https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57952
--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Why should there be any references to that? -march= is an ISA selection option, it says the compiler may use the instructions from the ISA. -ftree-vectorize is an optimization option (default at -O3 and -Ofast), which attempts to vectorize loops using the selected ISA if possible. The reason it isn't on at -O2 is that it doesn't always improve code, it can make code slower as well, and usually makes the code larger; it really depends on the loop etc.
