[llvm-dev] Contributing Bazel BUILD files similar to gn

[Johannes Doerfert via llvm-dev]

On 10/29/20 9:48 PM, Mehdi AMINI wrote:
> On Thu, Oct 29, 2020 at 7:30 PM Johannes Doerfert <
> johannesdoerfert at gmail.com> wrote:
>
>> I replied only selectively.
>>
>>
>> On 10/29/20 5:47 PM, Mehdi AMINI via llvm-dev wrote:
>>> On Thu, Oct 29, 2020 at 2:35 PM Chris Tetreault <ctetreau at quicinc.com>
>>> wrote:
>>>
>>>> Honestly, I’m hearing that some people would like the Bazel build system
>>>> to be in community master, and the argument basically boils down to
>> “It’ll
>>>> be fine. It’ll just sit there and mind its own business and you don’t
>> have
>>>> to care about it.”
>>>>
>>> Not really: this argument is only the answer to why it does not bear any
>>> weight on non-Bazel users, just like `gn` does already today.
>>>
>>> I think I explained the motivation to do it, but I can restate it: many
>>> LLVM contributors need to collaborate on this piece of infrastructure
>> that
>>> is very specific to LLVM and enabling some users of LLVM: the natural
>> place
>>> of collaboration is the monorepo.
>>>
>>>
>>>>> So why are we doing it? I mentioned this in another answer: this is
>>>> mainly to provide a collaboration space for the support of OSS projects
>>>> using Bazel interested to use LLVM (and some subprojects). …
>>>>
>>>>
>>>>
>>>> Which could be handled by having it in an external public repo.
>>>>
>>> Sure, just like almost every new code could be handled in an external
>> repo.
>>> However when many LLVM contributors are interested to collaborate on
>>> something highly coupled to LLVM it seems like the natural place to do
>> it.
>>> Also I don't know for Qualcomm, but most companies will want you to sign
>> a
>>> CLA if they provide this "external repo" where we can collaborate, and
>>> other parties won't be able to collaborate. The LLVM project is in
>> general
>>> seen as quite "neutral" for collaborating.
>>>
>>>
>>>>> Having them in-tree means that we can publish every day (or more) a git
>>>> hash that we validate with Bazel on private bots (like `gn`) and every
>>>> project can use to clone the LLVM monorepo and integrate in their build
>>>> flow easily.
>>>>
>>>>
>>>>
>>>> You could still publish this info: “Today, the head of llvm-bazel is
>>>> confirmed to work with LLVM monorepo sha [foo]”. I don’t think two git
>>>> clones is significantly harder than one.
>>>>
>>> For a developer at their desk, you could say it is just an inconvenience
>>> that can be worked around (scripting, etc.).
>>> For the project on the other hand, Bazel has native support to clone a
>> repo
>>> and build it itself as dependency.  For example TensorFlow has many
>>> dependencies, and it just points to a commit in the source repo:
>>>
>> https://github.com/tensorflow/tensorflow/blob/master/tensorflow/workspace.bzl#L689-L697
>>> You can see how it is convenient to update the SHA1 there and have it
>> just
>>> work for any Bazel user.
>>>
>>>
>>>
>>>> I submit that in a way this is simpler because you can always advertise
>>>> the head of the bazel repo. If the Bazel build system were in the
>> community
>>>> repo, then you might have to tell users to use an older version of the
>>>> bazel build if a fix went into the monorepo in the afternoon, but the
>> next
>>>> morning’s nightly finds that the most recent sha that passes the tests
>> is
>>>> prior to that fix.
>>>>
>>> This is not different from "a commit broke the ARM bootstrap and a user
>> who
>>> checked out the repo at the time will be broken". From this point of view
>>> this configuration is no different than any other, except that we don't
>>> revert or notify the author of a breaking change, a set of volunteers
>>> monitor a silent bot and fix-forward as needed, like `gn`.
>>> It is just much easier to have a bot publishing the "known good" revision
>>> of the monorepo.
>>>
>>>
>>>> I guess my concern is that I’m not really hearing a compelling (to my
>> ear)
>>>> argument for this inclusion.
>>>>
>>> Sure, but if other contributors have a strong interest, and you don't
>>> really have a strong objection here that we need to address, we should be
>>> able to get past that?
>> Wouldn't your argument hold for anything that "just lives" in the mono
>> repo but doesn't impact people? I mean, where is the line for stuff that
>> some contributors have "strong interest" in and others can't really
>> "hear a compelling argument for inclusion"? People raise concerns here
>> and from where I am sitting they are brushed over easily and more
>> aggressively as the thread progresses (up to the email I respond to).
>>
> Sorry, I invite you to reread the thread again and revisit your impression:
> Tom and Renato expressed clear concerns, and I believe I really tried to
> listen and address these with concrete proposals to mitigate:
> http://lists.llvm.org/pipermail/llvm-dev/2020-October/146182.html
> However there is not much I can do to address folks who object because
> "they don't see the interest" in it, this isn't a productive way of moving
> forward with such proposal IMO.
>
>
>
>>>> I guess it would make the lives of google employees easier?
>>>>
>>> I explained before that Google internal integration flow is likely better
>>> without this at the moment, TensorFlow itself is also in a reasonably
>> good
>>> spot at the moment. But Google is also not a monolithic place, some
>> people
>>> are working on small independent projects that they are open-sourcing,
>> and
>>> would like to be able to use LLVM.
>>>
>>>>    Then what’s to stop every large org from committing their internal
>> stuff
>>> to master?
>>>
>>>
>>>
>>> If their "internal stuff" is highly-coupled to LLVM, has zero-cost
>>> maintenance on the community, and is something that multiple other
>> parties
>>> can benefit and established members of the community want to maintain and
>>> collaborate on, why not?
>> Let's be honest, nothing has "zero-cost".
>
> I hope you're not implying I'd be dishonest here right?

