llvm dev - Feb 2020 - Status of the ARC backend

What do you see as the pros and cons of making it a stable target?  Does anyone
else have any concerns about doing so?

-Chris
> On Feb 14, 2020, at 7:59 AM, Nico Weber via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> +better dylanmckay address
> 
> On Fri, Feb 14, 2020 at 10:58 AM Nico Weber <thakis at chromium.org
<mailto:thakis at chromium.org>> wrote:
> Hi,
> 
> There was a thread a few days ago about the expectations for experimental
targets. At the moment, the only experimental target is AVR. It's been in
the tree for a long time now, and generally seems well-behaved.
> 
> Should we just make it a normal target?
> 
> Nico
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Pros:
- LLVM's release binaries contain AVR support :)
- It'd happen to remove the only backend that's currently marked
experimental, which imho makes the build config easier to understand

Cons:
- Everyone gets to pay the cost for maintaining AVR for cross-cutting
changes. From the last 3 months, this seems to happen once or twice a
month. We have a bit over 100 commits/day, so that seems fine.
- By default all backends get linked, so all binaries get larger by the
size of the AVR backend (but people who care probably already have an
explicit list of enabled targets)

Mixed:
- The AVR backend will likely grow more users, which might expose bugs :)
>From an outsider's perspective (mine), the AVR backend seems in better
shape than some non-experimental targets.

On Fri, Feb 14, 2020 at 12:13 PM Chris Lattner <clattner at nondot.org>
wrote:
> What do you see as the pros and cons of making it a stable target?  Does
> anyone else have any concerns about doing so?
>
> -Chris
>
> On Feb 14, 2020, at 7:59 AM, Nico Weber via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> +better dylanmckay address
>
> On Fri, Feb 14, 2020 at 10:58 AM Nico Weber <thakis at chromium.org>
wrote:
>
>> Hi,
>>
>> There was a thread a few days ago about the expectations for
experimental
>> targets. At the moment, the only experimental target is AVR. It's
been in
>> the tree for a long time now, and generally seems well-behaved.
>>
>> Should we just make it a normal target?
>>
>> Nico
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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On 02/14/2020 09:13 AM, Chris Lattner via llvm-dev
wrote:
> What do you see as the pros and cons of making it a stable target?  Does
anyone else have any concerns about doing so?
> 
My only concern with AVR is having active mantainers.  It doesn't seem
to have had much development in the last 6 months.

-Tom
> -Chris
> 
>> On Feb 14, 2020, at 7:59 AM, Nico Weber via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> +better dylanmckay address
>>
>> On Fri, Feb 14, 2020 at 10:58 AM Nico Weber <thakis at chromium.org
<mailto:thakis at chromium.org>> wrote:
>>
>>     Hi,
>>
>>     There was a thread a few days ago about the expectations for
experimental targets. At the moment, the only experimental target is AVR.
It's been in the tree for a long time now, and generally seems well-behaved.
>>
>>     Should we just make it a normal target?
>>
>>     Nico
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

On Fri, Feb 14, 2020 at 7:05 PM Tom Stellard <tstellar at redhat.com>
wrote:
> On 02/14/2020 09:13 AM, Chris Lattner via llvm-dev wrote:
> > What do you see as the pros and cons of making it a stable target? 
Does
> anyone else have any concerns about doing so?
> >
>
> My only concern with AVR is having active mantainers.  It doesn't seem
> to have had much development in the last 6 months.
>
https://reviews.llvm.org/p/dylanmckay/ looks reasonably active to me. And
it looks a lot more active than e.g. XCore :)

