I'm confused by this claim. Apple has an ARM64 device shipped to millions of people since September 2013.
It appears to me that they kept their compiler improvements a secret for an additional six months after the public release of a product using them, and so the fact that it's only taken an additional couple months to complete the contribution doesn't really strike me as fast.
What am I missing here?
LLVM gives you several opportunities to make decisions on how lowering from LLVM IR to machine code will occur, it is not likely that both backends would have been able to handle the same sets of preconditions for each other.
A very obvious example to consider: relocations.
Relocations are tied very closely to the final object file format. I doubt Apple's ARM64 backend had any provisions to deal with the ELF relocations in the psABI and I similarly doubt that the AArch64 backend had any understanding on what kind of relocations were available to it if it were to target Mach-O.
This license is much like BSD License, which doesn't require to release modification.
For Example, Sony uses LLVM as part of the PS4 Toolchain[2], but they also doesn't release their code.
[1] http://llvm.org/releases/2.8/LICENSE.TXT [2] http://llvm.org/devmtg/2013-11/slides/Robinson-PS4Toolchain....