[llvm-dev] [RFC] One or many git repositories?

Piotr Padlewski via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 22 13:18:31 PDT 2016

I have one reasone why we should not moe to monolithic repository - If you
do some light stuff like clang-tidy, that don't often require syncing with
clang, but you still want to have the most recent checks, then I don't see
a solution in monolithic repository.
And this is a real issue if you only have 2 or 4 core laptop to do work.
And I guess the the build system won't solve the problem, just a small
change in some llvm file will result in recompiling many files that
clang-tidy depends on.

2016-07-22 13:08 GMT-07:00 Richard Smith via llvm-dev <
llvm-dev at lists.llvm.org>:

> Having read through the entire thread and thought about this for a while,
> here are my thoughts:
>  * A single monolithic repository has quite a lot of advantages, some
> because of what it is (for instance, you can make atomic cross-project
> commits), and some because of what it isn't (keeping the repositories
> separate creates synchronization problems for version-locked components,
> and it's not clear to me that we have a good answer for these problems)
>  * A single repository from which we can build a complete LLVM toolchain,
> without requiring checking out a dozen components in seemingly-random
> locations, would be valuable. The default behavior for someone checking out
> and building the LLVM project should be that they get a complete,
> fully-functional toolchain.
>  * We need to preserve and maintain the easy ability to mix and match LLVM
> components with other components (other C runtime libraries, C++ ABI
> libraries, C++ standard libraries, linkers, debuggers, ...). That means
> that it needs to be obvious what the boundaries of the optional components
> are, which means that the current project layout (the one implied by the
> build system) is not good enough for a monolithic repository (LLVM tests
> will fail if you don't check out llvm/tools/opt, but we presumably want to
> explicitly support not checking out llvm/tools/clang) -- unless we have
> extensive documentation covering this, and even then there are likely to be
> discoverability issues.
> However, the move to git and the reorganization need not be done at the
> same time, and it seems vastly easier to reorganize *after* we move to a
> monolithic git repository -- it would then be essentially trivial for each
> person with organizational ideas to move the code around in their
> monolithic git repository, push it somewhere where we can all look at it,
> and for us to then make an informed choice about the layout, with a
> concrete example in front of us. Then we push the selected new layout; git
> supports this really nicely if all the parts are already in a single
> repository.
> So here's what I would suggest:
> - we move to a monolithic git repository on github
> - this monolithic repository contains all the LLVM subprojects necessary
> to build a complete toolchain, including libc++ and other pieces that are
> not version-locked to llvm or clang
> - the initial structure exactly matches the current layout implied by the
> build system (clang in tools/clang, lld in tools/lld, compiler-rt in
> runtimes/compiler-rt, libc++ in projects/libcxx, and so on)
> - after we transition to git, interested parties assemble and upload to
> github patches reorganizing the project structure, and we have another
> discussion about principles for the restructuring (including forming solid
> guidance for how to organize future additions to LLVM), with reference to
> the patches so we can look at the proposed new layout; we pick one and
> commit it
> The goal would be to have the new layout entirely settled by the time 4.0
> branches.
> On Wed, Jul 20, 2016 at 4:39 PM, Justin Lebar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Dear all,
>> I would like to (re-)open a discussion on the following specific question:
>>   Assuming we are moving the llvm project to git, should we
>>   a) use multiple git repositories, linked together as subrepositories
>> of an umbrella repo, or
>>   b) use a single git repository for most llvm subprojects.
>> The current proposal assembled by Renato follows option (a), but I
>> think option (b) will be significantly simpler and more effective.
>> Moreover, I think the issues raised with option (b) are either
>> incorrect or can be reasonably addressed.
>> Specifically, my proposal is that all LLVM subprojects that are
>> "version-locked" (and/or use the common CMake build system) live in a
>> single git repository.  That probably means all of the main llvm
>> subprojects other than the test-suite and maybe libc++.  From looking
>> at the repository today that would be: llvm, clang, clang-tools-extra,
>> lld, polly, lldb, llgo, compiler-rt, openmp, and parallel-libs.
>> Let's first talk about the advantages of a single repository.  Then
>> we'll address the disadvantages raised.
>> At a high level, one repository is simpler than multiple repos that
>> must be kept in sync using an external mechanism.  The submodules
>> solution requires nontrivial automation to maintain the history of
>> commits in the umbrella repo (which we need if we want to bisect, or
>> even just build an old revision of clang), but no such mechanisms are
>> required if we have a single repo.
>> Similarly, it's possible to make atomic API changes across subprojects
>> in a single repo; we simply can't do with the submodules proposal.
>> And working with llvm release branches becomes much simpler.
