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

[Renato Golin via llvm-dev]

On Thu, 29 Oct 2020 at 15:23, Tom Stellard via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> I'm a little concerned about having two 'unsupported' buildsystems
> living in tree, and I'm not sure what would stop us from continuing to
> add more.  I would feel better if we had a set of guidelines to define
> the criteria for adding a new buildsytem and also criteria for when we
> can remove them.


I have used Bazel and it doesn't seem to map well to CMake. It seems to be
in between CMake and Ninja with a lot of hard-coded dependencies that are
cumbersome to keep updating. I'm by no means an expert, and I could very
well be wrong, but supporting more than one build system is not trivial
(remember the autoconf days?).

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. 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.

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?).

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).

If the Bazel files can be completely ignored, then it's just more clutter.
But if other projects start to use more different build systems and we
start packing them all in LLVM, then we'll have a hard time knowing what we
build how. I can't really see this scaling.

Two-cents worth.
--renato
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