https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61333
--- Comment #13 from Jack Howarth <howarth.at.gcc at gmail dot com> --- (In reply to Dominique d'Humieres from comment #12) > > Is gcc really optimizing that low by default? ... > > AFAIK the default optimization in gcc is -O0. Now before drawing conclusions > you should answer my question in comment 8. Without optimization flags on a 16-core MacPro under darwin13, the timings for one, two and four OMP processes are… clang-3.4.1 (clang-omp/openmp) 82.913658 sec: 41.704652 sec: 22.256563 sec gcc 4.8.3 96.409755 sec: 50.521193 sec: 28.822280 sec gcc 4.9.0 96.341129 sec: 50.563898 sec: 28.850048 sec at -O1 clang-3.4.1 (clang-omp/openmp) 19.371284 sec: 9.743520 sec: 5.938325 sec gcc 4.8.3 38.253825 sec: 21.149855 sec: 13.426259 sec gcc 4.9.0 38.170274 sec: 21.022076 sec: 13.402209 sec at -O2 clang-3.4.1 (clang-omp/openmp) 15.621070 sec: 7.890557 sec: 5.384909 sec gcc 4.8.3 18.473835 sec: 11.278842 sec: 8.954324 sec gcc 4.9.0 18.468089 sec: 11.208950 sec: 8.949956 sec at -O3 clang-3.4.1 (clang-omp/openmp) 15.627173 sec: 8.073639 sec: 5.870642 sec gcc 4.8.3 18.535541 sec: 11.258917 sec: 8.951850 sec gcc 4.9.0 17.088016 sec: 10.685973 sec: 8.884664 sec at -Os clang-3.4.1 (clang-omp/openmp) 19.365366 sec: 9.732779 sec: 5.360228 sec gcc 4.8.3 19.523171 sec: 11.657896 sec: 8.993581 sec gcc 4.9.0 18.472308 sec: 11.224615 sec: 8.959600 sec at -Ofast clang-3.4.1 (clang-omp/openmp) 12.533942 sec: 6.371977 sec: 5.358282 sec gcc 4.8.3 15.593206 sec: 9.804462 sec: 8.581757 sec gcc 4.9.0 14.145020 sec: 9.089317 sec: 8.449206 sec
