Is there a way to enable vectorization only for some part of the code, like a pragma directive? Basically having as if the -ftree-vectorize is enabled only while compiling some part of the code? Pragma simd for example is not available with gcc...
The reason is that from benchmarking we saw that with -O3 (which enables vectorization) the timings were worse than with -O2. But there are some part of the code for which we would like the compiler to try vectorizing loops.
One solution I could use would be to restrict the compiler directive to one file.
Yes, this is possible. You can either disable it for the whole module or individual functions. You can't however do this for particular loops.
For individual functions use
__attribute__((optimize("no-tree-vectorize"))).
For whole modules -O3 automatic enables -ftree-vectorize. I'm not sure how to disable it once it's enabled but you can use -O2 instead. If you want to use all of -O3 except -ftree-vectorize then do this
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
And then include all the options except for -ftree-vectorize.
Edit: I don't see -fno-tree-vectorize in the man pages but it works anyway so you can do -O3 -fno-tree-vectorize.
Edit: The OP actually wants to enable vectorization for particular functions or whole modules. In that case for individual functions __attribute__((optimize("tree-vectorize"))) can be used and for whole modules -O2 -ftree-vectorize.
Edit (from Antonio): In theory there is a pragma directive to enable tree-vectorizing all functions that follow
#pragma GCC optimize("tree-vectorize")
But it seems not to work with my g++ compiler, maybe because of the bug mentioned here: How to enable optimization in G++ with #pragma. On the other hand, the function attribute works.