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

[Eric Christopher via llvm-dev]

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