ARMv8: few details.

It looks like few details about ARMv8 are starting to appear on the network. The root of this is presentation and videos about ARMv8 made by Richard Grisenthwaite and which are now linked from the ARM ISAs page. Please just scroll down and select ARMv8 Resources tab.
Anyway, I’d like to list a few details also here especially focused on details which affect user-land application writer. Small table should do the job I hope. Please note that with ARMv8, ARM started to name various ISAs as A32, which is classical ARM, T32, which is Thumb2 and A64 which is new ISA for ARM 64bit computing. So far ARMv7’s and ARMv8’s A32 and T32 ISAs looks similar.

32 bit ISAs A32, T32 A32, T32
64 bit ISAs A64
Number of GPs 13* 13* (A32, T32), 31** (A64)
ISNs encoding length (bits) 16-32 (T32), 32 (A32) 16-32 (T32), 32 (A32), 32 (A64)
NEON 64 bit regs 32 32
NEON 128 bit regs 16 32
Crypto ISNs (using NEON regs) AES, SHA-1, SHA-256

*: I count only R0-R12
**: PC and SP are no longer considered GPs

So as you can see, we get nearly twice the general purpose registers, twice the number of 128 bit registers in NEON and we also get some additional instructions to support some common cryptography operations. Besides this A64 also provides new load-acquire/store-release instructions to better support ARM weak-memory model in higher level programming languages.

Well, so from the point of view of GHC this might indeed be fun. The only pity is that we still depend on LLVM to come with A64 support first and then we’ll be able to use it in GHC.


2 thoughts on “ARMv8: few details.

  1. > The only pity is that we still depend on LLVM to come with A64 support first and then we’ll be able to use it in GHC

    He, I would say that’s an advantage. It means we don’t have to do any work ourselves :). 30 minutes of work to get 64bit support once LLVM has it? thats just great.

    • 🙂 David you are optimist! The question is when is apple going to enhance LLVM to support A64? Well, I don’t hold my breath for that. My only hope is that ARM will do this themself, but this is still not sure as LLVM is not that established project in the industry — in comparison with for example binutils/gcc which is lingua-franca for such architectural projects like ARMv8/A64 is…. So will we bet? LLVM A64 in 2015 is my choice. 🙂 (gcc/binutils 2012…)

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