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D56819.id182188.diff

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)
@@ -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 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
+ 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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