Index: llvm/trunk/CMakeLists.txt
===================================================================
--- llvm/trunk/CMakeLists.txt
+++ llvm/trunk/CMakeLists.txt
@@ -448,9 +448,12 @@
 set(LLVM_ABI_BREAKING_CHECKS "WITH_ASSERTS" CACHE STRING
   "Enable abi-breaking checks.  Can be WITH_ASSERTS, FORCE_ON or FORCE_OFF.")
 
-option(LLVM_FORCE_USE_OLD_HOST_TOOLCHAIN
+option(LLVM_FORCE_USE_OLD_TOOLCHAIN
        "Set to ON to force using an old, unsupported host toolchain." OFF)
 
+option(LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN
+       "Set to ON to only warn when using a toolchain which is about to be deprecated, instead of emitting an error." OFF)
+
 option(LLVM_USE_INTEL_JITEVENTS
   "Use Intel JIT API to inform Intel(R) VTune(TM) Amplifier XE 2011 about JIT code"
   OFF)
Index: llvm/trunk/cmake/modules/CheckCompilerVersion.cmake
===================================================================
--- llvm/trunk/cmake/modules/CheckCompilerVersion.cmake
+++ llvm/trunk/cmake/modules/CheckCompilerVersion.cmake
@@ -5,13 +5,18 @@
 include(CheckCXXSourceCompiles)
 
 set(GCC_MIN 4.8)
-set(GCC_WARN 4.8)
+set(GCC_SOFT_ERROR 5.1)
 set(CLANG_MIN 3.1)
-set(CLANG_WARN 3.1)
+set(CLANG_SOFT_ERROR 3.5)
 set(APPLECLANG_MIN 3.1)
-set(APPLECLANG_WARN 3.1)
-set(MSVC_MIN 19.0)
-set(MSVC_WARN 19.00.24213.1)
+set(APPLECLANG_SOFT_ERROR 6.0)
+set(MSVC_MIN 19.00.24213.1)
+set(MSVC_SOFT_ERROR 19.1)
+
+# Map the above GCC versions to dates: https://gcc.gnu.org/develop.html#timeline
+set(GCC_MIN_DATE 20130322)
+set(GCC_SOFT_ERROR_DATE 20150422)
+
 
 if(DEFINED LLVM_COMPILER_CHECKED)
   return()
@@ -22,21 +27,25 @@
   return()
 endif()
 
-function(check_compiler_version NAME NICE_NAME MINIMUM_VERSION WARN_VERSION)
+function(check_compiler_version NAME NICE_NAME MINIMUM_VERSION SOFT_ERROR_VERSION)
   if(NOT CMAKE_CXX_COMPILER_ID STREQUAL NAME)
     return()
   endif()
   if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS MINIMUM_VERSION)
     message(FATAL_ERROR "Host ${NICE_NAME} version must be at least ${MINIMUM_VERSION}, your version is ${CMAKE_CXX_COMPILER_VERSION}.")
-  elseif(CMAKE_CXX_COMPILER_VERSION VERSION_LESS WARN_VERSION)
-    message(WARNING "Host ${NICE_NAME} version must be at least ${WARN_VERSION} due to miscompiles from earlier versions, your version is ${CMAKE_CXX_COMPILER_VERSION}.")
+  elseif(CMAKE_CXX_COMPILER_VERSION VERSION_LESS SOFT_ERROR_VERSION)
+    if(LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN)
+      message(WARNING "Host ${NICE_NAME} version should be at least ${SOFT_ERROR_VERSION} because LLVM will soon use new C++ features which your toolchain version doesn't support. Your version is ${CMAKE_CXX_COMPILER_VERSION}. Ignoring because you've set LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN, but very soon your toolchain won't be supported.")
+    else()
+      message(FATAL_ERROR "Host ${NICE_NAME} version should be at least ${SOFT_ERROR_VERSION} because LLVM will soon use new C++ features which your toolchain version doesn't support. Your version is ${CMAKE_CXX_COMPILER_VERSION}. You can temporarily opt out using LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN, but very soon your toolchain won't be supported.")
+    endif()
   endif()
 endfunction(check_compiler_version)
 
