alignment - Does AArch64 support unaligned access?

Question

Does AArch64 support unaligned access natively? I am asking because currently ocamlopt assumes "no".

Answer 1

Providing the hardware bit for strict alignment checking is not turned on (which, as on x86, no general-purpose OS is realistically going to do), AArch64 does permit unaligned data accesses to Normal (not Device) memory with the regular load/store instructions.

However, there are several reasons why a compiler would still want to maintain aligned data:

• Atomicity of reads and writes: naturally-aligned loads and stores are guaranteed to be atomic, i.e. if one thread reads an aligned memory location simultaneously with another thread writing the same location, the read will only ever return the old value or the new value. That guarantee does not apply if the location is not aligned to the access size - in that case the read could return some unknown mixture of the two values. If the language has a concurrency model which relies on that not happening, it's probably not going to allow unaligned data.
• Atomic read-modify-write operations: If the language has a concurrency model in which some or all data types can be updated (not just read or written) atomically, then for those operations the code generation will involve using the load-exclusive/store-exclusive instructions to build up atomic read-modify-write sequences, rather than plain loads/stores. The exclusive instructions will always fault if the address is not aligned to the access size.
• Efficiency: On most cores, an unaligned access at best still takes at least 1 cycle longer than a properly-aligned one. In the worst case, a single unaligned access can cross a cache line boundary (which has additional overhead in itself), and generate two cache misses or even two consecutive page faults. Unless you're in an incredibly memory-constrained environment, or have no control over the data layout (e.g. pulling packets out of a network receive buffer), unaligned data is still best avoided.
• Necessity: If the language has a suitable data model, i.e. no pointers, and any data from external sources is already marshalled into appropriate datatypes at a lower level, then there's really no need for unaligned accesses anyway, and it makes the compiler's life that much easier to simply ignore the idea altogether.

I have no idea what concerns OCaml in particular, but I certainly wouldn't be surprised if it were "all of the above".