[PATCH] D56819: Document toolchain update policy

Wed Jan 16 15:35:05 PST 2019

jfb created this revision.
Herald added subscribers: llvm-commits, dexonsmith, jkorous.

Capture the current agreed-upon toolchain update policy based on the following
discussions:

- LLVM dev meeting 2018 BoF "Migrating to C++14, and beyond!" llvm.org/devmtg/2018-10/talk-abstracts.html#bof3
- A Short Policy Proposal Regarding Host Compilers lists.llvm.org/pipermail/llvm-dev/2018-May/123238.html
- Using C++14 code in LLVM (2018) lists.llvm.org/pipermail/llvm-dev/2018-May/123182.html
- Using C++14 code in LLVM (2017) lists.llvm.org/pipermail/llvm-dev/2017-October/118673.html
- Using C++14 code in LLVM (2016) lists.llvm.org/pipermail/llvm-dev/2016-October/105483.html
- Document and Enforce new Host Compiler Policy llvm.org/D47073 <https://reviews.llvm.org/D47073>
- Require GCC 5.1 and LLVM 3.5 at a minimum llvm.org/D46723 <https://reviews.llvm.org/D46723>


Repository:
  rL LLVM

https://reviews.llvm.org/D56819

Files:
  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 currently 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)
 
@@ -259,6 +260,41 @@
 symptom is an error about cyclic dependencies.  We recommend upgrading to a
 newer version of Gold.
 
+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 ones.
+
+  * 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
+    and this document. 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.
+
+
 Getting a Modern Host C++ Toolchain
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -336,7 +372,7 @@
 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,


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