Adrian Sampson: The Apple ISA

Following instructions -- see the full post for some intriguing speculation

"But recently, Apple announced that Watch apps would be delivered as LLVM IR, not machine code: the ISA [instruction set architecture] is completely hidden from third-party developers. Apple pundits have inferred that the bitcode development presages ARM Macs or, even less plausibly, x86 phones. But it’s more likely Apple will move away from legacy ISAs.

Apple’s outsourcing of this crucial piece at the heart of its systems looks increasingly incongruous as the need for interoperability with the broader ARM ecosystem evaporates. In the Apple stack, ARM today acts as a serialization format for LLVM to convey information to the Apple microarchitecture. And it’s certainly not the best serialization format: a traditional von Neumann ISA like ARM incurs a semantic gap; the architecture wastes time and energy rediscovering facts that the compiler already knew."

Adrian Sampson: The Apple ISA