[llvm-dev] Scalable Vector Types in IR - Next Steps?

[Graham Hunter via llvm-dev]

Hi Eric and Chandler,

I appreciate your concerns; I don't think the impact will be that great, but then it's
rather easy for me to keep SVE in mind when working on other parts of the codebase
given how long I've spent working on it.

Are there any additional constraints on the scalable types you think would alleviate
your concerns a little? At the moment we will prevent scalable vectors from being
included in structs and arrays, but we could add more (at least to start with) to
avoid potential hidden problems.

I'm also trying to come up with an idea of how much impact we have in our downstream
implementation; most places where there is divergence are in the AArch64 backend (as you'd
expect), followed by the generic SelectionDAG code -- but lowering and legalization for
current instructions should (hopefully) be a one-off.

Do you have any specific parts of the codebase you're interested in a report into the
extent of changes?

-Graham


> On 18 Mar 2019, at 17:39, Eric Christopher via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> On Fri, Mar 15, 2019 at 1:55 PM Chandler Carruth via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> 
>> On Fri, Mar 15, 2019 at 11:22 AM Finkel, Hal J. via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>> 
>>> On 3/15/19 10:58 AM, David Greene wrote:
>>>> Renato Golin <rengolin at gmail.com> writes:
>>>> 
>>>>> On Fri, 15 Mar 2019 at 15:30, Finkel, Hal J. via llvm-dev
>>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>>> I've talked with a number of people about this as well, and I think that
>>>>>> I understand the objections. I'm happy that ARM followed through with
>>>>>> the alternate set of patches. Regardless, however, unless those who had
>>>>>> wished to object still wish to object, and then actually do so, we now
>>>>>> clearly have a good collection of contributors actively desiring to do
>>>>>> code review, and we should move forward (i.e., start committing patches
>>>>>> once they're judged ready).
>>>>> Let's start by closing the three flying revisions, so that people that
>>>>> weren't involved in the discussion don't waste time looking at them.
>>>> See the reply I just posted to Hal.  I am not sure we've made a decision
>>>> to abandon the current patches.  We may in fact decide that, but I
>>>> haven't seen consensus for doing so yet.  In fact I've seen the opposite
>>>> -- that people want to move forward with the scalable types.
>>> 
>>> 
>>> I agree with David. We should move forward with native support for
>>> scalable types.
>> 
>> 
>> Sorry I haven't been as available as usual for the past few weeks, but FWIW, I still am unconvinced that scalable vector types belong in the IR.
>> 
>> I think this adds complexity to LLVM's IR to serve a niche use case without proven benefit to a broad spectrum of hardware or software. I think the complexity is significant and will be a net drag on all parts of the IR and IR-level transformations. But I don't really think it is useful to re-hash all these debates. Nothing relevant has changed in the years this has been discussed.
>> 
>> That said, if I'm the only one who feels this way (and is willing to actually state this publicly), I'm not going to stop progress.
>> 
> 
> You're not, and I'm in the same position here. I don't think there's a
> really good answer for how this is going to affect a lot of the IR and
> IR-level transformations from a maintainability perspective. It mostly
> seems like this is a "we need this for the new ISA support" and while
> I don't see a lot of compelling use case here and a lot of downside
> that there...
> 
> -eric
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