Long story short, I did not try to imply you were dishonest.

I'm saying that the sentence "has zero-cost maintenance on the community"
cannot be true in a general sense but only in a narrow one. I believe that
everything has cost. I added, "let's be honest", because the cost is not
obvious and one can easily overlook it. However, I assumed we all know
there has to be one as it would otherwise conflict with some universal
law or something. The way I see it you acknowledge the existence in a few
other places.



>
>> It seems unhelpful to pretend it does. (FWIW, I explained a simple
>> scenario that would make the bazel
>> inclusion "costly" in my previous mail.)
>>
> "zero-cost" is well defined: it is "as a community member: feel free to
> ignore, no one will bother you about it", and a subset of the community
> signed up for the maintenance.
> I think it is also helpful to be concrete here: we have existing data and
> history with `gn`, it isn't hypothetical.
>
> To be sure I address your previous email, that was about user expectations
> right? i.e. was it this part:
>
>> people will assume we (=the LLVM community) maintain(s) a bazel build,
> which can certainly be a benefit but also a cost", e.g., when the build is
> not properly maintained, support is scarce, etc. and emails come in
> complaining about it (not thinking of prior examples here.)
>
> Isn't this similar to the concerns from Renato here:
> http://lists.llvm.org/pipermail/llvm-dev/2020-October/146179.html ?
> I acknowledge this as very valid concerns and offered some possibility to
> mitigate: http://lists.llvm.org/pipermail/llvm-dev/2020-October/146188.html
>
>
>
>>
>>> I mentioned it before, but Bazel is not something internal or specific to
>>> Google: it isn't (actually there are many incompatibilities between Bazel
>>> and the internal system), 400 people attended the Bazel conference last
>>> year. I attended this conference 3 years ago when I was at Tesla trying
>> to
>>> deploy Bazel internally. Many other companies are using Bazel,
>> open-source
>>> projects as well. Feel free to watch the talks online about SpaceX
>>> <https://www.youtube.com/watch?v=t_3bckhV_YI> or Two Sigma and Uber
>>> <https://www.youtube.com/watch?v=_bPyEbAyC0s> for example
>> Let's not conflate "using bazel" and "benefit for LLVM", the former
>> is not up for debate here. (I mean, a lot of people use autoconf but
>> we got rid of it anyway).
>>
> I doubt we wouldn't have got rid of Autoconf if a chunk of the community
> offered to maintain it at "no cost" (again see definition).

