[Bug libstdc++/54786] New: Missing fence in std::atomic<T>::store() triggers wrong reordering.

[ozabluda at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54786

             Bug #: 54786
           Summary: Missing fence in std::atomic<T>::store() triggers
                    wrong reordering.
    Classification: Unclassified
           Product: gcc
           Version: 4.6.3
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ozabluda@gmail.com
                CC: hans_boehm@hp.com
              Host: x86_64-linux-gnu
            Target: x86_64-linux-gnu
             Build: x86_64-linux-gnu


Created attachment 28335
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28335
Preprocessed source

In the standard publish idiom:

#include <atomic>

double d;
std::atomic<int> aflag(0);

void publish() {
   d = 1.0;
   aflag = 1;
}


store to d is reordered (sunk) after store to aflag:

_Z7publishv:
.LFB382:
    .cfi_startproc
    movabsq    $4607182418800017408, %rax
    movl    $1, aflag(%rip) #<== store to aflag
    movq    %rax, d(%rip)   #<== store to d
    mfence
    ret


The reason for it is that in include/c++/4.6/bits/atomic_2.h store() is missing
needed __sync_synchronize() before non-atomic write:

      void
      store(__int_type __i, memory_order __m = memory_order_seq_cst)
      {
        __glibcxx_assert(__m != memory_order_acquire);
        __glibcxx_assert(__m != memory_order_acq_rel);
        __glibcxx_assert(__m != memory_order_consume);

        if (__m == memory_order_relaxed)
          _M_i = __i;
        else
          {
            // write_mem_barrier(); //OZ: Missing __sync_synchronize();  
            _M_i = __i;
            if (__m == memory_order_seq_cst)
              __sync_synchronize();
          }
      }


Incidentally, load() has unnecessary barrier before non-atomic read:

      __int_type
      load(memory_order __m = memory_order_seq_cst) const
      {
        __glibcxx_assert(__m != memory_order_release);
        __glibcxx_assert(__m != memory_order_acq_rel);

        __sync_synchronize(); //OZ: unnecessary
        __int_type __ret = _M_i;
        __sync_synchronize();
        return __ret;
      }

And by unnecessary, I don't mean just for "publish", but ever.

============================================================================

$ g++-4.6 -v -save-temps --std=c++0x -O2 -c reorder.cpp 
Using built-in specs.
COLLECT_GCC=g++-4.6
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.6.3-1ubuntu5' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.6 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++0x' '-O2' '-c' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.6/cc1plus -E -quiet -v -imultilib .
-imultiarch x86_64-linux-gnu -D_GNU_SOURCE reorder.cpp -mtune=generic
-march=x86-64 -std=c++0x -O2 -fpch-preprocess -fstack-protector -o reorder.ii
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/4.6
 /usr/include/c++/4.6/x86_64-linux-gnu/.
 /usr/include/c++/4.6/backward
 /usr/lib/gcc/x86_64-linux-gnu/4.6/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.6/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++0x' '-O2' '-c' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.6/cc1plus -fpreprocessed reorder.ii -quiet
-dumpbase reorder.cpp -mtune=generic -march=x86-64 -auxbase reorder -O2
-std=c++0x -version -fstack-protector -o reorder.s
GNU C++ (Ubuntu/Linaro 4.6.3-1ubuntu5) version 4.6.3 (x86_64-linux-gnu)
    compiled by GNU C version 4.6.3, GMP version 5.0.2, MPFR version 3.1.0-p3,
MPC version 0.9
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C++ (Ubuntu/Linaro 4.6.3-1ubuntu5) version 4.6.3 (x86_64-linux-gnu)
    compiled by GNU C version 4.6.3, GMP version 5.0.2, MPFR version 3.1.0-p3,
MPC version 0.9
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 65b5171ac1bd7b3f07dbea6bdb24be3d
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++0x' '-O2' '-c' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
 as --64 -o reorder.o reorder.s
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++0x' '-O2' '-c' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'