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

[Chris Tetreault via llvm-dev]

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.

Thanks,
   Christopher Tetreault

From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> On Behalf Of Renato Golin via llvm-dev
Sent: Thursday, October 29, 2020 9:06 AM
To: tstellar at redhat.com<mailto:tstellar at redhat.com>
Cc: Mehdi Amini <aminim at google.com<mailto:aminim at google.com>>; LLVM Dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; Stella Laurenzo <laurenzo at google.com<mailto:laurenzo at google.com>>; Tres Popp <tpopp at google.com<mailto:tpopp at google.com>>; Geoffrey Martin-Noble <gcmn at google.com<mailto:gcmn at google.com>>; Thomas Joerg <tjoerg at google.com<mailto:tjoerg at google.com>>
Subject: [EXT] Re: [llvm-dev] Contributing Bazel BUILD files similar to gn

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