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

[Troy Johnson via llvm-dev]

> 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.

I view the situation differently, but I'm still relatively new to llvm-dev and may be too unfamiliar with the threshold for inclusion in trunk, so please help educate me.  I see people from more than one organization saying that they'd like to see this in trunk.  No one wants it to be a drag on all transforms because no one wants to have to rewrite a ton of code.  So it would seem that there are two possible outcomes: it gets merged into trunk and the interested parties try hard to not adversely impact all of LLVM because that's in their best interest, too, OR it isn't merged and then we have potentially the same multiple organizations maintaining support for this on the side, out of trunk.  This community tries to avoid the latter situation, right?  I've always thought of LLVM trunk as where code ends up that is useful to multiple orgs and then each org maintains their own local patches for stuff that no one else would want (or that they can't share for competitive reasons).

-Troy
________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Chandler Carruth via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Friday, March 15, 2019 3:55:26 PM
To: Finkel, Hal J.
Cc: Francesco Petrogalli via llvm-dev; Chandler Carruth; David Greene; Chris Lattner; nd; Maxim Kuvyrkov
Subject: Re: [llvm-dev] Scalable Vector Types in IR - Next Steps?

On Fri, Mar 15, 2019 at 11:22 AM Finkel, Hal J. via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
On 3/15/19 10:58 AM, David Greene wrote:
> Renato Golin <rengolin at gmail.com<mailto: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<mailto: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.

-Chandler
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