g++ how to do partial link time optimization (-flto) - g++

Question

I man g++ and read carefully about the lto part. Now I want to know how to do partial link time optimization just like -ipo-c in icpc compiler. For example:
g++ -O2 -flto -c a.cpp
g++ -O2 -flto -c b.cpp
Now it will generate a.o and b.o which contain GIMPLE (one of GCC's internal representations). I want to combine a.o and b.o to generate a real object c.o file. That means just lto over two cpp file not the whole program. Any idea?
The reason is that I need to combine fortran code and c++ code together, so the final link step is
ifort -nofor-main -cxxlib -fexceptions f.o a.o b.o
f.o is generated by ifort. Because ifort has no idea what GIMPLE is. So the final link step will fail.

Answer 1

Just use g++ to link it.
$ g++ --version
g++ (GCC) 4.8.3
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gfortran --version
GNU Fortran (GCC) 4.8.3
Copyright (C) 2013 Free Software Foundation, Inc.
GNU Fortran comes with NO WARRANTY, to the extent permitted by law.
You may redistribute copies of GNU Fortran
under the terms of the GNU General Public License.
For more information about these matters, see the file named COPYING
$ cat a.cpp
#include <stdio.h>
extern "C" void foo_() { puts("foo"); }
$ cat b.cpp
#include <stdio.h>
extern "C" void bar_() { puts("bar"); }
$ cat f.f90
program f
implicit none
external foo
external bar
call foo()
call bar()
end program
$ g++ -O2 -flto -c a.cpp
$ g++ -O2 -flto -c b.cpp
$ gfortran -O2 -flto -c f.f90
$ g++ -O2 -flto a.o b.o f.o -lgfortran
$ ./a
foo
bar