Does GCC have a pragma to enforce auto-vectorization? [duplicate]

Question

Here my very simple question. With ICC I know it is possible to use #pragma SIMD to force vectorization of loops that the compiler chooses not to vectorize. Is there something analogous in GCC? Or, is there any plan to add this feature in a future release?

Quite related, what about forcing vectorization with Graphite?

Answer 1

As long as gcc is allowed to use SSE/SSE2/etc instructions, the compiler will in general produce vector instructions when it realizes that it's "worthwhile". Like most things in compilers, this requires some luck/planning/care from the programmer to avoid the compiler thinking "maybe this isn't safe" or "this is too complicated, I can't figure out what's going on". But quite often, it's successful if you are using a reasonably modern version of gcc (4.x versions should all do this).

You can make the compiler use SSE or SSE2 instructions by adding -msse or -msse2 (etc. for later SSE extensions). -msse2 is default in x86-64.

I'm not aware of any way that you can FORCE this, however. The compiler will either do this because it's happy that it's a good solution, or it wont.

Sorry, can't answer about Graphite.