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

[Aaron Ballman]

This is the latest version of this patch -- with some additional
features as requested. Specifically, this is using the existing
__has_attribute syntax in an extended style, and implements a
feature-test macro (__has_feature(__has_attribute_syntax)) which
allows you to determine whether the extended syntax is supported or
not. It adds some additional documentation explaining this.

~Aaron

On Thu, Jan 23, 2014 at 10:23 AM, Aaron Ballman <aaron at aaronballman.com> wrote:
> 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!
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