[Bug c++/58272] New: unnecessary vtables emission for pure abstract classes

[Bug c++/58272] New: unnecessary vtables emission for pure abstract classes

[froydnj at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58272

            Bug ID: 58272
           Summary: unnecessary vtables emission for pure abstract classes
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: froydnj at gcc dot gnu.org
                CC: hubicka at ucw dot cz

Consider the following testcase:

class Interface {
public:
  virtual int f() = 0;
};

class Concrete : public Interface {
public:
  virtual int f();
};

int
Concrete::f()
{
  return 2;
}

Concrete*
do_stuff()
{
  Concrete* c = new Concrete();
  return c;
}

int
call_f()
{
  Interface* i = do_stuff();
  return i->f();
}

compiling this on x86-64 with:

g++ -fno-exceptions -S -o - -O2 vtables.cpp -fno-rtti

produces output with a vtable for Interface, which is completely unused in the
compilation unit.  (Still does it when omitting -fno-rtti, so it's not a
missing check for typeinfo emission or similar.)

Mozilla has a fair number of these and even though one can eliminate them via
--gc-sections, they still take up time to assemble, write to disk, etc. etc. 
Honza and I were talking about them and we did not know whether it was an ABI
requirement that they be emitted or merely a bug in the compiler.  G++ appears
to have some smarts about emitting the vtable for Concrete, so I am somewhat
surprised that it doesn't have similar smarts for the vtable for Interface.

[Bug c++/58272] unnecessary vtables emission for pure abstract classes

[hubicka at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58272

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hubicka at gcc dot gnu.org

--- Comment #1 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
Current mainline seems to optimize Interace out completely. If this is correct,
we probably ought to add it to testsuite?

[Bug c++/58272] unnecessary vtables emission for pure abstract classes

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58272

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|UNCONFIRMED                 |RESOLVED
      Known to work|                            |4.9.0
           Keywords|                            |missed-optimization
   Target Milestone|---                         |4.9.0
      Known to fail|                            |4.1.2, 4.4.7, 4.7.1, 4.8.1,
                   |                            |4.8.5

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Been fixed since 4.9.0.