (which I would interpret as removing). The member
still takes up space, so if this patch does the same I think this
effect should be made clear here.
The intent is to match the behaviour of clang sufficiently closely that GCC
can
make more effective use of the system headers on Darwin platforms (and, of
course, allow for the same kind of life cycle markup on other platforms).
Like attribute deprecated, I suspect attribute unavailable in GCC
will also be subject to the same catch 22 of marking up both a type
and its uses in APIs. E.g., in:
struct __attribute__ ((deprecated)) A { ... };
struct B {
// Clang accepts this w/o warning, GCC warns.
struct A a __attribute__ ((deprecated));
int i;
};
With unavailable, the problem will be made worse due to the error.
so this is a bug in GCC’s availability attributes (not specifically caused by
this patch) - addressing it for deprecation would naturally carry over to the
unavailable case.
To be generally usable, I think GCC needs to change to behave more
like Clang. As a motivating example, consider the deprecated POSIX
getitimer API:
struct itimerval { int it_interval, it_value; };
int getitimer (int, struct itimerval*);
int setitimer (int, const struct itimerval*, struct itimerval*);
All three names are deprecated and so using each one alone outside
the header that declares them should trigger a deprecation message;
the declarations themselves must not. If/when the APIs are removed,
marking them unavailable in the headers must likewise not trigger
errors.
Is there a PR for this ?
(IMO it’s still a separate issue, curing the problem for deprecation will
also
cure it for unavailability).
thanks for the review,
Iain
