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

[Alp Toker]

On 13/01/2014 13:23, Aaron Ballman wrote:
> On Sun, Jan 12, 2014 at 11:43 PM, Kal <b17c0de at gmail.com> wrote:
>> Am 13.01.14 03:43, schrieb Aaron Ballman:
>>
>> New patch attached with some suggestions implemented. Changes include:
>>
>> 1) Updated the cmake file because the name of the tablegen file was
>> updated (this should have been in the initial patch)
>> 2) Lexing unknown syntax now attempts better recovery. This allows us
>> to be a bit more future-proof
>> 3) Unknown syntax is now handled as a warning instead of an error.
>> However, malformed __has_attribute is still treated as an error (for
>> instance, __has_attribute[haha] will be an error, but
>> __has_attribute([haha]) will be a warning). The warning may warrant
>> its own diagnostic group instead of piggy-backing on UnknownAttribute.
>> 4) C++11 language option checking was moved into tablegen instead of
>> the preprocessor
>> 5) Updated test cases for the new warning, future-proofing and the
>> movement of C++11 language option checking
>>
>> What this patch does not address is:
>>
>> * It's still using __has_attribute while we discuss an improved name
>> * The default __has_attribute(ident) functionality still checks for
>> all attribute names (changing this would be a separate patch,
>> regardless)
>>
>> One other thing to consider with regards to naming -- another approach
>> we could take would be to not encode the syntax directly in the
>> feature macro, but instead use separate macros (with its analogous
>> currently-proposed syntax in comments):
>>
>> __has_gnu_attribute(ident)  // __has_attribute(__attribute__((ident)))
>> __has_declspec_attribute(ident)  // __has_attribute(__declspec(ident))
>> __has_generalized_attribute(ident) // __has_attribute([[ident]])
>> __has_keyword_attribute(ident)  // __has_attribute(ident)
>>
>>
>> What about the following instead (could also swap the order of these
>> arguments):
>>
>> __has_attribute(gnu,         ident) //
>> __has_attribute(__attribute__((ident)))
>> __has_attribute(declspec,    ident) // __has_attribute(__declspec(ident))
>> __has_attribute(generalized, ident) // __has_attribute([[ident]])
>> __has_attribute(keyword,     ident) // __has_attribute(ident)
>>
> Thank you for the input!
>
> Unfortunately, it would cause backwards compatibility problems as the
> current implementation has no fallback mode for syntaxes it doesn't
> understand (so this would break existing code, and have no migration
> path forward). At the very least, the name would have to change.
> Personally, if we are using a single identifier for the feature test
> macro, I would prefer to key off the distinct attribute syntax instead
> of another contextualized identifier. Users already know how to write
> the attribute, so the syntax is something already known. A new
> contextual identifier is one more thing to learn and remember.

Yes, for the alternative proposal (3) it only makes sense to name the 
macros individually for each attribute type. There's no value in 
expecting a name in the first macro argument and the comma form would be 
incompatible.

As a tweak to this latest alternative proposal, there's also no longer a 
need for the spelling to be written because each macro can test for the 
attribute name precisely.

Also instead of 'ident', it's strictly speaking a 'name' in the case of 
C++11 generalized attributes given that they can be namespaced (gnu, 
omp?). These are the forms and names I'd suggest:

    |__has_attribute(deprecated) // Test for GNU attributes only, macro
    always defined (minor tightening of semantics)|||
    |||__has_declspec(deprecated) // Macro only defined in MSVC
    extension mode|||
    |||__has_generalized_attribute(gnu::deprecated)||// Macro only
    defined in C++11 mode|||
    |||__has_simple_attribute(alignas) // Macro always defined, but
    mostly useful for C++11 keyword attributes like 'alignas'|


Overall I think it'll be either this or Aaron's proposal to have a 
single unified feature macro that uses the spellings.

This one has the benefit of simpler parse rules in the preprocessor 
where we don't have access to balanced token and recovery facilities, so 
is less "magical".

On the other hand Aaron's original proposal has the benefit of being 
visually closer to the attribute as written.

Both this one and Aaron's patch are now forward compatible and well 
specified. Overall I'm on the fence as to which is preferable, slightly 
favouring Aaron's original proposal.

Alp.


>
> ~Aaron
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