On Reddit they have linked an article that shows auto vectorization in GCC 4.7:

http://locklessinc.com/articles/vectorize/

http://www.reddit.com/r/programming/comments/tz6ml/autovectorization_with_gcc_47/

GCC is good, it knows many tricks, it contains a lot of pattern matching code and other code to allow such vectorizations, and that C code is almost transparent & standard (restrict is standard, and I think __builtin_assume_aligned isn't too much hard to #define away when not available. And something like --fast-math is available on most compilers (despite Walter doesn't like it)). So it's good to optimize legacy C code too.

But this article also shows why such strategy is not usable for serious purposes. If small changes risk turning off such major optimizations, you can't rely much on them. More generally, writing low-level code and hoping the compiler recovers that high level semantics of the code is a bit ridiculous. It's way better to express that semantics in a more direct way, in a standard way that's understood by all compilers of a language (this also because the code shown in that article has very simple semantics).