Detect '-fvisibility=hidden' during pre process or compile time

Detect '-fvisibility=hidden' during pre process or compile time

[Max Sagebaum]

Hello @ all,

Short question: Is there a way to detect the option '-fvisibility=hidden' during the preprocessor or the compile time, such that a warning to the user can be issued, that the library might not support it?

Long question:
We develop a header only library where we have a static structure member inside of a class. We require that this static member is seen by all operations on this class.

If our library is included with '-fvisibility=hidden' then we get multiple symbols of the same static member. We tried to fix this by declaring the static member with '__attribute__((visibility("default")))' but for members which are structs this does not seem to work.  I filed an bug report about this and got the answer, that  this is not a bug but the behavior is as desired.  (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104631)<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104631>

Since we can not change the visibility of the static member and do not want to change the visibility of the while library. (Since there is the real use case, that our library is only used in a different library and all the symbols do not need to be exposed.) We wanted to ask if it is possible to issue a warning during compile time or in the preprocessor that the  '-fvisibility=hidden' option is used in our library.

Thanks

Max

Re: Detect '-fvisibility=hidden' during pre process or compile time

[Alexander Monakov]

On Fri, 22 Apr 2022, Max Sagebaum wrote:

> Hello @ all,
> 
> Short question: Is there a way to detect the option '-fvisibility=hidden'
> during the preprocessor or the compile time, such that a warning to the user
> can be issued, that the library might not support it?

No (note that visibility can change during optimization, like when during LTO
the compiler learns that external references will not be possible).

> Long question: We develop a header only library where we have a static
> structure member inside of a class. We require that this static member is seen
> by all operations on this class.
> 
> If our library is included with '-fvisibility=hidden' then we get multiple
> symbols of the same static member. We tried to fix this by declaring the
> static member with '__attribute__((visibility("default")))' but for members
> which are structs this does not seem to work.  I filed an bug report about
> this and got the answer, that  this is not a bug but the behavior is as
> desired.
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104631

Well, it is not a matter of a scalar vs a struct type. The issue is that the
containing class is a template, and when you instantiate it, template parameter
has hidden visibility. I've posted a short example to the above bug.


> Since we can not change the visibility of the static member and do not want to
> change the visibility of the while library. (Since there is the real use case,
> that our library is only used in a different library and all the symbols do
> not need to be exposed.)

You can change the visibility of the member if you don't require that its type
is a template parameter.

Alexander

Re: Detect '-fvisibility=hidden' during pre process or compile time

[Max Sagebaum]

Hello Alexander,

thank you for the answer. Unfortunately, the type needs to be a template parameter.  Is there any other way to change the visibility of a member or in addition change the visibility of a template parameter?

Thanks

Max

On Fri, 2022-04-22 at 18:09 +0300, Alexander Monakov wrote:
On Fri, 22 Apr 2022, Max Sagebaum wrote:

Hello @ all,

Short question: Is there a way to detect the option '-fvisibility=hidden'
during the preprocessor or the compile time, such that a warning to the user
can be issued, that the library might not support it?

No (note that visibility can change during optimization, like when during LTO
the compiler learns that external references will not be possible).

Long question: We develop a header only library where we have a static
structure member inside of a class. We require that this static member is seen
by all operations on this class.

If our library is included with '-fvisibility=hidden' then we get multiple
symbols of the same static member. We tried to fix this by declaring the
static member with '__attribute__((visibility("default")))' but for members
which are structs this does not seem to work.  I filed an bug report about
this and got the answer, that  this is not a bug but the behavior is as
desired.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104631

Well, it is not a matter of a scalar vs a struct type. The issue is that the
containing class is a template, and when you instantiate it, template parameter
has hidden visibility. I've posted a short example to the above bug.


Since we can not change the visibility of the static member and do not want to
change the visibility of the while library. (Since there is the real use case,
that our library is only used in a different library and all the symbols do
not need to be exposed.)

You can change the visibility of the member if you don't require that its type
is a template parameter.

Alexander


--

Dr. Max Sagebaum

Chair for Scientific Computing,
TU Kaiserslautern,
Bldg/Geb 34, Paul-Ehrlich-Strasse,
67663 Kaiserslautern, Germany

Phone: +49 (0)631 205 5638
Fax:   +49 (0)631 205 3056
Email: max.sagebaum@scicomp.uni-kl.de<mailto:max.sagebaum@scicomp.uni-kl.de>
URL:   www.scicomp.uni-kl.de

Re: Detect '-fvisibility=hidden' during pre process or compile time

[Alexander Monakov]

On Fri, 22 Apr 2022, Max Sagebaum wrote:

> Hello Alexander,
> 
> thank you for the answer. Unfortunately, the type needs to be a template
> parameter.  Is there any other way to change the visibility of a member or in
> addition change the visibility of a template parameter?

You can change visibility of the type used as the template parameter; in the
small example in the Bugzilla, you could write

struct __attribute__((visibility("default"))) S {
};

or with a pragma:

#pragma GCC visibility push(default)
struct S {
};
#pragma GCC visibility pop

If in your actual library such 'struct S' has many members and you want to avoid
making them default-visibility, you could move some of them into a subobject
(which then may have hidden visibility).

Alexander