Thanks for that. I didn't realise that ARM arches retained such a high level of source compatibility despite continually breaking binary compatibility.
I've only dealt with ARM in a few areas like mutex/locking asm helpers, where the best sequence to use is different for different sub-architectures.
I can see that the break of source compatibility would make v8/ARM64 a bigger challenge if perfect source-level ASM compatibility has been the norm up until now. It still sounds more like an ia32-to-x86_64 move than an ia32-to-itanic move, though.