[llvm-dev] Contributing Bazel BUILD files similar to gn
Eric Astor via llvm-dev
llvm-dev at lists.llvm.org
Thu Oct 29 20:18:37 PDT 2020
I *am* a Googler, though not directly involved with the teams that maintain
the internal LLVM build. I happen to be a big fan of Bazel - and mostly
build LLVM with the internal Bazel build, rather than the external CMake,
because the better caching and remote-build-farm support is such an
enormous help. (Also, I find the CMake build & build options kind of
impenetrable.) However, I'm writing this particular email on my personal
account, with personal resources, well past the close of business; my
Google hat is firmly on the shelf, and I'm speaking as just an individual
contributor.
When I first started contributing to LLVM, *I was confused by the GN
build's existence*. I didn't understand who was supposed to maintain it,
whether I should use it, what the benefits were... you name it.
I agree with some of the first comments on this thread. I'd suggest that we
set aside the question of contributing Bazel BUILD files into the LLVM
repository for now, and start by proposing a general policy around
alternate/unsupported build systems in relation to the main repository. (GN
can have an exception if needed.) The fact that the GN build is basically
working, and doesn't confuse too many people, is a data point - but going
from 1 alternate build system to 2 seems like a good point to pause and set
an actual set of constraints and goals. Eventually, someone may want a
third, and we should know what the guidelines are so we don't hash out the
decision from scratch again!
I don't think I could draft the RFC in question - I don't have enough
experience with the community yet to judge what's really needed - but I'd
be glad to help out with it. The idea should be to minimize the cost (to
nearly zero) for both experienced LLVM contributors and new LLVM
contributors. A few requirements I'd suggest, mostly put together from this
thread:
- CMake should be able to build (and test!) everything the alternate
build system can, at all times.
- There must be a clear group who want to maintain the alternate build
system.
- The alternate build system's files should be isolated in a separate
directory, with a README explaining that this is an alternate build system
for LLVM, maintained by its own smaller community - and is not supported by
the community at large.
- The alternate build system must have independent buildbots, which do
not email the larger community; people can opt into being emailed about
this. (And should, if they're contributing to it!)
- If the buildbots are red for an extended time, we should put out a
call for maintainers to fix the issues; if not answered in a reasonable
time, we shouldn't be afraid to delete the alternate build system.
I *do *also see the argument for the git submodule approach. It looks like
a .gitmodules file would theoretically let a repository of Bazel BUILD
files specify exactly which LLVM commit it currently tracks - and you could
fetch the corresponding updates in both with a single command. I think that
addresses the main point I noticed brought up on this side of the argument.
Any RFC here probably needs to present pros & cons of both approaches.
We'll need to hash those out in general discussion before people start
looking for consensus, so people understand what they're deciding on.
Best,
- Eric
On Thu, Oct 29, 2020 at 10:40 PM Eric Christopher via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
>
> On Thu, Oct 29, 2020 at 9:44 PM Johannes Doerfert via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> (see below)
>>
>>
>> On 10/28/20 6:18 PM, Geoffrey Martin-Noble via llvm-dev wrote:
>> > Hi all,
>> >
>> > tl;dr: We'd like to contribute Bazel BUILD files for LLVM and MLIR in a
>> > side-directory in the monorepo, similar to the gn build.
>> >
>> > Some of us have been working on open-source Bazel BUILD files for the
>> LLVM
>> > Project. You may have seen us hanging out in the #build-systems discord
>> > channel. As you may know, Google uses Bazel internally and has
>> maintained a
>> > Bazel BUILD of LLVM for years. Especially with the introduction of
>> MLIR,
>> > we've got more and more OSS projects with a Bazel BUILD depending on
>> LLVM
>> > (e.g. IREE <https://github.com/google/iree> and TensorFlow
>> > <https://github.com/tensorflow/tensorflow>). We're also not the only
>> ones
>> > using Bazel: e.g. PlaidML also has a Bazel BUILD of LLVM that they've
>> borrowed
>> > from TF
>> > <https://github.com/plaidml/plaidml/blob/master/vendor/llvm/llvm.BUILD
>> >.
>> > Each of these projects has to jump through some weird hoops to keep
>> their
>> > version of the Bazel BUILD files in sync with the code, which
>> requires some
>> > fragile combination of scripts and human intervention. Instead, we'd
>> like
>> > to move general-purpose Bazel BUILD files into the LLVM Project
>> monorepo.
>> > We expect to follow the model of the GN build where these will be
>> > maintained by interested contributors rather than expecting the general
>> > community to maintain them.
>> >
>> > To facilitate and test this we've been developing a standalone
>> repository
>> > that just has the Bazel BUILD files. It symlinks together the directory
>> > trees on top of a submodule as we would need in the monorepo to to
>> avoid
>> > in-tree BUILD files. The configuration is at
>> > https://github.com/google/llvm-bazel. We now have those in a good
>> place and
>> > think they would be useful upstream.
