You can see what optimizations are used by GCC with the following command:
$ gcc -Q --help=target
For instance, on my machine, GCC does not enable sse4.1 by default:
$ gcc -Q --help=target | grep msse4.1
-msse4.1 [disabled]
However, it is supported because it appears in /proc/cpuinfo
. And indeed, if I ask GCC to optimize the generated code for my machine, it enables sse4.1:
$ gcc -Q --help=target -march=native | grep msse4.1
-msse4.1 [enabled]