-check_compiler_version("GNU" "GCC" ${GCC_MIN} ${GCC_WARN})
-check_compiler_version("Clang" "Clang" ${CLANG_MIN} ${CLANG_WARN})
-check_compiler_version("AppleClang" "Apple Clang" ${APPLECLANG_MIN} ${APPLECLANG_WARN})
-check_compiler_version("MSVC" "Visual Studio" ${MSVC_MIN} ${MSVC_WARN})
+check_compiler_version("GNU" "GCC" ${GCC_MIN} ${GCC_SOFT_ERROR})
+check_compiler_version("Clang" "Clang" ${CLANG_MIN} ${CLANG_SOFT_ERROR})
+check_compiler_version("AppleClang" "Apple Clang" ${APPLECLANG_MIN} ${APPLECLANG_SOFT_ERROR})
+check_compiler_version("MSVC" "Visual Studio" ${MSVC_MIN} ${MSVC_SOFT_ERROR})
 
 if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
   if (CMAKE_CXX_SIMULATE_ID MATCHES "MSVC")
@@ -45,20 +54,39 @@
     endif()
     set(CLANG_CL 1)
   elseif(NOT LLVM_ENABLE_LIBCXX)
-    # Test that we aren't using too old of a version of libstdc++
-    # with the Clang compiler. This is tricky as there is no real way to
-    # check the version of libstdc++ directly. Instead we test for a known
-    # bug in libstdc++4.6 that is fixed in libstdc++4.7.
+    # Test that we aren't using too old of a version of libstdc++.
     set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
     set(OLD_CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES})
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++0x")
     check_cxx_source_compiles("
-#include <atomic>
-std::atomic<float> x(0.0f);
-int main() { return (float)x; }"
-      LLVM_NO_OLD_LIBSTDCXX)
-    if(NOT LLVM_NO_OLD_LIBSTDCXX)
-      message(FATAL_ERROR "Host Clang must be able to find libstdc++4.8 or newer!")
+#include <iosfwd>
+#if defined(__GLIBCXX__)
+#if __GLIBCXX__ < ${GCC_MIN_DATE}
+#error Unsupported libstdc++ version
+#endif
+#endif
+int main() { return 0; }
+"
+      LLVM_LIBSTDCXX_MIN)
+    if(NOT LLVM_LIBSTDCXX_MIN)
+      message(FATAL_ERROR "libstdc++ version must be at least ${GCC_MIN}.")
+    endif()
+    check_cxx_source_compiles("
+#include <iosfwd>
+#if defined(__GLIBCXX__)
+#if __GLIBCXX__ < ${GCC_SOFT_ERROR_DATE}
+#error Unsupported libstdc++ version
+#endif
+#endif
+int main() { return 0; }
+"
+      LLVM_LIBSTDCXX_SOFT_ERROR)
+    if(NOT LLVM_LIBSTDCXX_SOFT_ERROR)
+      if(LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN)
+        message(WARNING "libstdc++ version should be at least ${GCC_SOFT_ERROR} because LLVM will soon use new C++ features which your toolchain version doesn't support. Ignoring because you've set LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN, but very soon your toolchain won't be supported.")
+      else()
+        message(FATAL_ERROR "libstdc++ version should be at least ${GCC_SOFT_ERROR} because LLVM will soon use new C++ features which your toolchain version doesn't support. You can temporarily opt out using LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN, but very soon your toolchain won't be supported.")
+      endif()
     endif()
     set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
     set(CMAKE_REQUIRED_LIBRARIES ${OLD_CMAKE_REQUIRED_LIBRARIES})
Index: llvm/trunk/docs/CMake.rst
===================================================================
--- llvm/trunk/docs/CMake.rst
+++ llvm/trunk/docs/CMake.rst
@@ -578,6 +578,10 @@
   may not compile at all, or might fail at runtime due to known bugs in these
   toolchains.
 
+**LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN**:BOOL
+  If enabled, the compiler version check will only warn when using a toolchain
+  which is about to be deprecated, instead of emitting an error.
+
 CMake Caches
 ============
 
Index: llvm/trunk/docs/GettingStarted.rst
===================================================================
--- llvm/trunk/docs/GettingStarted.rst
+++ llvm/trunk/docs/GettingStarted.rst
@@ -170,7 +170,7 @@
 Package                                                     Version      Notes
 =========================================================== ============ ==========================================
 `GNU Make <http://savannah.gnu.org/projects/make>`_         3.79, 3.79.1 Makefile/build processor
-`GCC <http://gcc.gnu.org/>`_                                >=4.8.0      C/C++ compiler\ :sup:`1`
+`GCC <http://gcc.gnu.org/>`_                                >=5.1.0      C/C++ compiler\ :sup:`1`
 `python <http://www.python.org/>`_                          >=2.7        Automated test suite\ :sup:`2`
 `zlib <http://zlib.net>`_                                   >=1.2.3.4    Compression library\ :sup:`3`
 =========================================================== ============ ==========================================
@@ -228,6 +228,15 @@
 standards<CodingStandards>`. To enforce this language version, we check the most
 popular host toolchains for specific minimum versions in our build systems:
 
