interlocked pointer exchanges cause warnings with mingw64

[anonymous]

With a gcc build targeting Win64 (specifically ​http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win64/Release%20for%20GCC%204.4.0/mingw-w64_x86-64_linux64-4.4.0-1.tar.bz2/download), I'm seeing warnings like this:

/home/martind/work/unicorn/include/x86_64_mingw32w64/boost/thread/win32/interlocked_read.hpp: In function 'void boost::detail::interlocked_write_release(void* volatile*, void*)': /home/martind/work/unicorn/include/x86_64_mingw32w64/boost/thread/win32/interlocked_read.hpp:71: error: cast from 'void*' to 'long int' loses precision ... /home/martind/work/unicorn/include/x86_64_mingw32w64/boost/thread/win32/basic_timed_mutex.hpp: In member function 'void* boost::detail::basic_timed_mutex::get_event()': /home/martind/work/unicorn/include/x86_64_mingw32w64/boost/thread/win32/basic_timed_mutex.hpp:152: error: cast from 'void*' to 'long int' loses precision cc1plus: warnings being treated as errors

The warnings come from the casts here in boost/detail/interlocked.hpp:

# define BOOST_INTERLOCKED_COMPARE_EXCHANGE_POINTER(dest,exchange,compare) \

((void*)BOOST_INTERLOCKED_COMPARE_EXCHANGE((long volatile*)(dest),(long)(exchange),(long)(compare)))

# define BOOST_INTERLOCKED_EXCHANGE_POINTER(dest,exchange) \

((void*)BOOST_INTERLOCKED_EXCHANGE((long volatile*)(dest),(long)(exchange)))

On Win64, I see that sizeof(long) == 4 but sizeof(void*) == 8, so the compiler is right to warn. Elsewhere in this file, InterlockedCompareExchangePointer and InterlockedExchangePointer are conditionally used to implement these macros. Comments I found elsewhere suggested that this is done conditionally because these functions aren't available in very old platform SDKs and, indeed, MSDN says that they only became available with Windows 2000. Perhaps Win64 can be assumed to have these functions available, as is assumed earlier in the file when compiling with MSVC. gcc for Win64 is a relatively new development without wide deployment, which could explain why the branch that handles CYGWIN etc doesn't yet cope.

In the attached patch, I've used the same condition for detecting Win64 as used in the MSVC part of the file. That means that I've changed the behavior for IA64 as well as AMD64, despite having no means to test IA64.

I see interlocked.hpp is used by interprocess, smart_ptr and thread but I'm not sure which Component it "belongs" to, so I've left that set to None.

[mdorey@…]

use Interlocked(Compare)?ExchangePointer for mingw64

[viboes]

Can this be assigned to shared_ptr component?

[Marshall Clow]

>Can this be assigned to shared_ptr component?
I don't know; if I was sure, I would have done so....

Peter - what do you think?

[Peter Dimov]

These particular macros have, I believe, been added by Anthony Williams for use in Boost.Thread. So maybe Anthony is the person who needs to review and test the patch.

[Anthony Williams]

I've applied the patch to trunk.