[cfe-dev] Where do attributes get merged?

[Aaron Ballman]

On Wed, Dec 10, 2014 at 1:32 PM, Robinson, Paul
<Paul_Robinson at playstation.sony.com> wrote:
>> -----Original Message-----
>> From: aaron.ballman at gmail.com [mailto:aaron.ballman at gmail.com] On Behalf
>> Of Aaron Ballman
>> Sent: Wednesday, December 10, 2014 5:50 AM
>> To: David Majnemer
>> Cc: Robinson, Paul; cfe-dev at cs.uiuc.edu Developers (cfe-dev at cs.uiuc.edu)
>> Subject: Re: [cfe-dev] Where do attributes get merged?
>>
>> Yes, you're looking for Sema::mergeDeclAttributes. You're also correct
>> that the current behavior only works when the attributes are on the
>> same declaration.
>
> Awesome, thanks guys. Exactly what I needed.
>
>> However, from memory, I don't think that situation of yours should
>> result in well-formed code. IIRC, the definition must not attempt to
>> declare new attributes (aside from some keyword-like ones) that do not
>> also appear on the declaration.
>
> Hmm I poked around in online docs for both clang and gcc, I'm not finding
> any statement like that.

I may have been mis-remembering as well. ;-)

> Looking at the code, the behavior seems to be:
> - If we've already seen a definition, a new declaration can't add any
>   attributes. Except for a couple of special cases.
> - Otherwise, attributes that are on the old declaration, and are inheritable,
>   are propagated to the new declaration (again with a few special cases).
>
> The use case for __attribute__((optnone)) is such that having it on the
> definition but not the declaration would be the normal case.

GNU-style attributes generally are not definition-only. In fact, I
believe we only allow that as an extension for attributes known to
GCC. Since optnone isn't a GCC attribute (at this time), it's not
technically an extension, but I suspect calling it the "normal case"
is a bit of a stretch.

>  I think it
> would be reasonable to say that when optnone conflicts with something else,
> the attribute on the definition wins.
>
> How does that sound?

That may be feasible for optnone, but I'm not certain I like it in the
general sense. Attributes appearing on the definition but not the
declaration can change the behavior in a substantial way -- for
instance, a noreturn attribute. Also, this behavior will depend on the
spelling of the attribute given. A __declspec cannot be added to a
function definition if it is absent on a function declaration, but the
reverse isn't true.

~Aaron