NEON is ARM's advanced SIMD instruction set extension for benefiting multimedia and signal processing workloads. ARM NEON has been present going back to the 32-bit Cortex-A8/A9 days and has been supported by the 32-bit compiler back-ends for GCC and Clang. While LLVM/Clang has had initial AArch64 support for several months, the 64-bit ARM back-end hasn't been able to generate NEON instructions.
As of today with this Clang commit and this LLVM commit, the 64-bit ARM support includes this very performance beneficial advanced SIMD instruction set.
This 64-bit ARM NEON support is another important new feature for the LLVM 3.4 release that's likely to be out before year's end.