http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54861
Bug #: 54861 Summary: std::atomic_signal_fence(std::memory_order_seq_cst) issues unnecessary mfence Classification: Unclassified Product: gcc Version: 4.7.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassig...@gcc.gnu.org ReportedBy: ozabl...@gmail.com Host: x86_64-linux-gnu Target: x86_64-linux-gnu Build: x86_64-linux-gnu Created attachment 28394 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28394 Preprocessed source #include <atomic> void f() { std::atomic_signal_fence(std::memory_order_seq_cst); } f(): .LFB321: .cfi_startproc mfence # <== unnecessary mfence ret .cfi_endproc The reason is that std::atomic_signal_fence() (in bits/atomic_base.h) simply calls std::atomic_thread_fence(), where mfence is indeed necessary. C++11 29.8.8 says: extern "C" void atomic_signal_fence(memory_order order) noexcept; Effects: equivalent to atomic_thread_fence(order), except that the resulting ordering constraints are established only between a thread and a signal handler executed in the same thread. 8 Note: compiler optimizations and reorderings of loads and stores are inhibited in the same way as with atomic_thread_fence, but the hardware fence instructions that atomic_thread_fence would have inserted are not emitted. C11 7.17.4.2 has an identical language. =============================== $ g++-4.7 -v -save-temps --std=c++0x -O2 -c atomic_fences.cpp Using built-in specs. COLLECT_GCC=g++-4.7 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.7.0-7ubuntu3' --with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs --enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.7 --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++/4.7 --libdir=/usr/lib --enable-nls --disable-bootstrap --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 4.7.0 (Ubuntu/Linaro 4.7.0-7ubuntu3) COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++11' '-O2' '-c' '-shared-libgcc' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/4.7/cc1plus -E -quiet -v -imultiarch x86_64-linux-gnu -D_GNU_SOURCE atomic_fences.cpp -mtune=generic -march=x86-64 -std=c++11 -O2 -fpch-preprocess -fstack-protector -o atomic_fences.ii ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/include/c++/4.7 /usr/include/c++/4.7/x86_64-linux-gnu /usr/include/c++/4.7/backward /usr/lib/gcc/x86_64-linux-gnu/4.7/include /usr/local/include /usr/lib/gcc/x86_64-linux-gnu/4.7/include-fixed /usr/include/x86_64-linux-gnu /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++11' '-O2' '-c' '-shared-libgcc' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/4.7/cc1plus -fpreprocessed atomic_fences.ii -quiet -dumpbase atomic_fences.cpp -mtune=generic -march=x86-64 -auxbase atomic_fences -O2 -std=c++11 -version -fstack-protector -o atomic_fences.s GNU C++ (Ubuntu/Linaro 4.7.0-7ubuntu3) version 4.7.0 (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 4.7.0-7ubuntu3) version 4.7.0 (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: 33e4f0c75e4f7f1ac5eae22180dee261 COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++11' '-O2' '-c' '-shared-libgcc' '-mtune=generic' '-march=x86-64' as --64 -o atomic_fences.o atomic_fences.s COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.7/:/usr/lib/gcc/x86_64-linux-gnu/4.7/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.7/:/usr/lib/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.7/:/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.7/../../../:/lib/:/usr/lib/ COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++11' '-O2' '-c' '-shared-libgcc' '-mtune=generic' '-march=x86-64'