At the "EuroLLVM" meeting last week in Edinburgh, Scotland, developers met to figure out the approach of ARM 64-bit support within LLVM/Clang now that Apple has open-sourced their code-base while various other organizations and community developers have focused on their own 64-bit ARM back-end. Both sides have agreed that the code-bases are to be merged, but the main plan was figuring out the most reasonable approach for combining the two code-bases.
Last week's meeting reaffirmed that the ARM64 back-end is the merge target. The "ARM64" back-end is Apple's back-end while the "AArch64" back-end is the existing 64-bit ARM code. Developers will be working to merge the extra functionality of the existing back-end into the newly open-sourced back-end as soon as possible to avoid code fragmentation and problems. This merging of the two 64-bit ARM back-ends might not happen before the LLVM 3.5 release this summer, but when it does the ARM64 (Apple) back-end will be renamed to AArch64 for ARM's correct naming scheme.
When the two code-bases are complete, no significant regressions can be found from code correctness to features to stability and performance. For those wishing to learn more about the ongoing 64-bit ARM LLVM work can be discovered via this mailing list thread.