>
> -Tom
>
> > -Chris
> >
> >> On Feb 14, 2020, at 7:59 AM, Nico Weber via llvm-dev <
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
> >>
> >> +better dylanmckay address
> >>
> >> On Fri, Feb 14, 2020 at 10:58 AM Nico Weber <thakis at
chromium.org
> <mailto:thakis at chromium.org>> wrote:
> >>
> >>     Hi,
> >>
> >>     There was a thread a few days ago about the expectations for
> experimental targets. At the moment, the only experimental target is AVR.
> It's been in the tree for a long time now, and generally seems
well-behaved.
> >>
> >>     Should we just make it a normal target?
> >>
> >>     Nico
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
>
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On 2020-02-14, Nico Weber via llvm-dev wrote:
>Pros:
>- LLVM's release binaries contain AVR support :)
>- It'd happen to remove the only backend that's currently marked
>experimental, which imho makes the build config easier to understand
>
>Cons:
>- Everyone gets to pay the cost for maintaining AVR for cross-cutting
>changes. From the last 3 months, this seems to happen once or twice a
>month. We have a bit over 100 commits/day, so that seems fine.
>- By default all backends get linked, so all binaries get larger by the
>size of the AVR backend (but people who care probably already have an
>explicit list of enabled targets)
>
>Mixed:
>- The AVR backend will likely grow more users, which might expose bugs :)
>
>From an outsider's perspective (mine), the AVR backend seems in better
>shape than some non-experimental targets.
>
>On Fri, Feb 14, 2020 at 12:13 PM Chris Lattner <clattner at
nondot.org> wrote:
>
>> What do you see as the pros and cons of making it a stable target? 
Does
>> anyone else have any concerns about doing so?
>>
>> -Chris
>>
>> On Feb 14, 2020, at 7:59 AM, Nico Weber via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>> +better dylanmckay address
>>
>> On Fri, Feb 14, 2020 at 10:58 AM Nico Weber <thakis at
chromium.org> wrote:
>>
>>> Hi,
>>>
>>> There was a thread a few days ago about the expectations for
experimental
>>> targets. At the moment, the only experimental target is AVR.
It's been in
>>> the tree for a long time now, and generally seems well-behaved.
>>>
>>> Should we just make it a normal target?
>>>
>>> Nico
ARC is another experimental target Nico forgot to mention in
https://lists.llvm.org/pipermail/llvm-dev/2020-February/139158.html .

What is its status?

Parachuting here, collating many responses into one, forgive me.

On Fri, 14 Feb 2020 at 18:58, Nico Weber via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Pros:
> - LLVM's release binaries contain AVR support :)
> - It'd happen to remove the only backend that's currently marked
experimental, which imho makes the build config easier to understand
I think well maintained and well behaved backends with real users (no
matter how many) should be built by default once they fulfilled their
experimental cooling period successfully.
> Cons:
> - Everyone gets to pay the cost for maintaining AVR for cross-cutting
changes. From the last 3 months, this seems to happen once or twice a month. We
have a bit over 100 commits/day, so that seems fine.
> - By default all backends get linked, so all binaries get larger by the
size of the AVR backend (but people who care probably already have an explicit
list of enabled targets)
Those are the costs of every backend, and as they go, AVR seems pretty
cheap in comparison, so I don't see a problem here.
> (Tom) My only concern with AVR is having active mantainers.  It doesn't
seem to have had much development in the last 6 months.
I agree this may be a sign of potential bit-rot, but as long as it has
real users and the maintainer is up for fixing the eventual bugs quick
enough, it's ok to have a slow paced back-end, especially without
heavy corporate backing.

For example:
> (Dylan) I re-ran the test suite with
'-DLLVM_TARGET_ARCH=avr-unknown-unknown' and now the only errors I get
are JIT errors related to no JIT compiler enabled for my host machine, x86. It
seems the few failing generic CodeGen tests have since been fixed, along with
the ~80 or so DebugInfo tests no doubt fixed by one of Ayke's recent
changes.
This looks like an active sub-community to me. The JIT "errors" are
not really errors. When we turned the ARM backend built by default it
had no JIT support at all either. Just mark them as ignored in LIT.

The buildbot (http://lab.llvm.org:8014/builders/llvm-avr-linux) is
green for quite a while, which is also positive.

--renato