Re: [Acl-devel] [RFC] Add attribute visibility("default") to API functions

[Yury Gribov]

On 02/17/2016 07:24 AM, Mike Frysinger wrote:
> On 17 Feb 2016 00:57, Andreas Grünbacher wrote:
> > 2016-02-16 21:50 GMT+01:00 Mike Frysinger <address@hidden>:
> > > On 16 Feb 2016 23:18, Yury Usishchev wrote:
> > > > Ok, the patch is attached.
> > >
> > > i'm fine with this (although i haven't tested it yet).
> > > let's see what Andreas has to say.
> >
> > This patch looks simple enough, but I really don't want to have the
> > ACL_API_EXPORT define in the public headers: it doesn't add anything
> > for users, it just makes the header messier to read. An ACL_API_EXPORT
> > or similar macro can just as well be added where each function is
> > defined, with the same net result.

Still many packages already use this approach (just grep your 
/usr/include for 'define.*EXPORT.*visibility'.

> unfortunately, it cannot.  the visibility has to be set on the prototype
> so that when other functions see it, gcc knows how to call it and what
> optimizations to use.

We need to distinguish where the call is done from.

My understanding is that visibility annotations are indeed useless when 
calling external library function because linker will generate a PLT 
call regardless.  But they are very important for intra-library calls 
because compiler will generate direct call and also optimize much more 
agressively (especially under LTO).

The real problem here is that the same library header is typically used 
both by external users and by library internals.  AFAIK maintainers 
typically annotate it and users have to live with one more (useless) 
public macro.

> -mike