>> >
>> > # Details
>> >
>> > ## What
>> >
>> > Bazel BUILD files for the LLVM, MLIR, and Clang (PR out for review
>> > <https://github.com/google/llvm-bazel/pull/72>) subprojects,
>> potentially
>> > expanding to others, as needed. Basically everything currently at
>> > https://github.com/google/llvm-bazel.
>> >
>> > ## Where
>> >
>> > In https://github.com/google/llvm-bazel the BUILD files live in a
>> single
>> > directory tree matching the structure of the overall llvm-project
>> > directory. For users, @llvm-project is a single Bazel repository
>> >
>> <https://docs.bazel.build/versions/master/build-ref.html#repositories>
>> that
>> > includes both LLVM and MLIR subprojects. To maintain this structure, we
>> > would probably want to put a `bazel` directory in the monorepo's utils
>> > directory <https://github.com/llvm/llvm-project/tree/master/utils>,
>> which
>> > currently only contains a directory for arcanist. This is different
>> from
>> > gn, which is under the LLVM subproject's utils directory
>> > <https://github.com/llvm/llvm-project/tree/master/llvm/utils/gn>. We
>> could
>> > similarly put the Bazel BUILD files under llvm/utils/bazel but have
>> them be
>> > for the entire llvm project (the subsets that are supported). This
>> seems
>> > like an odd structure to me, but I know that the CMake build for LLVM
>> > also builds
>> > the other subprojects
>> >
>> <
>> https://github.com/llvm/llvm-project/blob/529ac33197f6/llvm/tools/CMakeLists.txt#L34-L41
>> >,
>> > so maybe this would be preferable.
>> >
>> > Alternatively we could split each subproject into a separate Bazel
>> > repository and put the Bazel build files under each subproject. I think
>> > this fragments the configuration of the BUILD without much benefit.
>> >
>> > ## Configurations
>> >
>> > We currently have configurations for Linux GCC and Clang, MacOS GCC and
>> > Clang, and Windows MSVC. Support for other configurations can be added
>> > as-desired, but supporting all possible LLVM build configurations is
>> not
>> > the goal.
>> >
>> > ## Support
>> >
>> > Support would be similar to the gn build. Contributors could optionally
>> > update the Bazel BUILD files as part of their patches, but would be
>> under
>> > no obligation to do so.
>> >
>> > ## Preserving History
>> >
>> > I don't *think* the history of llvm-bazel is interesting enough to try
>> to
>> > merge it into the monorepo and I was planning to submit this as a
>> single
>> > patch, but please let me know if you disagree.
>> >
>> > ## Benefits to the community
>> >
>> > -
>> >
>> > Projects that depend on LLVM and use the Bazel build system can
>> avoid
>> > duplicating fragile effort. We'll spend more time contributing to
>> LLVM
>> > instead :-D
>> > -
>> >
>> > Bazel is stricter than CMake in many ways (e.g. it requires that
>> even
>> > header dependencies be declared) and can catch layering issues
>> very easily.
>> > There's even an optional layering_check feature we could turn on
>> if its use
>> > would benefit the community. (though currently the existing
>> problematic
>> > layering makes it a burden to maintain on our own). Even without
>> that
>> > additional check, as I've been keeping the Bazel build green, I've
>> found
>> > and fixed a number of layering issues in the past couple weeks (e.g.
>> > https://reviews.llvm.org/rGb49787df9a
>> > <https://reviews.llvm.org/rGb49787df9a535f03761c340dca7ec3ec1155133d>
>> > and https://reviews.llvm.org/rGc17ae2916c
>> > <https://reviews.llvm.org/rGc17ae2916ccf45a0c1717bd5f11598cc4fff342a
>> >).
>> >
>> >
>> > Here's a patch <https://reviews.llvm.org/D90352> adding the Bazel
>> build
>> > system. It's basically just `cp -r llvm-bazel/llvm-bazel
>> > llvm-project/utils/bazel`.
>>
>> Doesn't the last paragraph mean all benefits derived from this can be
>> described either as:
>> (1) users do not need to clone the llvm-bazel git repo but get the
>> files in llvm-project, or
>> (2) "interested contributors" could send patches to llvm-project
>> instead of llvm-bazel to update the bazel build.
>>
>>
> Absolutely. This could happen. The main reason behind this is to make
> integating among a number of llvm based projects that use bazel (TF and
> TF-based projects primarily, though it sounds like FB's internal process
> would be helped as their system is similar to bazel).
>
>
>> TBH, I have no interest in using bazel nor anything against it being
>> merged per se. I just find it curious that we merge another build system
>> "at no cost" for the community (I think I picked that up in the thread
>> but I might have imagined the phrasing). I mean, there is always "a
>> cost"* so it boils down to determine if the benefit is worth it.
>>
>>
> As far as I can think the cost is...
>
>
>>
>> * i.a., 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.)
>>
>>
> ... this. If the system becomes a source of problems or user complaints
> then I think it's absolutely reasonable to remove it.
>
> -eric
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