Re: clang and static linking?

On Sat, Nov 10, 2012 at 01:33:40AM +0100, Dimitry Andric wrote:
> On 2012-11-10 00:25, Dimitry Andric wrote:
> ...
> > The more difficult way out is to not define any duplicate functions in
> > libc.a and libm.a.  For the shared libraries, this should not be a
> > problem, since the dynamic linker will figure out which of the two
> > copies will get precedence.  The functions must stay available for
> > backwards compatibility reasons anyway.

> > For static libraries, this compatibility seems to be unnecessary, as
> > they will only be used to link new programs.  Therefore, it would
> > probably be best to remove the whole isnan.o member from libc.a, and
> > move all the isnan functions to libm.a instead.

> > Currently, isnan() is commented out in lib/msun/src/s_isnan.c, maybe we
> > can enable it whenever PIC is not defined?  Then we could simply skip
> > building lib/libc/gen/isnan.c for libc.a.

> More concretely, here is a patch that seems to achieve the above:

I see this has already been committed, but anyway...

> - Only define isnan, isnanf, __isnan and __isnanf in libc.so, not in
>    libc.a and libc_p.a.

OK, but please add a comment about this.

> - Define isnan in libm.a and libm_p.a, not in libm.so.  I don't think
>    there is a need to define __isnan in the .a files, so I left that out.

Removing symbols from a .so causes subtle ABI breakage and is not needed
for fixing static linking.

More concretely, dlsym of isnan on libm.so will stop working and a
different version of isnan will be chosen if the search list is libm.so,
libother.so, libc.so and libother.so contains another isnan.

> Index: lib/libc/gen/isnan.c
> ===================================================================
> --- lib/libc/gen/isnan.c	(revision 242841)
> +++ lib/libc/gen/isnan.c	(working copy)
> _at__at_ -35,6 +35,7 _at__at_
>   *     binary compat until we can bump libm's major version number.
>   */
>  
> +#ifdef PIC
>  __weak_reference(__isnan, isnan);
>  __weak_reference(__isnanf, isnanf);
>  
> _at__at_ -55,3 +56,4 _at__at_ __isnanf(float f)
>  	u.f = f;
>  	return (u.bits.exp == 255 && u.bits.man != 0);
>  }
> +#endif /* PIC */
> Index: lib/msun/src/s_isnan.c
> ===================================================================
> --- lib/msun/src/s_isnan.c	(revision 242841)
> +++ lib/msun/src/s_isnan.c	(working copy)
> _at__at_ -30,8 +30,9 _at__at_
>  
>  #include "fpmath.h"
>  
> -/* Provided by libc */
> -#if 0
> +/* Provided by libc.so */
> +#ifndef PIC
> +#undef isnan
>  int
>  isnan(double d)
>  {
> _at__at_ -40,7 +41,7 _at__at_ isnan(double d)
>  	u.d = d;
>  	return (u.bits.exp == 2047 && (u.bits.manl != 0 || u.bits.manh != 0));
>  }
> -#endif
> +#endif /* !PIC */
>  
>  int
>  __isnanf(float f)

-- 
Jilles Tjoelker