It broke, ppl complained, and nobody wanted to fix it. That is the
kind of technical debt (aka. cost) you can accumulate.


>
>> That said, I think the original question is highly relevant. As I also
>> mentioned somewhere above, where do we draw the line is the key to this
>> RFC at the end of the day. A lot of the arguments I hear pro integration
>> apply to various other things that currently live out-of-tree, some of
>> which were proposed and not integrated.
>
> Can you provide more concrete reference to these things that could have
> been integrated in similar "zero cost" fashion?
> I'm all for consistency, and the only point of comparison here is `gn`.

Let's say RV, in a subfolder not build by default. Or any other
project that was proposed for inclusion without being build by
default. (I remember also the discussion if we can/should add
isl to llvm, pre-mono repo.)


>
>
>
>> I think we should not dismiss
>> this easily, no matter on which side of the argument you are this time.
>>
>> ~ Johannes
>>
>>
>>
>>>
>>> I'm not trying to convince anyone to use Bazel, it has drawbacks, but the
>>> point here is to recognize that this is about OpenSource communities that
>>> Bazel is serving: these are users, some of us in the LLVM community are
>>> trying to provide these users with a reasonably good integration story,
>> and
>>> we're ready to pay the cost for everyone.
>>>
>>>
>>>
>>>> *From:* Mehdi AMINI <joker.eph at gmail.com>
>>>> *Sent:* Thursday, October 29, 2020 2:00 PM
>>>> *To:* Chris Tetreault <ctetreau at quicinc.com>
>>>> *Cc:* Sterling Augustine <saugustine at google.com>; Mehdi Amini <
>>>> aminim at google.com>; LLVM Dev <llvm-dev at lists.llvm.org>; Stella
>> Laurenzo <
>>>> laurenzo at google.com>; Tres Popp <tpopp at google.com>; Geoffrey
>> Martin-Noble
>>>> <gcmn at google.com>; Thomas Joerg <tjoerg at google.com>
>>>> *Subject:* [EXT] Re: [llvm-dev] Contributing Bazel BUILD files similar
>> to
>>>> gn
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Thu, Oct 29, 2020 at 1:24 PM Chris Tetreault via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>> The problem is that once it’s in community LLVM, it becomes the
>>>> community’s problem.  The expectation is that individual contributors do
>>>> not break anything in upstream.
>>>>
>>>>
>>>>
>>>> I would expect that the community by now has concrete experience with
>> `gn`
>>>> gained over a few years demonstrating that this hasn't been a problem to
>>>> have this in-tree, without a burden of support on the community.
>>>>
>>>> In particular, I think that a salient point is the guarantee that no
>>>> public bot would be testing it (I mean here by "no public bot" that no
>> bot
>>>> would email you when you break it).
>>>>
>>>>
>>>>
>>>> Why else would you contribute it to the LLVM monorepo? If the goal is
>> just
>>>> to enable external-to-google orgs to collaborate on it, why not
>> contribute
>>>> it as a new repo separate from LLVM? You wouldn’t need to ask anybody’s
>>>> permission to do this.
>>>>
>>>>
>>>>
>>>> Yes, we could do this, and you are correct that in many cases a
>> motivation
>>>> to upstream a component is to make sure it is maintained by the
>> community
>>>> and works out of the box.
>>>>
>>>> In this case it is slightly different: we are OK with people to break
>>>> this. We are already maintaining these files out-of-tree for our own
>>>> purposes, and this has been the case for years as Sterling mentions. I
>>>> would even suspect that for Google internal build integration, it is
>>>> actually easier to have these files internal only rather than
>> unsupported
>>>> upstream.
>>>>
>>>> So why are we doing it? I mentioned this in another answer: this is
>> mainly
>>>> to provide a collaboration space for the support of OSS projects using
>>>> Bazel interested to use LLVM (and some subprojects).
>>>>
>>>> Having them in-tree means that we can publish every day (or more) a git
>>>> hash that we validate with Bazel on private bots (like `gn`) and every
>>>> project can use to clone the LLVM monorepo and integrate in their build
>>>> flow easily. Another repo, submodules, etc. are not making this
