[Bug tree-optimization/60899] New: undef reference generated with -fdevirtualize-speculatively

[xinliangli at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60899

            Bug ID: 60899
           Summary: undef reference generated with
                    -fdevirtualize-speculatively
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xinliangli at gmail dot com

Build the following code with the following command line:

g++ -O2 -fdisable-tree-einline a.cc a_m.cc 

results in:

/tmp/cci31j3N.o: In function `D::doit()':
a.cc:(.text._ZN1D4doitEv[_ZN1D4doitEv]+0x5): undefined reference to `A::foo()'
collect2: error: ld returned 1 exit status

It builds fine when devirtualization is disabled:

-O2 -fno-devirtualization-speculatively -fdisable-tree-einline 


The problem is there is no instantiation of any class A instances (final or
subclass) in the program, so vtables and A::foo are all eliminated. The
reference to A::foo is from D::doit. In a successful build, there are no D
instances either, so D::doit won't be emitted. However with speculative
devirtualization, D::doit may be speculatively referenced even though there are
no D instances.

What happens is that during ipa-inline, goo is inlined into D::doit, the
virtual call to foo should become an direct call to A::foo, but the new edge is
not discovered. Since there is no call edge to A::foo, A::foo gets removed
right after ipa-inline (before inline transform). However during inline
transform, gimple-fold-call converts the virtual call into a direct call.


The test case is extracted from a very large real program. The explicit
reference to D::doit in bar is to demonstrate the problem -- in the real
program, the reference is from spec-devirt.


//a.h
struct B {
 virtual int foo() = 0;
 int goo() { return foo(); }
 int i;
};

struct A  : public B {
 A() : i(0)  {}
 int foo() { return 1;}
 int i;
};
struct A2  : public B {
 int foo() { return 2;}
};

struct DI {
  virtual int doit() = 0;
};

struct D : public DI {
   virtual int doit () { return m.goo(); }

   A m;
};

// a.cc

#include "a.h"

int cond;
int bar (DI* ap) {
   if (cond) return static_cast<D*>(ap)->D::doit();  // Mimic speculative
devirtualization
   return ap->doit();
}


// a_m.cc

#include "a.h"

int cond;
int bar (DI* ap) {
   if (cond) return static_cast<D*>(ap)->D::doit();  // Mimic speculative
devirtualization
   return ap->doit();
}