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ARM64 Vs ARM32 -- What's Different For Linux Programmers? (edn.com)
New submitter DebugN writes: When ARM introduced 64-bit support to its architecture, it aimed for Linux application compatibility with prior 32-bit software on its architecture. But for Linux programmers, there remain some significant differences that can affect code behavior. If you are a Linux programmer working with — or will soon be working with — 64-bit, you might want to know what those differences are, and this useful EDN article says it all.
This will have absolutely no effect to the majority of programmers that use a higher level language such as Java or Objective-C.
As the article shows examples, only Assembly and C have changes from the 32 bit version, which are to be expected. Not a big surprise for anyone. I am sure my Python code will run the same as it did on ARM32.
The instruction set for ARM64 is a bit more like MIPS than ARM. I don't think it would matter to an end user, but this new 64-bit mode has made for a lot of extra work for compiler developers as many of the optimizations developed over the years are no longer effective for 64-bit code. And the bits of operating system and library code that was written for 32-bit or Thumb has to be rewritten as if ARM64 was a totally alien architecture.
It came as a big surprise really, because years ago when ARM Thumb was added as an extension it was somewhat compatible with the old instruction set, at least in the assembler syntax. That made it relatively easy to port between the two modes. Thumb mode instructions that are half as big, but still operate on 32-bit values, but those instructions are more limited in terms of what they can do and what arguments they will accept. On the Game Boy Advance, the thumb operations could run twice as fast as the non-thumb operations because the 16-bit bus for cartridges was relatively slow compared to the CPU performance.
ps - I prefer to call it aarch64 or ARMv8.
“Common sense is not so common.” — Voltaire