[llvm-dev] Compile for ARM SVE with the latest LLVM

[Renato Golin via llvm-dev]

Itaru-san,

I can't comment on what Arm announces, but upstream LLVM will never
announce "complete support" for any feature. As all other upstream
open source projects, the usage is "as is" and no promises are ever
made.

However, we will *always* be clear of what works, what doesn't and
what we're working on for the future, a process which you are more
than welcome to participate and help us define the roadmap and
implement the features.

There will never be promises of support (there will be bugs, which we
will try our best to fix), nor of delivery (what we work on and what
gets released are two different things).

Furthermore, there is no intention of upstream LLVM to "match" any
downstream toolchain support. Not only there are too many downstream
LLVM-based toolchains (Arm has at least 2) to be practical, but it
would pervert the notion of open source fairness to become a
second-class "product".

We will always strive to have the very best quality of implementation
of the largest set of features we can, while keeping maintenance cost
at the lowest (amortisation of cost in a multi-dimensional space), but
that's a problem that will never be "solved". That's why we keep
trying. :)

Regarding upstream SVE support, what I said before is as good guess as
any of what happens next. It will depend on the people upstreaming the
changes, the interaction of those changes with existing code (both
upstream and downstream) as well as feedback from other parts of the
community (ex. RISCV).

My hope is that by the next freeze we'll have enough IR support to
generate some code, but not enough to compile inline asm, ACLE or
automatic vectorisation. Ie. it will be a proof of concept, but hardly
usable.

I think we should prioritise each one of the remaining steps above in
order, so that we can make best use of our time. This is a slow
process, and we should not speed it up artificially, at the expense of
quality and maintainability.

Almost three years ago, when the first email reached the list, I said
this would be at least two years from the first accepted commit and
people called me a pessimist. We've past our first year and there will
be at least another.

For now, use GCC or, if you have access, the Arm compiler. Both
generate good SVE code and show real changes in real benchmarks.

cheers,
--renato


On Wed, 18 Sep 2019 at 07:26, Itaru Kitayama <itaru.kitayama at gmail.com> wrote:
>
> Will,
> When you guys announce the SVE codegen support is merged in the trunk, hopefully in the near future, that means all of the features/functions
> that can currently be found in the current release of Arm Compiler for Linux, correct? We were sitting down this afternoon and wondering about
> this a bit thus I am asking.
>
> On Thu, Sep 12, 2019 at 5:44 PM Will Lovett <Will.Lovett at arm.com> wrote:
>>
>> Hi Itaru,
>>
>>
>>
>> (and thanks for the replies on this Renato).
>>
>>
>>
>> We (Arm HPC) initially put up that github branch as an aid to the discussions on upstream SVE support.  The branch was very old (LLVM-5 based!), and various aspects of our implementation have changed significantly, following discussions with the community.  For this reason, we felt that branch was unhelpful at this point, so have removed it.
>>
>>
>>
>> The team are working hard on getting SVE codegen support into LLVM, but it’s not there yet.  For now, this leaves the following options for testing vectorization with SVE:
>>
>> Get an up to date GNU compiler, which has SVE vectorization support
>> Use Arm Compiler for Linux (the new name for Arm Compiler for HPC)
>>
>>
>>
>> If you’re interested in the latter option, feel free to give me a mail (will.lovett at arm dot com).  Depending on your use-case, there may be scope for arranging a license.
>>
>>
>>
>> Thanks,
>>
>>
>>
>> Will.
>>
>>
>>
>>
>>
>> Will Lovett
>>
>> Technical Product owner
>>
>> Compilers & Libraries
>> Arm Allinea Studio
>>
>>
>>
>> On 11/09/2019, 22:31, "llvm-dev on behalf of Itaru Kitayama via llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of llvm-dev at lists.llvm.org> wrote:
>>
>>
>>
>> Renato et al.
>>
>> In the meantime, is there an out of tree branch I mean, other than LLVM trunk so I can test how much I can SVE vectorize my code with it?
>>
>>
>>
>> Arm seemed to gave taken down the GitHub branch for sometime.
>>
>>
>>
>> On Wed, Sep 11, 2019 at 20:41 Renato Golin <rengolin at gmail.com> wrote:
>>
>> On Wed, 11 Sep 2019 at 06:13, Itaru Kitayama via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > Hi,
>> > I'm still seeing the same ie trying to produce SVE instructions but
>> > resulted in NEONs with Clang as of Sep. 10th.
>> > http://lists.llvm.org/pipermail/llvm-dev/2019-March/130788.html
>>
>> Hi Itaru-san,
>>
>> IR SVE implementation is ongoing, CC'ing Graham who is leading that effort.
>>
>> I'm hoping we can have something working before the next release is
>> branched (Dec/Jan), but I don't expect it to be fully featured (inline
>> asm, full ACLE, auto-vec in all NEON cases, etc).
>>
>> cheers,
>> --renato
>>
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