I'm not sure what parts are clang and what parts are LLVM, but I recently did some tests comparing g++ to clang++ for auto-vectorization and they were very on par. I'd even say clang was a little better than g++ with full optimizations turned on on average.

This was on an AVX2 machine, testing the (auto)-vectorization performance of expression templates. Anything with a compile-time unknown stride or a random gather failed horribly with both. Using (semi-)explicit vectorization turned out to be much faster still.