[cfe-dev] [PATCH] New syntax and functionality for __has_attribute

[Aaron Ballman]

Before I start in on this again, I wanted to make sure that there was
a consensus that this functionality was desirable. I believe the
answer (based on some conversations in IRC and here on the lists) was
tentatively "yes."

As far as I can tell, the work left to be done on this is to add a
feature test for __has_attribute_syntax, write the documentation for
it and that's about it? The syntax-based form is the preferable
nomenclature because it leaves the door open for testing parameters at
some point in the future, and with the __has_attribute_syntax feature
test, it is both backwards and forwards compatible.

~Aaron

On Mon, Jan 13, 2014 at 8:49 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Mon, Jan 13, 2014 at 5:44 PM, Aaron Ballman <aaron at aaronballman.com>
> wrote:
>>
>> On Mon, Jan 13, 2014 at 8:39 PM, Richard Smith <richard at metafoo.co.uk>
>> wrote:
>> > On Mon, Jan 13, 2014 at 5:31 PM, Aaron Ballman <aaron at aaronballman.com>
>> > wrote:
>> >>
>> >> On Mon, Jan 13, 2014 at 8:20 PM, Richard Smith <richard at metafoo.co.uk>
>> >> wrote:
>> >> > On Sun, Jan 12, 2014 at 4:49 PM, Aaron Ballman
>> >> > <aaron at aaronballman.com>
>> >> > wrote:
>> >> >>
>> >> >> On Sun, Jan 12, 2014 at 7:38 PM, Sean Silva <silvas at purdue.edu>
>> >> >> wrote:
>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> > On Sun, Jan 12, 2014 at 6:53 PM, Aaron Ballman
>> >> >> > <aaron at aaronballman.com>
>> >> >> > wrote:
>> >> >> >>
>> >> >> >> > That's good news -- thanks for confirming.
>> >> >> >> >
>> >> >> >> > The feature detection macro itself will still need to have a
>> >> >> >> > different
>> >> >> >> > name
>> >> >> >> > (or some other mechanism) so it can be used compatibly with
>> >> >> >> > existing
>> >> >> >> > clang
>> >> >> >> > deployments, because _has_attribute() currently emits a parse
>> >> >> >> > error
>> >> >> >> > instead
>> >> >> >> > of gracefully returning 0 when passed the new argument syntax:
>> >> >> >> >
>> >> >> >> > tmp/attr2.cpp:1:5: error: builtin feature check macro requires
>> >> >> >> > a
>> >> >> >> > parenthesized identifier
>> >> >> >> > #if __has_attribute(__attribute__((weakref)))
>> >> >> >> >     ^
>> >> >> >>
>> >> >> >> Good catch. Unfortunately, __has_attribute is really the best
>> >> >> >> identifier for the macro, so I am loathe to let it go.
>> >> >> >>
>> >> >> >> Due to the current design of __has_attribute, we can't get away
>> >> >> >> with
>> >> >> >> "
>> >> >> >> magic" by expanding the non-function-like form into a value that
>> >> >> >> could
>> >> >> >> be tested. So we would really have to pick a new name if we are
>> >> >> >> worried about this.
>> >> >> >>
>> >> >> >> Suggestions on the name are welcome.
>> >> >> >
>> >> >> >
>> >> >> > Ok, I'll bite:
>> >> >> >
>> >> >> > __has_attribute_written_as([[foo]])
>> >> >> > __has_attribute_syntax([[foo]])
>> >> >> > __has_attribute_spelling([[foo]])
>> >> >>
>> >> >> I kind of like __has_attribute_syntax, truth be told.
>> >> >
>> >> >
>> >> > Have we given up on using the name __has_attribute too soon? Users of
>> >> > the
>> >> > new syntax could write:
>> >> >
>> >> > // Probably already in project's porting header
>> >> > #ifndef __has_feature
>> >> > #define __has_feature(x) 0
>> >> > #endif
>> >> >
>> >> > #if __has_feature(__has_attribute_syntax)
>> >> > #define MY_HAS_ATTRIBUTE(...) __has_attribute(__VA_ARGS__)
>> >> > #else
>> >> > #define MY_HAS_ATTRIBUTE(...) 0
>> >> > #endif
>> >> >
>> >> > If it's given a different name, they instead would write something
>> >> > like:
>> >> >
>> >> > #ifdef __has_attribute_syntax
>> >> > #define MY_HAS_ATTRIBUTE(...) __has_attribute_syntax(__VA_ARGS__)
>> >> > #else
>> >> > #define MY_HAS_ATTRIBUTE(...) 0
>> >> > #endif
>> >> >
>> >> > So I don't think the change-in-syntax argument holds water.
>> >>
>> >> So are you proposing that we would have a different name for the
>> >> purposes of the __has_feature macro? Eg)
>> >> __has_feature(__has_attribute_syntax) is 1 for the proposed
>> >> functionality, and 0 otherwise?
>> >
>> >
>> > It's a possibility. It could be that a new name is a better approach,
>> > but
>> > both directions seem to be feasible.
>>
>> I'll ponder; I rather like keeping the existing name.
>
>
> By the same argument, it's possible to add extra arguments to
> __has_attribute, if we have a __has_feature check for the new syntax.
>
>>
>> >>
>> >> > Also, supporting arguments in the attributes is useful in some cases
>> >> > --
>> >> > it's
>> >> > not true that they don't make sense in a feature-checking facility.
>> >> > For
>> >> > instance:
>> >> >
>> >> >   __has_attribute( __attribute__((format)) )
>> >> >
>> >> > ... doesn't tell me whether __attribute__((format, gnu_scanf, 1, 2)
>> >> > will
>> >> > work (and I'd expect that the format attribute will gain additional
>> >> > archetypes in future).
>> >>
>> >> That's true, but the example also demonstrates why it's kind of
>> >> nonsensical. What do the 1, 2 represent for the purposes of
>> >> __has_attribute?
>> >
>> >
>> > They represent themselves. Suppose we added support for a format
>> > attribute
>> > with negative indices, or with three indices, or something -- this
>> > syntax
>> > would allow the user to determine if that syntax is available.
>> >
>> >> Can they be elided? If so, can we come up with
>> >> declarative rules as to when they can be elided?
>> >
>> >
>> > If you could omit them, how would you tell whether an attribute could be
>> > used without arguments?
>> >
>> > Again, I'm not saying we should go in this direction, but I don't think
>> > we
>> > should dismiss it without consideration -- we probably don't want to
>> > find we
>> > need a third form of __has_attribute later =)
>>
>> That's one of the reasons Alp's suggestion for forwards compatibility
>> is so nice -- if implemented properly, we could add parameter support
>> at a later date (presuming we stick with the attribute syntax style
>> approach).
>>
>> I'd like to avoid attempting to preprocess parameters for this patch,
>> but had intended to leave the door open for the future. So it wasn't
>> entirely without consideration. ;-)
>
>
> =) OK then!