[llvm-dev] Policy on support tiers, take 2

Geoffrey Martin-Noble via llvm-dev llvm-dev at lists.llvm.org
Mon Nov 2 12:27:16 PST 2020

I'm not super familiar with all the various subprojects, but I'd be a
little hesitant to put e.g. libc and flang in the same category as vim
bindings or the gn build :-D I think I've had failing pre-merge checks from
both flang and polly before, so at least in practice they don't seem to be
following the guidelines you mention here. I don't know whether this means
that they should just be moved up to tier 1, or whether they are actually
less-supported than some of the more established projects. If their current
behavior should change, then that seems like a bigger discussion.

I also think it might be better to make this focus specifically on the
monorepo. Incubator projects already have a clear policy and a lot of the
fine points here only make sense in the context of the monorepo. e.g. bots
with blamelists targeting only a subcommunity doesn't make much sense for a
separate repo. If you committed the the mlir-npcomp repo I think you should
get notified if you broke something :-D

Apologies if the below falls into the category of "writing", but here are
some additional thoughts:

I also think that the distinction we previously had with tier 2 vs tier 3
was useful in terms of differentiating things that need quite a bit of
promised support to be allowed in the monorepo because they're bigger, more
complicated, and require constant maintenance (e.g build systems) vs things
that can be checked in without much discussion because they are small,
simple, and degrade gracefully (e.g. editor bindings). Maybe separate tiers
isn't the right way to do that, but I think we should make that distinction
clear. Like if someone wants to check in bindings for their editor of
choice, a simple patch seems appropriate, whereas if someone
(hypothetically ;-P) wants to propose a secondary build system it should at
least be discussed on the mailing list. Maybe we can just include language
in the description of tier 2, like:

When adding components intending for tier 2 status, the level of discussion
and support commitment required is proportional to the size and complexity
of the component. For example:
1. editor bindings can be just be added as a patch following the normal
review process
2. more complex components like a secondary build system should start as an
RFC that details how this component will be supported
3. Experimental backends have an entire section on their introduction (link)

On Mon, Nov 2, 2020 at 11:58 AM Renato Golin <rengolin at gmail.com> wrote:

> Ok, so after some feedback, here's an updated version. Separate thread as
> the previous got split.
> People seem to agree on the overall terms, but there was confusion on the
> difference between tier 2 and 3 as well as clarification on what projects
> go where.
> I have joined tiers 2 and 3 and made explicit the three criteria they fit
> into, with the requirements more formally explained.
> Please review the sub-project lists on the two tiers, I'm not so sure
> about them.
> Once we're happy with the "what", I'll send a review for a new doc so we
> can discuss the writing and format there (ignore it for now).
> Here it goes:
> *** Tier 1: the core compiler, officially supported by the community.
> Rationale:
>  * Common code that supports most projects, upstream and downstream forks
> and forms the toolchain itself.
>  * Includes everything we release on all architectures and OSs we have
> releases on.
> What:
>  * LLVM itself, clang/tools, compiler-rt, libcxx/abi/unwind, lld, lldb,
> openmp, mlir.
>  * Basically everything inside the mono-repo that is not in tier 2.
>  * Builds on all first class citizen combinations of targets x OSs (incl.
> test-suite).
>  * The CMake build infrastructure and release scripts.
>  * Phabricator & Buildbot infrastructure.
>  * The test-suite repository.
> Requirements:
>  * Follow all core development policies on code quality, reviews, reverts,
> etc.
>  * Noisy green buildbots, emailing all developers.
>  * Most not be broken by changes in tier 2 (ex. if they require tier 1
> changes).
>  * Bitrot will downgrade areas to tier 2 or be removed, depending if a
> sub-community picks up support and has a timeline to fix.
> *** Tier 2: side projects that integrate with the compiler and that
> *should* work at all times.
> Rationale:
>  * Code that is making its way into LLVM core (tier 1) via
> experimental/incubator roadmaps, or;
>  * Code that isn't meant to be in LLVM core, but has a sub-community that
> maintains it long term, or;
>  * Code that is making its way out of LLVM core (legacy) and that is a
> strong candidate for removal.
> What:
>  * Experimental targets/options.
>  * Experimental mono-repo projects (flang, libc, libclc, parallel-libs,
> polly, beduginfo-tests?, pstl?)
>  * Incubator projects (circt, mlir-npcomp, etc).
>  * Legacy tools (lnt).
>  * Alternative build systems (gn, bazel).
>  * Tool support (gdb scripts, editor configuration, helper scripts).
> Requirements:
>  * Follow all core development policies on code quality, reviews, reverts,
> etc.
>  * Infrastructure that only notify its sub-community.
>  * Most not break tier 1, promptly reverting if it does, with discussions
> to be done offline before reapply.
>  * Leaner policy on bots being red for longer, as long as the
> sub-community has a roadmap to fix.
>  * Leaner policy on bitrot, as long as it doesn't impact tier 1 or other
> tier 2 projects.
>  * Should be easy to remove (either separate path, or clear impact in
> code).
>  * Must have a document making clear status, level of support and, if
> applicable, roadmap into tier 1 / out of LLVM.
> cheers,
> --renato
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