diff --git a/CMakeLists.txt b/CMakeLists.txt
index 168ec6a6cff..b224fd9d364 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -383,9 +383,12 @@ option(LLVM_ENABLE_EXPENSIVE_CHECKS "Enable expensive checks" OFF)
 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)
diff --git a/cmake/modules/CheckCompilerVersion.cmake b/cmake/modules/CheckCompilerVersion.cmake
index 67f533f9aba..b1cb5527422 100644
--- a/cmake/modules/CheckCompilerVersion.cmake
+++ b/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 @@ if(LLVM_FORCE_USE_OLD_TOOLCHAIN)
   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 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
     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})
diff --git a/docs/CMake.rst b/docs/CMake.rst
index 9725a0673d1..eb219c58560 100644
--- a/docs/CMake.rst
+++ b/docs/CMake.rst
@@ -578,6 +578,10 @@ LLVM-specific variables
   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
 ============
 
diff --git a/docs/GettingStarted.rst b/docs/GettingStarted.rst
index 126b84288b0..f528bc6014f 100644
--- a/docs/GettingStarted.rst
+++ b/docs/GettingStarted.rst
@@ -170,7 +170,7 @@ uses the package and provides other details.
 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 @@ 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 3.5
+* 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 @@ The first step is to get a recent GCC toolchain installed. The most common
 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 @@ For more details, check out the excellent `GCC wiki entry`_, where I got most
 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