[llvm-dev] Contributing Bazel BUILD files similar to gn
Johannes Doerfert via llvm-dev
llvm-dev at lists.llvm.org
Thu Oct 29 20:58:57 PDT 2020
On 10/29/20 10:38 PM, Mehdi AMINI wrote:
> On Thu, Oct 29, 2020 at 8:10 PM Johannes Doerfert <
> johannesdoerfert at gmail.com> wrote:
>
>>
>> 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.
>>
>
> Yes, I know you :)
> (actually I thought I included a wink smiley above, but apparently not,
> sorry about that)
>
>
>>
>> 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.
>
>
> I don't know what RV is?
Sorry, the region vectorizer [0,1]. Came to mind because it is the last
thing I wished we had upstream so I could use it without forking under a
cmake flag.
[0] https://github.com/cdl-saarland/rv
[1] http://llvm.org/devmtg/2016-11/Slides/Moll-RV.pdf
>
>
>> 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 am not sure I agree that we can compare new "projects" (or
something like
> ISL) with "utilities for LLVM users".
> I would expect a more comparable situation to me to be:
> - the gdb scripts in llvm/utils/gdb-scripts/prettyprinters.py
> - IDE visualizer in llvm/utils/LLVMVisualizers
> - The Visual Studio Code syntax highlighting for LLVM IR and TableGen in
> llvm/utils/vscode ; and similar for kate, jedit, vim, textmate, ...
> - the gn files in llvm/utils/gn
>
> The general theme here is that these are not "new projects" in
themselves:
> they are highly coupled to LLVM itself and only allow a specific
subset of
> users to plug their tool/workflow into LLVM at a given revision.
> Also all of these are "zero cost" in that they may be "broken" and
> maintained with best effort (I don't think we revert someone breaking any
> of the visualizer or syntax highlighter?). And none of these are really
> core to LLVM, and each could be in a separate repo where the interested
> parties could maintain it.
If I want to use isl, RV, project XYZ from an in-tree pass, you cannot
upstream it if the dependences are not upstream or properly hooked up.
Both things have been very hard to get into upstream llvm in the past.
I'm aware this is a build system we are talking about so it's a bit
different
but conceptually we should have better guidelines for integration of
code not
build by default, especially the code that is not planned to be enabled by
default any time soon.
Eric mentioned in a follow up that he is more inclined to accept such
code, at
least that is what I read. I am actually as well, probably always was ;)
I have no problem with gn, bazel, ... but I want us to be similarly open to
other projects that are used by the community and benefit from integration
without burdening everyone.
~ Johannes
>
> Best,
>
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