+* Clang 3.5
+* Apple Clang 6.0
+* GCC 5.1
+* Visual Studio 2017
+
+The below versions currently soft-error as we transition to the new compiler
+versions listed above. The LLVM codebase is currently known to compile correctly
+with the following compilers, though this will change in the near future:
+
 * Clang 3.1
 * Apple Clang 3.1
 * GCC 4.8
@@ -283,33 +292,36 @@
 distribution on which users have struggled with the version requirements is
 Ubuntu Precise, 12.04 LTS. For this distribution, one easy option is to install
 the `toolchain testing PPA`_ and use it to install a modern GCC. There is
-a really nice discussions of this on the `ask ubuntu stack exchange`_. However,
-not all users can use PPAs and there are many other distributions, so it may be
-necessary (or just useful, if you're here you *are* doing compiler development
-after all) to build and install GCC from source. It is also quite easy to do
-these days.
+a really nice discussions of this on the `ask ubuntu stack exchange`_ and a
+`github gist`_ with updated commands. However, not all users can use PPAs and
+there are many other distributions, so it may be necessary (or just useful, if
+you're here you *are* doing compiler development after all) to build and install
+GCC from source. It is also quite easy to do these days.
 
 .. _toolchain testing PPA:
   https://launchpad.net/~ubuntu-toolchain-r/+archive/test
 .. _ask ubuntu stack exchange:
-  http://askubuntu.com/questions/271388/how-to-install-gcc-4-8-in-ubuntu-12-04-from-the-terminal
+  https://askubuntu.com/questions/466651/how-do-i-use-the-latest-gcc-on-ubuntu/581497#58149
+.. _github gist:
+  https://gist.github.com/application2000/73fd6f4bf1be6600a2cf9f56315a2d91
 
-Easy steps for installing GCC 4.8.2:
+Easy steps for installing GCC 5.1.0:
 
 .. code-block:: console
 
-  % wget https://ftp.gnu.org/gnu/gcc/gcc-4.8.2/gcc-4.8.2.tar.bz2
-  % wget https://ftp.gnu.org/gnu/gcc/gcc-4.8.2/gcc-4.8.2.tar.bz2.sig
+  % gcc_version=5.1.0
+  % wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2
+  % wget https://ftp.gnu.org/gnu/gcc/gcc-${gcc_version}/gcc-${gcc_version}.tar.bz2.sig
   % wget https://ftp.gnu.org/gnu/gnu-keyring.gpg
-  % signature_invalid=`gpg --verify --no-default-keyring --keyring ./gnu-keyring.gpg gcc-4.8.2.tar.bz2.sig`
+  % signature_invalid=`gpg --verify --no-default-keyring --keyring ./gnu-keyring.gpg gcc-${gcc_version}.tar.bz2.sig`
   % if [ $signature_invalid ]; then echo "Invalid signature" ; exit 1 ; fi
-  % tar -xvjf gcc-4.8.2.tar.bz2
-  % cd gcc-4.8.2
+  % tar -xvjf gcc-${gcc_version}.tar.bz2
+  % cd gcc-${gcc_version}
   % ./contrib/download_prerequisites
   % cd ..
-  % mkdir gcc-4.8.2-build
-  % cd gcc-4.8.2-build
-  % $PWD/../gcc-4.8.2/configure --prefix=$HOME/toolchains --enable-languages=c,c++
+  % mkdir gcc-${gcc_version}-build
+  % cd gcc-${gcc_version}-build
+  % $PWD/../gcc-${gcc_version}/configure --prefix=$HOME/toolchains --enable-languages=c,c++
   % make -j$(nproc)
   % make install
 
@@ -317,7 +329,7 @@
 of this information from.
 
 .. _GCC wiki entry:
-  http://gcc.gnu.org/wiki/InstallingGCC
+  https://gcc.gnu.org/wiki/InstallingGCC
 
 Once you have a GCC toolchain, configure your build of LLVM to use the new
 toolchain for your host compiler and C++ standard library. Because the new