[Bug c++/38828] New: g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[Bug c++/38828] New: g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[ronan dot lehy at probayes dot com]

I'm compiling the attached source and inspecting symbols in it:
  g++-4.3 -v -save-temps -Wall -Wextra -O2 -fno-inline-small-functions
mylib.cpp -c -o mylib.o
  nm mylib.o | c++filt | grep 'serialize<'

With -O2 -fno-inline-small-functions (as above), nm gives (as expected):
 00000000 W void
boost::serialization::serialize<boost::archive::xml_iarchive>(boost::archive::xml_iarchive&,
A&, unsigned int)
 00000000 W void
boost::serialization::serialize<boost::archive::xml_iarchive>(boost::archive::xml_iarchive&,
B&, unsigned int)
 00000000 W void
boost::serialization::serialize<boost::archive::xml_oarchive>(boost::archive::xml_oarchive&,
A&, unsigned int)
 00000000 W void
boost::serialization::serialize<boost::archive::xml_oarchive>(boost::archive::xml_oarchive&,
B&, unsigned int)

With just -O2, some symbols are missing:
 00000000 W void
boost::serialization::serialize<boost::archive::xml_iarchive>(boost::archive::xml_iarchive&,
B&, unsigned int)
 00000000 W void
boost::serialization::serialize<boost::archive::xml_oarchive>(boost::archive::xml_oarchive&,
B&, unsigned int)

The disappearing functions are explicit template instantiations, that are not
marked static, or inline. With g++-4.1 and g++-4.2, no symbols are missing.

The g++ output follows.

Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.3.2-1ubuntu11'
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3
--program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --enable-mpfr --enable-targets=all --enable-checking=release
--build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.3.2 (Ubuntu 4.3.2-1ubuntu11) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-O2' '-c' '-o'
'mylib.o' '-shared-libgcc' '-mtune=generic'
 /usr/lib/gcc/i486-linux-gnu/4.3.2/cc1plus -E -quiet -v -D_GNU_SOURCE mylib.cpp
-D_FORTIFY_SOURCE=2 -mtune=generic -Wall -Wextra -O2 -fpch-preprocess -o
mylib.ii
ignoring nonexistent directory "/usr/local/include/i486-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/i486-linux-gnu/4.3.2/../../../../i486-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/4.3
 /usr/include/c++/4.3/i486-linux-gnu
 /usr/include/c++/4.3/backward
 /usr/local/include
 /usr/lib/gcc/i486-linux-gnu/4.3.2/include
 /usr/lib/gcc/i486-linux-gnu/4.3.2/include-fixed
 /usr/include/i486-linux-gnu
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-O2' '-c' '-o'
'mylib.o' '-shared-libgcc' '-mtune=generic'
 /usr/lib/gcc/i486-linux-gnu/4.3.2/cc1plus -fpreprocessed mylib.ii -quiet
-dumpbase mylib.cpp -mtune=generic -auxbase-strip mylib.o -O2 -Wall -Wextra
-version -fstack-protector -o mylib.s
GNU C++ (Ubuntu 4.3.2-1ubuntu11) version 4.3.2 (i486-linux-gnu)
        compiled by GNU C version 4.3.2, GMP version 4.2.2, MPFR version 2.3.2.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: dc8f858cacda993dd8f70837f0d9565d
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-O2' '-c' '-o'
'mylib.o' '-shared-libgcc' '-mtune=generic'
 as -V -Qy -o mylib.o mylib.s
GNU assembler version 2.18.93 (i486-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.18.93.20081009
COMPILER_PATH=/usr/lib/gcc/i486-linux-gnu/4.3.2/:/usr/lib/gcc/i486-linux-gnu/4.3.2/:/usr/lib/gcc/i486-linux-gnu/:/usr/lib/gcc/i486-linux-gnu/4.3.2/:/usr/lib/gcc/i486-linux-gnu/:/usr/lib/gcc/i486-linux-gnu/4.3.2/:/usr/lib/gcc/i486-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/i486-linux-gnu/4.3.2/:/usr/lib/gcc/i486-linux-gnu/4.3.2/:/usr/lib/gcc/i486-linux-gnu/4.3.2/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/i486-linux-gnu/4.3.2/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wall' '-Wextra' '-O2' '-c' '-o'
'mylib.o' '-shared-libgcc' '-mtune=generic'


-- 
           Summary: g++ 4.3.2: -O2 without -fno-inline-small-functions makes
                    some template instantiations disappear
           Product: gcc
           Version: 4.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ronan dot lehy at probayes dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[ronan dot lehy at probayes dot com]

------- Comment #1 from ronan dot lehy at probayes dot com  2009-01-13 16:35 -------
Created an attachment (id=17088)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17088&action=view)
Preprocessed source


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[ronan dot lehy at probayes dot com]

------- Comment #2 from ronan dot lehy at probayes dot com  2009-01-13 16:39 -------
(In reply to comment #1)
> Created an attachment (id=17088)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17088&action=view)

I had to trim a bit the source to be make the preprocessed source fit into 1000
ko. With it, the expected nm output (-O2 -fno-inline-small-functions) is:

00000000 W void
boost::serialization::serialize<boost::archive::xml_iarchive>(boost::archive::xml_iarchive&,
A&, unsigned int)
 00000000 W void
boost::serialization::serialize<boost::archive::xml_iarchive>(boost::archive::xml_iarchive&,
B&, unsigned int)

And the one with missing symbols (-O2) is:
 00000000 W void
boost::serialization::serialize<boost::archive::xml_iarchive>(boost::archive::xml_iarchive&,
B&, unsigned int)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[ronan dot lehy at probayes dot com]

------- Comment #3 from ronan dot lehy at probayes dot com  2009-01-13 16:42 -------
Created an attachment (id=17089)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17089&action=view)
Original source (#includes Boost headers).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2009-01-13 20:43 -------
serialize with an empty body is a pure function so it will be can be optimized
away without any effects.  I don't see the issue here really.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2009-01-13 20:46 -------
Also since it is not explicitly instatinated, the template does not need to be
in the object file really.

