[C++-sig] Symbol visibility patch for GCC v3.4

[Niall Douglas]

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If people could test this patch and let me know, I'd be most 
grateful.

Some might consider this off-topic, so I'll give you a quick run down 
of the improvements - with my Boost.Python built bindings, my shared 
object went from >200,000 symbols down to just 19,738 resulting in 
the six minute load time dropping to a few seconds. If you examine 
the dynamic table with nm, the output is vastly cleaner with only 
what is actually required being exported - this will beat any linker 
version script method hands down. Also, because the compiler now 
knows what symbols don't need to run through the relocation table, it 
can substantially optimise code output with better inlining, half the 
overhead of calling a function, much better ability to totally elide 
unnecessary code and many other benefits. See Ulrich Drepper's "How 
to Write Shared Libraries" article.

To install, get a GCC v3.4 from CVS. Apply patch. Find the header 
file boost/boost/python/detail/config.hpp. Modify the dllexport 
section to:

#if defined(BOOST_PYTHON_DYNAMIC_LIB)
#  if (defined(_WIN32) || defined(__CYGWIN__))
#     if defined(BOOST_PYTHON_SOURCE)
#        define BOOST_PYTHON_DECL __declspec(dllexport)
#        define BOOST_PYTHON_BUILD_DLL
#     else
#        define BOOST_PYTHON_DECL __declspec(dllimport)
#     endif
#  elif (defined(__GNUC__) && __GNUC__ >= 3 && __GNUC_MINOR__ >=4)
#     if defined(BOOST_PYTHON_SOURCE)
#        define BOOST_PYTHON_DECL __attribute__ 
((visibility("default")))
#        define BOOST_PYTHON_BUILD_DLL
#     else
#        define BOOST_PYTHON_DECL
#     endif
#  endif
#endif

Adjust the Jamfile to include a <cxxflags>-fvisibility=hidden for the 
dll boost_python. Recompile everything using -fvisibility=hidden for 
your binding source files too. You should see a substantial 
improvement in link times and your shared objects will have reduced 
in size, sometimes by tens of megabytes.

Everything should run just as before. If it doesn't, please let me 
know - everything on my end runs just as before.

Cheers,
Niall





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