https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57796
--- Comment #10 from vincenzo Innocente <vincenzo.innocente at cern dot ch> --- added a self contained "benchmark" on my machine [innocent@vinavx3 ctest]$ c++ -Ofast -Wall SparseOnly.c -march=native ; time ./a.out 0.496u 0.000s 0:00.49 100.0% 0+0k 0+0io 0pf+0w [innocent@vinavx3 ctest]$ c++ -O2 -Wall SparseOnly.c -march=native ; time ./a.out 0.411u 0.000s 0:00.41 100.0% 0+0k 0+0io 0pf+0w [innocent@vinavx3 ctest]$ c++ -O3 -Wall SparseOnly.c -march=native ; time ./a.out 0.413u 0.000s 0:00.41 100.0% 0+0k 0+0io 0pf+0w