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: unassig...@gcc.gnu.org
ReportedBy: ozabl...@gmail.com
CC: hans_bo...@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'
