Index: llvm/CMakeLists.txt
===================================================================
--- llvm/CMakeLists.txt
+++ llvm/CMakeLists.txt
@@ -23,6 +23,9 @@
 if(NOT DEFINED LLVM_VERSION_SUFFIX)
   set(LLVM_VERSION_SUFFIX svn)
 endif()
+if (NOT DEFINED LLVM_IGNORE_HOST_COMPILER_VERSION_CHECK)
+  set(LLVM_IGNORE_HOST_COMPILER_VERSION_CHECK OFF)
+endif()
 
 if (NOT PACKAGE_VERSION)
   set(PACKAGE_VERSION
Index: llvm/cmake/modules/CheckCompilerVersion.cmake
===================================================================
--- llvm/cmake/modules/CheckCompilerVersion.cmake
+++ llvm/cmake/modules/CheckCompilerVersion.cmake
@@ -1,19 +1,37 @@
-# Check if the host compiler is new enough. LLVM requires at least GCC 4.8,
-# MSVC 2015 (Update 3), or Clang 3.1.
-
+# Check if the host compiler is new enough.
+# See docs/GettingStarted.rst for the policy to update the minimum requirements
 include(CheckCXXSourceCompiles)
 
+# These should be identical, except for one release cycle where we "warn"
+# about the coming change in requirement in the following release.
+set(GCC_MIN 4.8)
+set(GCC_WARN 4.8)
+set(CLANG_MIN 3.1)
+set(CLANG_WARN 3.1)
+
 if(NOT DEFINED LLVM_COMPILER_CHECKED)
   set(LLVM_COMPILER_CHECKED ON)
 
   if(NOT LLVM_FORCE_USE_OLD_TOOLCHAIN)
     if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
-      if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
-        message(FATAL_ERROR "Host GCC version must be at least 4.8!")
+      if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS GCC_MIN)
+        message(FATAL_ERROR "GCC ${CMAKE_CXX_COMPILER_VERSION} is unsupported "
+                "(minimum version is GCC ${GCC_MIN}).
+      elseif(CMAKE_CXX_COMPILER_VERSION VERSION_LESS GCC_WARN
+         AND NOT LLVM_IGNORE_HOST_COMPILER_VERSION_CHECK)
+        message(FATAL_ERROR "GCC ${CMAKE_CXX_COMPILER_VERSION} will be unsupported"
+                " in the next release, the future requirement will be GCC ${GCC_WARN}."
+                " This warning can be ignored with the cmake option LLVM_IGNORE_HOST_COMPILER_VERSION_CHECK.")
       endif()
     elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
-      if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.1)
-        message(FATAL_ERROR "Host Clang version must be at least 3.1!")
+      if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS CLANG_MIN)
+        message(WARNING "Clang ${CMAKE_CXX_COMPILER_VERSION} is unsupported "
+                "(minimum version is Clang ${CLANG_MIN}).")
+      elseif(CMAKE_CXX_COMPILER_VERSION VERSION_LESS CLANG_WARN
+         AND NOT LLVM_IGNORE_HOST_COMPILER_VERSION_CHECK)
+        message(FATAL_ERROR "Clang ${CMAKE_CXX_COMPILER_VERSION} will be unsupported"
+                " in the next release, the future requirement will be GCC ${GCC_WARN}."
+                " This warning can be ignored with the cmake option LLVM_IGNORE_HOST_COMPILER_VERSION_CHECK.")"
       endif()
 
       if (CMAKE_CXX_SIMULATE_ID MATCHES "MSVC")
Index: llvm/docs/CMake.rst
===================================================================
--- llvm/docs/CMake.rst
+++ llvm/docs/CMake.rst
@@ -573,6 +573,11 @@
   options, which are passed to the CCACHE_MAXSIZE and CCACHE_DIR environment
   variables, respectively.
 
+**LLVM_IGNORE_HOST_COMPILER_VERSION_CHECK**:BOOL
+  If enabled, the CMake checks for Clang and GCC minimum host compiler versions
+  will be ignored. This flag will likely be removed for LLVM 9.0 due to a bump
+  in the required C++ version. This defaults to OFF.
+
 CMake Caches
 ============
 
Index: llvm/docs/GettingStarted.rst
===================================================================
--- llvm/docs/GettingStarted.rst
+++ llvm/docs/GettingStarted.rst
@@ -220,10 +220,9 @@
 ------------------------------------------------------
 
 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:
@@ -232,10 +231,14 @@
 * GCC 4.8
 * Visual Studio 2015 (Update 3)
 
-Anything older than these toolchains *may* work, but will require forcing the
-build system with a special option and is not really a supported host platform.
-Also note that older versions of these compilers have often crashed or
-miscompiled LLVM.
+For GCC and Clang, LLVM's guideline is to support all major compiler versions
+released at least 3 years from the date in which the previous version was
+branched. Before dropping support for an older version of a compiler, we will
+consider the benefits in terms of features available for development with the
+availability of more recent versions on the most widely used platforms (for
+example considering Ubuntu LTS, etc.). When we decide to bump the minimum
+version of any compiler, we will issue a cmake warning for one release cycle
+before enforcing the new minimum.
 
 For less widely used host toolchains such as ICC or xlC, be aware that a very
 recent version may be required to support all of the C++ features used in LLVM.