[cfe-dev] [llvm-dev] GN build roundtable summary; adding GN build files to the repo

Mon Nov 5 13:25:53 PST 2018

If the files can exist in an isolated subdirectory, what's the benefit to
having them checked in with LLVM rather than in a separate project checked
out alongside or nested (the same way clang and other projects are checked
into subdirectories of the LLVM tree)?

If they're in a separate tree then it doesn't matter if they bitrot -
there's no cost to the LLVM project to that project/tree continuing to
exist.

(I guess one reason would be that these files may be version locked with
the LLVM source tree, and lining those up may be difficult unless it's in
the same repository? If that's the case, then, yeah, the difference between
a subdirectory of the llvm repo, or a separate repo in the llvm-project
repository... much of a muchness

Are you looking at this only for LLVM proper, or also Clang? Would each
project (LLVM, Clang, etc) then have their own "experimental/gn"
directories?)

(one of the things I'd be worried about is not maintenance/improvement of
the CMake stuff - but just the casual breakages that happen when files are
added/moved - if a substantial number of LLVM developers started using GN
as their main build system, the number of incidental breakages of the CMake
build seems likely to rise - though minor/short term breaks like that
aren't usually a huge drag - all the buildbots complain, fixes are usually
pretty quick/obvious for anyone to make, etc)

On Mon, Nov 5, 2018 at 1:16 PM Nico Weber <thakis at chromium.org> wrote:

> If I read this correctly, there isn't much opposition to landing the gn
> files as long as it's very clear that regular devs aren't supposed to
> update them and that it's clear that they're experimental
>
> The main concerns I've heard so far:
>
> - Having two build systems is confusing. I can see this, but I think
> putting the gn files below llvm/experimental/gn (instead of right into the
> source, like I currently have) and updating GN.rst to explicitly say
> "Reviewers should not ask patch authors to update gn files in addition to
> the cmake files" will address this.
>
> - Having a few people care about the GN build means these people won't
> improve the cmake build. I think this has some merit (but I did clean up
> parts of the cmake build a bit while reading all our cmake code to create
> the gn build for example, and I have several ideas about improving the
> cmake build I want to implement at some point; and it's also not a given
> that the folks who may or may not end up working on the GN build would have
> worked on the cmake build). However, this is true independently of where
> the GN build files are stored.
>
> - GN isn't a cmake replacement, for distro reasons and whatnot. I
> wholeheartedly agree with this. Maybe GN will become better here, but cmake
> is ubiquitous _today_, and it's backed by a company who's interested in
> keeping it around, so it will be around for a long time. I think this is
> cmake's biggest strength. That's why I'm not proposing on replacing the
> cmake build. If the GN build at some point is way better (compiler-rt,
> lldb, etc added; out-of-tree build support; GN itself gets a real distro
> story; ...) then this _may_ be different in a few years, at which point we
> could reconsider. I think this is unlikely to happen.
>
> (I still don't see that any of these problems are being solved by having
> this in an overlay or a separate fork.)
>
> And again, It's a real possibility that we check this in and it turns out
> the people who are interested in don't feel it's all that useful, it
> bitrots, and we delete it again. That's cool, no harm done. I think we
> should have a very high standard for replacing the supported build system
> (cmake), but I think it's cool to have a low bar for people experimenting
> with unsupported build systems, as long as they get deleted if not used. If
> someone wanted to a, say, llvm/experimental/meson to see how that would
> feel, I think that'd be super cool too for example.
>
> On Fri, Nov 2, 2018 at 2:06 AM Dean Michael Berris <dean.berris at gmail.com>
> wrote:
>
>> I think this is how, after moving to a monorepo, it may be feasible to
>> get this in a separate fork.
>>
>> Granted the Git Monorepo + GitHub happens, we can even make it so that
>> the GN build is a branch on the official git repository, which can
>> track the mainline development.
>> On Fri, Nov 2, 2018 at 3:49 AM David Blaikie via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >
>> > Any easy way to do this as some kind of overlay, so they GN files
>> wouldn't live in the LLVM repository, but in a separate one?
>> >
>> > (this might avoid some of the community discussion - though would
>> perhaps still likely have the issue I see as most significant: With a
>> sufficient number of developers using GN, the rate of build breaks due to
>> those developers missing CMake file updates might rise to be a bit of a
>> drag on the LLVM project - though I don't think that's likely to ever be a
>> huge deal, just an annoyance)
>> >
>> > On Wed, Oct 31, 2018 at 11:19 AM Nico Weber via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>> >>
>> >> Hi,
>> >>
>> >> first things first: If you're happy with cmake, you can stop reading
>> now. Nobody is proposing that LLVM moves off cmake, and nobody is proposing
>> anything that's causing people using cmake more work.
>> >>
>> >> At the LLVM conference, I gave a lightning talk [1] about using GN [2]
>> to build LLVM and clang. cmake is great for many use cases, but in my
>> opinion local day-to-day development isn't one of them. So I wrote GN build
>> files for LLVM and clang, enough to make `ninja check-llvm check-clang
>> check-lld` build everything needed for these three test suites and that all
>> tests pass. This works on Linux, Mac, Win hosts targeting X86, ARM,
>> AArch64. You can see them at [3].
>> >>
>> >> I had been worried that it would be a lot of work to keep the build
>> files up to date, but I've been using this for all my LLVM/clang/lld
>> development the last 8 months, and it turned out to not be a big problem --
>> LLVM's build files don't change very often, and GN build files are a
>> pleasure to work with in my opinion.
>> >>
>> >> I gave the lightning talk just to talk about my personal workflow, but
>> there was a lot of interest. We had a roundtable on the next day, and about
>> 20 people said they'd be interested in using this for their development
>> too. The main request was that the .gn files are checked in upstream, so
>> that we can collaborate on keeping them working. Two to three orgs even
>> said they'd be interested in moving their buildbots to GN.
>> >>
>> >> As mentioned at the top, the intention here is not to replace cmake,
>> only to offer an opt-in alternative for people who are interested in it.
>> Keeping the GN build going would be the responsibility of people using it,
>> not of the general LLVM community. If this fails to find use and bitrots,
>> we can easily remove it again.
>> >>
>> >> Are there any concerns with checking in GN files? I've put some
>> initial docs for the GN build at https://reviews.llvm.org/D53944 ; it
>> describes what the GN build is and is not, what its advantages are (speed
>> and easier configurability), and some points about the philosophy behind
>> the LLVM GN build.
>> >>
>> >> If folks are generally ok with GN files living in-tree, I'll start
>> sending patches for gradually adding gn files through the regular review
>> process.
>> >>
>> >> If having a BUILD.gn file in every directory being confusing is a
>> concern, GN has the concept of a "secondary tree" so that all GN files
>> could be below llvm/gn/tree/{llvm,clang,lld,...}.
>> >>
>> >> Cheers,
>> >> Nico
>> >>
>> >> 1: https://llvm.org/devmtg/2018-10/talk-abstracts.html#lt2
>> >> 2: https://gn.googlesource.com/gn , https://is.gd/gn_intro
>> >> 3:
>> https://github.com/llvm-project/llvm-project-20170507/compare/master...nico:gn
>> , click "Files Changed" to see the GN files.
>> >> _______________________________________________
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>> >
>> > _______________________________________________
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>>
>>
>>
>> --
>> Dean
>>
>
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