Question for maintainers: ARCv3 port feasibility

[Andrew Stubbs]

On 16/05/2025 01:23, Paul Koning wrote:
> 
> 
>> On May 15, 2025, at 8:06 PM, Oleg Endo via Gcc <gcc@gcc.gnu.org> wrote:
>>
>> Hi,
>>
>> On Thu, 2025-05-15 at 17:41 +0100, Andrew Stubbs wrote:
>>> Dear GCC Maintainers and Steering Committee,
>>>
>>> I'm currently doing a feasibility study and effort estimate for
>>> upstreaming the existing ARCv3 out-of-tree port [1].
>>>
>>> Question: Is there likely to be any objection to adding a new "arc64"
>>> port in addition to the existing "arc" port?
>>>
>>> At this point, I would like to check that the general approach is likely
>>> to be accepted at the end of the project. Or, at least not rejected for
>>> this most fundamental of reasons.
>>>
>>> The ARCv3 port has been written as a new backend because it is not just
>>> a simple evolution of the ARC architecture and starting afresh made more
>>> sense to the developers at the time.  I'm aware that there are some
>>> precedents for this (sh64, ia64, aarch64), so I think it's probably
>>> fine, right?
>>>
>>
>> SH5/SH64 was actually part of the original SH port, not a separate
>> standalone port.  It made the port more complex and convoluted, but probably
>> made sense since the it was backwards compatible with the original SH ISA
>> (it was like ARM Thumb -- jump to select between classic SH ISA and new SH5
>> ISA, from what I remember).
>>
>> Best regards,
>> Oleg Endo
> 
> Also: ia64 is a separate ISA.  If you were thinking about x86-64, that's part of i386 ("ia-32").  Similarly, "mips" is both flavors (and all four ABIs). So there are several examples in both directions.

Apparently I hallucinated the sh64 being a separate port, which is weird 
because I worked on that stuff (20 years ago). Probably it was separate 
in some other tool(?)

I'm aware that IA64 Itanium was "slightly" more different to IA32 than 
current x86_64 is, but still it kind of sits in this space.

Anyway, I don't think ARC/ARC64 is as different as IA32/IA64 but 
probably about the same amount of difference as Arm/AArch64. Unlike x86, 
you can't run v2 binaries on v3 hardware. I don't know the ISAs well, so 
I can't (yet) speak with authority on this point, but I'm very sure that 
(at this point) it'll be less work to keep arc64 independent, at least 
for GCC.

Andrew