>> possible /
>>>> practical.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> *From:* Sterling Augustine <saugustine at google.com>
>>>> *Sent:* Thursday, October 29, 2020 1:14 PM
>>>> *To:* Chris Tetreault <ctetreau at quicinc.com>
>>>> *Cc:* Renato Golin <rengolin at gmail.com>; tstellar at redhat.com; Mehdi
>> Amini
>>>> <aminim at google.com>; LLVM Dev <llvm-dev at lists.llvm.org>; Stella
>> Laurenzo <
>>>> laurenzo at google.com>; Tres Popp <tpopp at google.com>; Geoffrey
>> Martin-Noble
>>>> <gcmn at google.com>; Thomas Joerg <tjoerg at google.com>
>>>> *Subject:* [EXT] Re: [llvm-dev] Contributing Bazel BUILD files similar
>> to
>>>> gn
>>>>
>>>>
>>>>
>>>> On Thu, Oct 29, 2020 at 12:29 PM Chris Tetreault via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>> I think Renato has articulated quite well some concerns I have about
>> this
>>>> but was unable to express. I would very much prefer if we just focus on
>>>> using CMake effectively.
>>>>
>>>> ...
>>>>
>>>> For example, when trying to implement the same logic on both will not be
>>>> trivial. So, whenever we want to add some functionality or improve how
>> we
>>>> build LLVM with one system, we'll have to do so in multiple build
>> systems
>>>> that do not easily match each other.
>>>>
>>>>
>>>>
>>>> Google already does all of this work, and has for years. I think it is
>>>> fair to say that it hasn't been a burden on the community.
>>>>
>>>>
>>>>
>>>> If we don't try to match functionality, we'll segregate the community,
>>>> because people will be able to do X on build system A but not B, and the
>>>> similar features cluster together and then we have essentially two
>> projects
>>>> built from the same source code.
>>>>
>>>>
>>>>
>>>> As long as we keep CMake as the canonical system everything will be
>> fine.
>>>> It works perfectly well today, except that not everyone gets to see or
>> use
>>>> the bazel files. They exist right now; they work right now; and it
>> hasn't
>>>> been a burden on anyone but the people who care about bazel.
>>>>
>>>>
>>>>
>>>> Testing this, or worse, trying to fix a buildbot that is built with
>> Bazel
>>>> (and having to install Java JDK and all its dependencies) on
>> potentially a
>>>> hardware that you do not have access to, will be a nightmare to debug.
>> The
>>>> nature of post-commit testing, revert and review of LLVM will not make
>> that
>>>> simpler. Unless we treat the Bazel build as "not our problem" (which
>>>> defeats the point of having it?).
>>>>
>>>>
>>>>
>>>> Google makes it work like this today, with the rest of the project
>>>> treating it as "not our problem" because they don't even see that they
>>>> exist. The build bot issues would be real, but I think surmountable,
>> given
>>>> that Google already cleans up the bazel files, it just doesn't push
>> them.
>>>> Perhaps an explicit policy that cmake folks don't have to update the
>> bazel
>>>> files would be helpful.
>>>>
>>>>
>>>>
>>>> To make matters worse, our CMake files are not simple, and do not do all
>>>> of the things we want them to do in the way we understand completely.
>> There
>>>> is a lot of kludge that we carry and with that comes in two categories:
>> the
>>>> things that we hate and would love to fix, and the things that are fixes
>>>> that we have no idea are there. The former are the reasons why people
>> want
>>>> to start a new build system, the latter is why they soon realise that
>> was a
>>>> mistake (insert XKCD joke here).
>>>>
>>>>
>>>>
>>>> It wouldn't be starting a new build system, it would be making a
>>>> pre-existing, already extremely well functioning one, available to more
>>>> people.
>>>>
>>>>
>>>>
>>>> I can definitely see folks who use cmake not wanting more hassle--that
>> may
>>>> be a valid reason not to do it. But "it won't work" or "it's hard to
>> keep
>>>> up" or "it's too complicated" seem well refuted by a multi-year
>> existence
>>>> proof.
>>>>
>>>>
>>>>
>>>> _______________________________________________
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>>>> llvm-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>>
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