[Bug debug/81135] Extra debug info generated for unused extern declarations

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81135

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-06-20
                 CC|                            |aldyh at gcc dot gnu.org,
                   |                            |jason at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  Note that technically for

extern int j;
extern int i;
extern int bar (int);
extern int foo (int);

int main () { return foo (1) + i; }

we are still missing debuginfo for 'foo'.  If the module containing the
implementation was not built with -g then debugging experience in this
module is worse than necessary.

Note that 'foo' _is_ used here still neither GCC 5 nor GCC 6 create a DIE
for 'foo' but both generate one for 'i'.  GCC 6 now generates a DIE for 'j'
where GCC 5 didn't.  No compiler generates a DIE for 'bar'.

Same for GCC 7 + trunk.

I think we should have a mode where we generate debuginfo for all of the
extern declarations, used or not, and we should have a way to prune
unused extern decl DIEs.  Which means we should emit a DIE for 'foo' in
all cases (which will bloat info).  Otherwise why emit one for 'i'
(maybe have a mode to drop all extern DIEs which may be refered to via
the symtab).