ARM64 vs ARM32 -- What's different for Linux programmers

Isa Smith, Undo Software
EDN (October 23, 2015)

When ARM introduced 64-bit support to its architecture, it aimed for compatibility with prior 32-bit software. But for Linux programmers, there remain some significant differences that can affect code behaviour. Here are some we found and the workarounds we developed for them.

I had originally planned to call this article “What’s NEW in ARMv8 for Linux Programmers?” However, I think “what’s different” is much more apt. And, just for the record, by “ARMv8-A” I mean AArch64, with the A64 instruction set, also known as arm64 or ARM64. I’ve used AArch64 registers in the examples, but many of the issues I’ve described also happen in the ARMv8-A 32-bit execution state.

To help frame the problems discussed here, let me start by giving a little background on the sort of codebase we have here at Undo. Our core technology is a record and replay engine, which works by recording all non-deterministic input to a program and uses just-in-time compilation (JIT) to keep track of the program state. Our technology started on x86 (32 and 64-bit) and had progressed to have fairly complete, maturing support on ARM 32-bit when we began adapting it to work on AArch64. I joined the company after almost all of the low hanging fruit had been grabbed (as well as many rather higher up the tree, to be fair) leaving us with some tricky problems to tackle when it came to moving to ARMv8.

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