[llvm-dev] [RFC] AAP Backend

Tue Aug 30 09:59:21 PDT 2016

Given the large amount of feedback, I felt I should add some more
context/answer some of the question raised.

Internally, the work on AAP is led by me and assisted by Simon Cook. We
also have Andrew Burgess, Graham Markall and Jeremy Bennett who are also
working on the project.

There are no traditional end-users, but the target audience isn't the
same as for most backends. Despite this it is still an open platform and
the hardware design runs readily on low-cost FPGA boards used in education.

AAP is aimed at individuals or groups developing backends for deeply
embedded systems. It may not be possible to upstream these backends
because it would reveal too many architecture details, because it is
only for internal use, or because it would require too many generic
changes. For these architectures, there aren't any reference
architectures in the public LLVM code base on which to base their work.
Equally, there are no in-tree architectures which can serve as a vehicle
for any improvements they would otherwise share.

There are a lot of embedded architectures out there, and we think it is
desirable to lower the barrier of entry to make it easier for these to
go in tree.

As for the backend itself, the initial patches don't require any
significant changes to generic code, and should not cause a development
burden. As LLVM advances and we make changes to the backend, we expect
to maintain it appropriately to keep up.

Eventually we will make proposals which do touch on generic code, since
one of the key reason of AAP is to advance LLVM. These will of course go
through the normal review process, and would hopefully be accepted as
useful improvements to LLVM on the whole.

Thanks,
Edward Jones

On 26/08/16 19:54, Robinson, Paul via llvm-dev wrote:
> Re-reading the thread, it looks like there is a difference of opinion
> what "an active community behind the target" means: an active community
> of LLVM-target-maintainers, and/or an active community of end-users.
> 
> I'd think the immediate practical concern is that there is an active
> community of LLVM-target-maintainers, so that the maintenance burden
> does not fall unduly on the rest of the LLVM community.  Beyond that
> is the broader and vaguer question of whether there is anybody *else*
> who would benefit from the in-tree target, other than the maintainers.
> You could think of the former as a "cost" (to the LLVM community)
> question, and the latter as a "benefit" (beyond the community) question.
> Or even more succinctly:
> - does is hurt "us" to have this?
> - does it help "them" to have this?
> A secondary point to having an end-user community is that if the
> initial maintainers drop out for any reason, there is likely to be
> somebody to take their place, preserving the low-hurt characteristic.
> 
> As a straw man, if some of us ex-DEC folks wanted to resurrect the
> Alpha target, I'd think that would be pretty cool, but it's hard to
> point to anybody that could actually make use of an Alpha-target Clang.
> So while I suspect there are enough of us around that the "hurt" part
> could be low to nothing, there's really nobody on the "help" side.
> And after the Alpha fans die off, well, that's that.
> 
> I didn't pay enough attention to the Lanai debate to know whether
> Google has much of an end-user community there.  It seems likely the
> hurt side would not be a problem, even if the current maintainers
> rotate out for whatever reason then Google seems likely to rotate in
> somebody else to take their place.  That capacity would seem to make
> it less necessary to identify an end-user community of its own.
> 
> Regarding AAP, we've got a few target-maintainers, who say they do
> have target-users who could benefit.  Alex asked if the maintainers
> could better quantify their user base, which seems like a reasonable
> question.  Who is on the "help" side?  Would they be able to take
> over from the initial target-maintainers if necessary?
> 
> --paulr
> 
> 
>> -----Original Message-----
>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Mehdi
>> Amini via llvm-dev
>> Sent: Friday, August 26, 2016 10:05 AM
>> To: Renato Golin
>> Cc: LLVM Dev; Alex Rosenberg
>> Subject: Re: [llvm-dev] [RFC] AAP Backend
>>
>>
>>> On Aug 26, 2016, at 9:55 AM, Renato Golin <renato.golin at linaro.org>
>> wrote:
>>>
>>> On 26 August 2016 at 17:45, Mehdi Amini <mehdi.amini at apple.com> wrote:
>>>> “Major corporation” does not mean size to me, I read it as “having a
>> major involvement in the project”.
>>>
>>> Still, you're rejecting new developers because they haven't
>>> contributed much before.
>>
>> Can you quote exactly where *I*  *rejected* anything?
>>
>>
>>> But if their back-end is upstream, than they'll contribute code
>>> upstream for their changes on their back-end.
>>>
>>> However, if we don't allow their back-end to be upstream, they won't
>>> contribute to the project.
>>>
>>> Looks like a self-defeating argument, and one that still doesn't agree
>>> with the open source philosophy.
>>>
>>> Not to mention that it's an argument that is not required by the
>>> policy in any way.
>>
>> The policy you quote says *must be an active community behind the target”.
>> The question is *only* about this, so it seems right on point to me.
>>
>>
>>>> “the question is about who will use/develop/maintain this backend
>> upstream in LLVM?"
>>>
>>> They have answered that question. Ed and Simon will be the active
>>> maintainers. I imagine they have other developers around to help.
>>> I don't see *any* concern here, nor any violation of the policy. Like
>>> the Lanai back-end, the community is the maintainers. LGTM.
>>
>> See above quote from the policy.
>>
>>
>>>
>>>> "is there already an open-source community around this backend
>> somewhere?"
>>>
>>> This is not a requirement of the policy, nor was a requirement to any
>>> other back-end, so not a valid argument.
>>
>> Asking a question is not an argument.
>>
>>
>>>
>>> If we were to reject changes for not having an open source community
>>> elsewhere, we'd be chopping a very large parts of LLVM off.
>>
>> I didn’t write that, and *I* didn’t ask any rejection. In short my only
>> question is “what is the community around this”? (And I didn’t even start
>> the question but found it interesting).
>>
>> —
>> Mehdi
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