>> In addition, the single repository approach ties branches that contain
>> changes to subprojects (e.g. clang) to a specific version of llvm
>> proper.  This means that when you switch between two branches that
>> contain changes to clang, you'll automatically check out the right
>> llvm bits.
>> Although we can do this with submodules too, a single repository makes
>> it much easier.
>> As a concrete example, suppose you are working on some changes in
>> clang.  You want to commit the changes, then switch to a new branch
>> based on tip of head and make some new changes.  Finally you want to
>> switch back to your original branch.  And when you switch between
>> branches, you want to get an llvm that's in sync with the clang in
>> your working copy.
>> Here's how I'd do it with a monolithic git repository, option (b):
>>   git commit # old-branch
>>   git fetch
>>   git checkout -b new-branch origin/master
>>   # hack hack hack
>>   git commit # new-branch
>>   git checkout old-branch
>> Here's how I'd do it with option (a), submodules.  I've used git -C
>> here to make it explicit which repo we're working in, but in real life
>> I'd probably use cd.
>>   # First, commit to two branches, one in your clang repo and one in your
>>   # master repo.
>>   git -C tools/clang commit # old-branch, clang submodule
>>   git commit # old-branch, master repo
>>   # Now fetch the submodule and check out head.  Start a new branch in the
>>   # umbrella repo.
>>   git submodule foreach fetch
>>   git checkout -b origin/master new-branch
>>   git submodule update
>>   # Start a new branch in the clang repo pointing to the current head.
>>   git checkout -b -C tools/clang new-branch
>>   # hack hack hack
>>   # Commit both branches.
>>   git commit -C tools/clang # new-branch
>>   git commit # new-branch
>>   # Check out the old branch.
>>   git checkout old-branch
>>   git submodule update
>> This is twice as many git commands, and almost three times as much
>> typing, to do the same thing.
>> Indeed, this is so complicated I expect that many developers wouldn't
>> bother, and will continue to develop the way we currently do.  They
>> would thus continue to be unable to create clang branches that include
>> an llvm revision.  :(
>> There are real simplifications and productivity advantages to be had
>> by using a single repository.  They will affect essentially every
>> developer who makes changes to subprojects other than LLVM proper,
>> cares about release branches, bisects our code, or builds old
>> revisions.
>> So that's the first part, what we have to gain by using a monolithic
>> repository.  Let's address the downsides.
>> If you'll bear with a hypothetical: Imagine you could somehow make the
>> monolithic repository behave exactly like the N separate repositories
>> work today.  If so, that would be the best of both worlds: Those of us
>> who want a monolithic repository could have one, and those of us who
>> don't would be unaffected.  Whatever downsides you were worried about
>> would evaporate in a mist of rainbows and puppies.
>> It turns out this hypothetical is very close to reality.  The key is
>> git sparse checkouts [1], which let you check out only some files or
>> directories from a repository.  Using this facility, if you don't like
>> the switch to a monolithic repository, you can set up your git so
>> you're (almost) entirely unaffected by it.
>> If you want to check out only llvm and clang, no problem. Just set up
>> your .git/info/sparse-checkout file appropriately.  Done.
>> If you want to be able to have two different revisions of llvm and
>> clang checked out at once (maybe you want to update your clang bits
>> more often than you update your llvm bits), you can do that too.  Make
>> one sparse checkout just of llvm, and make another sparse checkout
>> just of clang.  Symlink the clang checkout to llvm/tools/clang.
>> That's it.  The two checkouts can even share a common .git dir, so you
>> don't have to fetch and store everything twice.
>> As far as I can tell, the only overhead of the monolithic repository
>> is the extra storage in .git.  But this is quite small in the scheme
>> of things.
>> The .git dir for the existing monolithic repository [2] is 1.2GB.  By
>> way of comparison, my objdir for a release build of llvm and clang is
>> 3.5G, and a full checkout (workdir + .git dirs) of llvm and clang is
>> 0.65G.
>> If the 1.2G really is a problem for you (or more likely, your
>> automated infrastructure), a shallow clone [3] takes this down to 90M.
>> The critical point to me in all this is that it's easy to set up the
>> monolithic repository to appear like it's a bunch of separate repos.
>> But it is impossible, insofar as I can tell, to do the opposite.  That
>> is, option (b) is strictly more powerful than option (a).
>> Renato has understandably pointed out that the current proposal is
>> pretty far along, so please speak up now if you want to make this
>> happen.  I think we can.
>> Regards,
>> -Justin
>> [1] Git sparse checkouts were introduced in git 1.7, in 2010. For more
>> info, see
>> http://jasonkarns.com/blog/subdirectory-checkouts-with-git-sparse-checkout/
>> .
>> As far as I can tell, sparse checkouts work fine on Windows, but you
>> have to use git-bash, see http://stackoverflow.com/q/23289006.
>> [2] https://github.com/llvm-project/llvm-project
>> [3] git clone --depth=1 https://github.com/llvm-project/llvm-project.git
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160722/ce99e985/attachment-0001.html>

More information about the llvm-dev mailing list