[Bug target/40068] GCC fails to apply dllexport attribute to typeinfo.

["dave dot korn dot cygwin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #5 from dave dot korn dot cygwin at gmail dot com  2009-05-10 11:17 -------
(In reply to comment #4)
> Hello Dave,

  Hi Danny!

> Rather than use DLL linkage (and so force client to resort to auto-import
> magic)
> why not just always emit the RTTI with one-only comdat linkage.

  I have your patch in the cygwin distro compiler where it works fine, but I am
concerned about what unforeseeable problems could arise by violating ODR in
this way.  I don't have any concrete evidence of any problem yet, it is just a
worry.

  Also, I don't think this is necessarily an either-or situation; we could add
my patch and have the typeinfo exported from the DLL, and also add yours so
that clients could link a comdat copy (which would override the import stub)
until we have a better solution for importing from the DLL.  Or I could follow
up with another patch that propagates dllimport attributes from class to
typeinfo.

There is of course this:

/* We leave typeinfo tables alone.  We can't mark TI objects as
     dllimport, since the address of a secondary VTT may be needed
     for static initialization of a primary VTT.  VTT's  of
     dllimport'd classes should always be link-once COMDAT.  */ 

          /* Do not import tinfo nodes if the class has dllimport attribute.
             Dllimports do not have a constant address at compile time, so
             static initialization of tables with RTTI fields is a problem.
             Set to comdat instead.   */

... but I do not see why this should be a problem in these days of auto-import
and pseudo-relocs; do you know more about what the actual problem is (or was)
here?  Is this not basically the same situation as something like 

----dll_A.c----

int foo;
int __attribute__ ((dllexport)) * bar = &foo;

----dll_B.c----

extern int __attribute__ ((dllexport)) * bar;
int __attribute__ ((dllexport)) * baz = &bar;

---------------

the example above?

  If it's possible to solve either in the compiler or further down the
toolchain, I would very much like to do so.

  Also, how come emitting the typeinfo .linkonce as we currently do isn't the
same as COMDAT for these purposes?

    cheers,
      DaveK


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40068