__has_attribute and pragmas

[Aaron Ballman]

Ping

On Mon, Nov 24, 2014 at 10:02 AM, Aaron Ballman <aaron at aaronballman.com> wrote:
> On Thu, Nov 20, 2014 at 7:22 PM, Richard Smith <richard at metafoo.co.uk> wrote:
>> On Thu, Nov 20, 2014 at 3:48 PM, Aaron Ballman <aaron at aaronballman.com>
>> wrote:
>>>
>>> On Thu, Nov 20, 2014 at 6:27 PM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>> > On Thu, Nov 20, 2014 at 1:38 PM, Aaron Ballman <aaron at aaronballman.com>
>>> > wrote:
>>> >>
>>> >> The __has_attribute implementation does not pay attention to the
>>> >> syntax supported by attributes -- instead, it looks to see whether an
>>> >> attribute is generally known with that spelling. Since pragmas can now
>>> >> be spelled as attributes, this means __has_attribute(loop) returns
>>> >> true because of the #pragma loop functionality. Same for unroll.
>>> >>
>>> >> Should __has_attribute ignore attributes spelled with a #pragma
>>> >> spelling?
>>> >
>>> >
>>> > I would go further: __has_attribute should probably only look for
>>> > GNU-syntax
>>> > attributes. We have __has_cpp_attribute for C++-syntax attributes now,
>>> > and I
>>> > don't think anyone is (yet) using this for __declspec, so now seems like
>>> > a
>>> > good time to make this change.
>>>
>>> A long, long while back, we discussed having a way to determine
>>> attributes by syntax, because that's sometimes important, as well as a
>>> general query mechanism.
>>>
>>> How about adding:
>>>
>>> __has_declspec_attribute
>>> __has_keyword_attribute
>>> __has_gnu_attribute
>>>
>>> and leaving __has_attribute generic across syntaxes?
>>>
>>> This also reduces the chances of breaking code by allowing the
>>> __has_attribute syntax to continue to work as it always has.
>>
>>
>> Back when we only had GNU attributes, that's all it detected. I'm not
>> convinced that people are actually using it for anything else,
>
> I believe I've seen some code using it for __declspec attributes.
> IIRC, we may have run into this with MinGW, which implements
> __declspec as a macro, replaced by __attribute__.
>
>> and I think
>> it would be surprising if it said an attribute was supported but that
>> attribute didn't work with GNU syntax. A generic-across-syntaxes
>> __has_attribute is basically useless.
>
> The more I think about cross-syntax attribute checking, the more I
> agree it's useless.
>
> I still think it would make sense to add the various forms of this,
> and I definitely think that pragmas should be excluded from
> __has_attribute.
>
> How does this sound as a path forward:
>
> * Change __has_attribute to only support GNU-style attributes. This
> has the potential to break code, so this will require careful watching
> of the lists.
> * Add __has_declspec_attribute & __has_keyword_attribute, that only
> apply to __declspecs and keywords.
>
> ~Aaron