[PATCH] D56819: Document toolchain update policy

[JF Bastien via Phabricator via llvm-commits]

jfb updated this revision to Diff 182190.
jfb added a comment.

- Remove extra space.


Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D56819/new/

https://reviews.llvm.org/D56819

Files:
  cmake/modules/CheckCompilerVersion.cmake
  docs/DeveloperPolicy.rst
  docs/GettingStarted.rst


Index: docs/GettingStarted.rst
===================================================================
--- docs/GettingStarted.rst
+++ docs/GettingStarted.rst
@@ -220,15 +220,16 @@
 ------------------------------------------------------
 
 LLVM is very demanding of the host C++ compiler, and as such tends to expose
-bugs in the compiler. We are also planning to follow improvements and
-developments in the C++ language and library reasonably closely. As such, we
-require a modern host C++ toolchain, both compiler and standard library, in
-order to build LLVM.
+bugs in the compiler. We also attempt to follow improvements and developments in
+the C++ language and library reasonably closely. As such, we require a modern
+host C++ toolchain, both compiler and standard library, in order to build LLVM.
 
-For the most popular host toolchains we check for specific minimum versions in
-our build systems:
+LLVM is written using the subset of C++ documented in :doc:`coding
+standards<CodingStandards>`. To enforce this language version, we check the most
+popular host toolchains for specific minimum versions in our build systems:
 
 * Clang 3.1
+* Apple Clang 3.1
 * GCC 4.8
 * Visual Studio 2015 (Update 3)
 
@@ -336,7 +337,7 @@
 from the loader similar to ``libstdc++.so.6: version `GLIBCXX_3.4.20' not
 found``. This means you need to tweak the -rpath linker flag.
 
-When you build Clang, you will need to give *it* access to modern C++11
+When you build Clang, you will need to give *it* access to modern C++
 standard library in order to use it as your new host in part of a bootstrap.
 There are two easy ways to do this, either build (and install) libc++ along
 with Clang and then use it with the ``-stdlib=libc++`` compile and link flag,
Index: docs/DeveloperPolicy.rst
===================================================================
--- docs/DeveloperPolicy.rst
+++ docs/DeveloperPolicy.rst
@@ -638,6 +638,47 @@
 status, but also markers to define bit-rot, and will be used to clean up the
 tree from unmaintained targets.
 
+.. _toolchain:
+
+Updating Toolchain Requirements
+-------------------------------
+
+We intend to require newer toolchains as time goes by. This means LLVM's
+codebase can use newer versions of C++ as they get standardized. Requiring newer
+toolchains to build LLVM can be painful for those building LLVM, it will
+therefore only be done through the following process:
+
+  * Generally, try to support LLVM and GCC versions from the last 3 years at a
+    minimum. This time-based guideline is not strict: we may support much older
+    compilers, or decide to support fewer versions.
+
+  * An RFC is sent to the `llvm-dev mailing list <http://lists.llvm.org/mailman/listinfo/llvm-dev>`_
+
+    - Detail upsides of the version increase (e.g. allow LLVM to use newer C++
+      language or library features; avoid miscompiles in particular compiler
+      versions, etc).
+    - Detail downsides on important platforms (e.g. Ubuntu LTS status).
+
+  * Once the RFC reaches consensus, update the CMake toolchain version checks as
+    well as the :doc:`getting started<GettingStarted>` guide. We want to
+    soft-error when developers compile LLVM. We say "soft-error" because the
+    error can be turned into a warning using a CMake flag. This is an important
+    step: LLVM still doesn't have code which requires the new toolchains, but it
+    soon will. If you compile LLVM but don't read the mailing list, we should
+    tell you!
+
+  * Ensure that at least one LLVM release has had this soft-error. Not all
+    developers compile LLVM tip-of-tree. These release-bound developers should
+    also be told about upcoming changes.
+
+  * Turn the soft-error into a hard-error after said LLVM release has branched.
+
+  * Update the :doc:`coding standards<CodingStandards>` to explicitly allow the
+    new features we've now unlocked.
+
+  * Start using the new features in LLVM's codebase.
+
+
 .. _copyright-license-patents:
 
 Copyright, License, and Patents
Index: cmake/modules/CheckCompilerVersion.cmake
===================================================================
--- cmake/modules/CheckCompilerVersion.cmake
+++ cmake/modules/CheckCompilerVersion.cmake
@@ -1,6 +1,6 @@
 # Check if the host compiler is new enough.
 # These versions are updated based on the following policy:
-#   llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library
+#   llvm.org/docs/DeveloperPolicy.html#toolchain
 
 include(CheckCXXSourceCompiles)
 


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