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