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

[Sterling Augustine via llvm-dev]

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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