[docs] Adding "new target" rules to dev policy

Making explicit our current policy to accept new targets as experimental and
later official. Every new target should follow these rules to be added,
and kept relevant in the upstream tree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278971 91177308-0d34-0410-b5e6-96231b3b80d8

docs/DeveloperPolicy.rst

@@ -169,6 +169,8 @@ on a patch, but only people with Subversion write access can approve it.
 There is a web based code review tool that can optionally be used
 for code reviews. See :doc:`Phabricator`.
 
+.. _code owners:
+
 Code Owners
 -----------
 
@@ -497,6 +499,8 @@ list, development list, or LLVM bug tracker component. If someone sends you
 a patch privately, encourage them to submit it to the appropriate list first.
 
 
+.. _IR backwards compatibility:
+
 IR Backwards Compatibility
 --------------------------
 
@@ -553,6 +557,85 @@ C API Changes
   release notes so that it's clear to external users who do not follow the
   project how the C API is changing and evolving.
 
+New Targets
+-----------
+
+LLVM is very receptive to new targets, even experimental ones, but a number of
+problems can appear when adding new large portions of code, and back-ends are
+normally added in bulk. Revisions of large pieces of code is hard, especially
+when the reviewers don't know the full implications of the new back-end with
+details (which is usually the case), makes for a very error prone process.
+
+For these reasons, new targets are *always* added as *experimental* until
+they can be proven stable, and then moved to non-experimental. The difference
+between both classes is that experimental targets are not built by default
+(need to be added to -DLLVM_TARGETS_TO_BUILD at CMake time).
+
+So, the basic rules for a back-end to be upstreamed in **experimental** mode are:
+
+* Every target must have a :ref:`code owner<code owners>`. The `CODE_OWNERS.TXT`
+  file has to be updated as part of the first merge. The code owner makes sure
+  that changes to the target get reviewed and steers the overall effort.
+
+* There must be an active community behind the target. This community
+  will be the maintainers of the target by providing buildbots, fixing
+  bugs, answering the LLVM community's questions and making sure the new
+  target doesn't break any of the other targets, or generic code. This
+  behavior is expected to continue throughout the lifetime of the
+  target's code.
+
+* The code must be free of contentious issues, for example, large
+  changes in how the IR behaves or should be formed by the front-ends,
+  unless agreed by the majority of the community via refactoring of the
+  (:doc:`IR standard<LangRef>`) **before** the merge of the new target changes,
+  following the :ref:`IR backwards compatibility`.
+
+* The code conforms to all of the policies laid out in this developer policy
+  document, including license, patent, and coding standards.
+
+* The target should have either reasonable documentation on how it
+  works (ISA, ABI, etc.) or a publicly available simulator/hardware
+  (either free or cheap enough), so that developers can validate
+  assumptions, understand constraints and review code that can affect
+  the target. Preferably both.
+
+In addition, the rules for a back-end to be marked as **official** are:
+
+* The target must have addressed every other minimum requirement and
+  have been stable in tree for at least 3 months. This cool down
+  period is to make sure that the back-end and the target community can
+  endure continuous upstream development for the foreseeable future.
+
+* The target's code must have been completely adapted to this policy
+  as well as the :doc:`coding standards<CodingStandards>`. Any exceptions that
+  were made to move into experimental mode must have been fixed **before**
+  becoming official.
+
+* The test coverage needs to be broad and well written (small tests,
+  well documented). The build target ``check-all`` must pass with the
+  new target built, and where applicable, the ``test-suite`` must also
+  pass without errors, in at least one configuration (publicly
+  demonstrated, for example, via buildbots).
+
+* Public buildbots need to be created and actively maintained, unless
+  the target requires no additional buildbots (ex. ``check-all`` covers
+  all tests). The more relevant and public the new target's CI infrastructure
+  is, the more the LLVM community will embrace it.
+
+To **continue** as a supported and official target:
+
+* The maintainer(s) must continue following these rules throughout the lifetime
+  of the target. Continuous violations of aforementioned rules and policies
+  could lead to complete removal of the target from the code base.
+
+* Degradation in support, documentation or test coverage will make the target as
+  nuisance to other targets and be considered a candidate for deprecation and
+  ultimately removed.
+
+In essences, these rules are necessary for targets to gain and retain their
+status, but also markers to define bit-rot, and will be used to clean up the
+tree from unmaintained targets.
+
 .. _copyright-license-patents:
 
 Copyright, License, and Patents