[Bug c++/58542] New: std::atomic<__int128_t>::store broken with -march=corei7

[2013.bugzilla.gcc.gnu.org at ingomueller dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58542

            Bug ID: 58542
           Summary: std::atomic<__int128_t>::store broken with
                    -march=corei7
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 2013.bugzilla.gcc.gnu.org at ingomueller dot net

The attached file shows how std::atomic<__int128_t>::store is broken when
compiled with --march=corei7. Then relevant code looks like this:

    std::atomic<__int128_t> i;
    i = -1;
    std::cout << std::hex << uint64_t(i>>64) << uint64_t(i) << std::endl;

Depending on the compile flags, the program outputs the correct or an incorrect
result:

    $ g++-4.8 -std=c++11 test.cpp -O1 -latomic -o test_right && ./test_right 
    ffffffffffffffffffffffffffffffff
    $ g++-4.8 -std=c++11 test.cpp -O1 -march=corei7 -latomic -o test_wrong &&
./test_wrong 
    0ffffffffffffffff

It looks like the upper 64bit are not stored.

I tested GCC 4.7.3 and 4.8.1, but I don't have a snapshot at hand -- sorry.