Can you give a better example of why do you think this is wrong besides a nm
testcase?  It might be due to that you are defining the template specialization
in the other TUs in which case, the code is invalid.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[ronan dot lehy at probayes dot com]

------- Comment #6 from ronan dot lehy at probayes dot com  2009-01-14 09:11 -------
Thanks a lot for considering this report!

(In reply to comment #5)
> Also since it is not explicitly instatinated, the template does not need to be
> in the object file really.

I believe this is instantiated with Archive = boost_xml_iarchive by the
BOOST_EXPORT macro.

(In reply to comment #5)
> serialize with an empty body is a pure function so it will be can
> be optimized away without any effects.  I don't see the issue here really.

But this function (as instantiated with Archive = boost_xml_iarchive by
BOOST_EXPORT) can be called from other .o files. Actually, it is, and linking
the .o files together into a shared library then fails.

> Can you give a better example of why do you think this is wrong besides a nm
> testcase?

I could show you how mylib1.o and mylib2.o fail to link into libmylib.so, since
serialize<boost_xml_iarchive>(boost_xml_iarchive&, A&, unsigned int) is
referenced by mylib2.o, and should be defined in mylib1.o, but is not. Do you
want that ?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[ronan dot lehy at probayes dot com]

------- Comment #7 from ronan dot lehy at probayes dot com  2009-01-14 09:14 -------
(In reply to comment #6)
> I believe this is instantiated with Archive = boost_xml_iarchive by the
> BOOST_EXPORT macro.

I mean BOOST_CLASS_EXPORT(), of course, sorry.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[ronan dot lehy at probayes dot com]

------- Comment #8 from ronan dot lehy at probayes dot com  2009-01-15 15:13 -------
If I add simple explicit instantiations in place or in addition to the
BOOST_CLASS_EXPORT() calls, all symbols are correctly exposed in all cases :
namespace boost {
    namespace serialization {
        template void
serialize<boost::archive::xml_iarchive>(boost::archive::xml_iarchive &, A &,
unsigned int);
        template void
serialize<boost::archive::xml_iarchive>(boost::archive::xml_iarchive &, B &,
unsigned int);
    } }

Therefore, there may be a problem related to the way BOOST_CLASS_EXPORT()
instantiates serialize<>(). BOOST_CLASS_EXPORT(A) expands to:

namespace boost { namespace archive { namespace detail {
      template<> const guid_initializer< A >
        guid_initializer< A >::instance("A");
      template std::pair<
        const export_generator<
          A, boost::archive::detail::known_archive_types::type> *,
        const guid_initializer< A > *>
      export_archives_invoke<
        A, boost::archive::detail::known_archive_types::type>
      (A &, boost::archive::detail::known_archive_types::type &);
    } } };


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[ronan dot lehy at probayes dot com]

------- Comment #9 from ronan dot lehy at probayes dot com  2009-01-15 16:12 -------
Following the twisted maze that is BOOST_CLASS_EXPORT() leads me to think that
it is (very) roughly equivalent to this:

void dummy(boost::archive::xml_iarchive & ar, A & a, B & b) {
    boost::serialization::serialize(ar, a, 0);
    boost::serialization::serialize(ar, b, 0);
}

Indeed, replacing my BOOST_CLASS_EXPORT() calls with this, I get the same
effect ("disappearing" symbols with -O2).

At this point, my question is: is triggering template instantiations in this
way supposed to be equivalent to explicit instantiations (as in comment #8)
with respect to symbols exposed to the outside? If the answer is no, thank you
very much for your help and sorry for the trouble.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828

[Bug c++/38828] g++ 4.3.2: -O2 without -fno-inline-small-functions makes some template instantiations disappear

[bangerth at gmail dot com]

------- Comment #10 from bangerth at gmail dot com  2009-01-30 15:37 -------
(In reply to comment #9)
> Following the twisted maze that is BOOST_CLASS_EXPORT() leads me to think that
> it is (very) roughly equivalent to this:
> 
> void dummy(boost::archive::xml_iarchive & ar, A & a, B & b) {
>     boost::serialization::serialize(ar, a, 0);
>     boost::serialization::serialize(ar, b, 0);
> }
> 
> Indeed, replacing my BOOST_CLASS_EXPORT() calls with this, I get the same
> effect ("disappearing" symbols with -O2).
> 
> At this point, my question is: is triggering template instantiations in this
> way supposed to be equivalent to explicit instantiations (as in comment #8)
> with respect to symbols exposed to the outside? If the answer is no, thank you
> very much for your help and sorry for the trouble.

The answer is no. The code requires the *implicit* instantiation of some code
but the compiler only needs to instantiate whatever it needs and is free
to eliminate what it doesn't. In fact, there is no guarantee that any code
visible outside this one object file is generated at all.

W.


-- 

bangerth at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bangerth at gmail dot com
             Status|WAITING                     |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38828