[llvm-branch-commits] [llvm-branch] r353512 - Merging r351387, r351765, and r353374:
Hans Wennborg via llvm-branch-commits
llvm-branch-commits at lists.llvm.org
Fri Feb 8 03:06:27 PST 2019
Author: hans
Date: Fri Feb 8 03:06:27 2019
New Revision: 353512
URL: http://llvm.org/viewvc/llvm-project?rev=353512&view=rev
Log:
Merging r351387, r351765, and r353374:
------------------------------------------------------------------------
r351387 | jfb | 2019-01-16 23:22:38 +0100 (Wed, 16 Jan 2019) | 7 lines
[NFC] Factor out + document build requirements
Summary: This change factors out compiler checking / warning, and documents LLVM_FORCE_USE_OLD_TOOLCHAIN. It doesn't introduce any functional changes nor policy changes, these will come late.
Subscribers: mgorny, jkorous, dexonsmith, llvm-commits
Differential Revision: https://reviews.llvm.org/D56799
------------------------------------------------------------------------
------------------------------------------------------------------------
r351765 | jfb | 2019-01-22 00:53:52 +0100 (Tue, 22 Jan 2019) | 24 lines
Document toolchain update policy
Summary:
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
- Require GCC 5.1 and LLVM 3.5 at a minimum
llvm.org/D46723
Subscribers: jkorous, dexonsmith, llvm-commits
Differential Revision: https://reviews.llvm.org/D56819
------------------------------------------------------------------------
------------------------------------------------------------------------
r353374 | jfb | 2019-02-07 06:20:00 +0100 (Thu, 07 Feb 2019) | 12 lines
Bump minimum toolchain version
Summary:
The RFC on moving past C++11 got good traction:
http://lists.llvm.org/pipermail/llvm-dev/2019-January/129452.html
This patch therefore bumps the toolchain versions according to our policy:
llvm.org/docs/DeveloperPolicy.html#toolchain
Subscribers: mgorny, jkorous, dexonsmith, llvm-commits, mehdi_amini, jyknight, rsmith, chandlerc, smeenai, hans, reames, lattner, lhames, erichkeane
Differential Revision: https://reviews.llvm.org/D57264
------------------------------------------------------------------------
Modified:
llvm/branches/release_80/ (props changed)
llvm/branches/release_80/CMakeLists.txt
llvm/branches/release_80/cmake/modules/CheckCompilerVersion.cmake
llvm/branches/release_80/docs/CMake.rst
llvm/branches/release_80/docs/DeveloperPolicy.rst
llvm/branches/release_80/docs/GettingStarted.rst
Propchange: llvm/branches/release_80/
------------------------------------------------------------------------------
--- svn:mergeinfo (original)
+++ svn:mergeinfo Fri Feb 8 03:06:27 2019
@@ -1,3 +1,3 @@
/llvm/branches/Apple/Pertwee:110850,110961
/llvm/branches/type-system-rewrite:133420-134817
-/llvm/trunk:155241,351325,351344-351345,351349,351351,351370,351381,351421,351426,351436,351475,351485,351753-351754,351910,351930,351932,352034,352204,352246,352374,352555,352770,352889,352945,353082,353155,353218,353304,353367
+/llvm/trunk:155241,351325,351344-351345,351349,351351,351370,351381,351387,351421,351426,351436,351475,351485,351753-351754,351765,351910,351930,351932,352034,352204,352246,352374,352555,352770,352889,352945,353082,353155,353218,353304,353367,353374
Modified: llvm/branches/release_80/CMakeLists.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/branches/release_80/CMakeLists.txt?rev=353512&r1=353511&r2=353512&view=diff
==============================================================================
--- llvm/branches/release_80/CMakeLists.txt (original)
+++ llvm/branches/release_80/CMakeLists.txt Fri Feb 8 03:06:27 2019
@@ -383,9 +383,12 @@ option(LLVM_ENABLE_EXPENSIVE_CHECKS "Ena
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)
Modified: llvm/branches/release_80/cmake/modules/CheckCompilerVersion.cmake
URL: http://llvm.org/viewvc/llvm-project/llvm/branches/release_80/cmake/modules/CheckCompilerVersion.cmake?rev=353512&r1=353511&r2=353512&view=diff
==============================================================================
--- llvm/branches/release_80/cmake/modules/CheckCompilerVersion.cmake (original)
+++ llvm/branches/release_80/cmake/modules/CheckCompilerVersion.cmake Fri Feb 8 03:06:27 2019
@@ -1,52 +1,94 @@
-# 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.
+# These versions are updated based on the following policy:
+# llvm.org/docs/DeveloperPolicy.html#toolchain
include(CheckCXXSourceCompiles)
-if(NOT DEFINED LLVM_COMPILER_CHECKED)
- set(LLVM_COMPILER_CHECKED ON)
+set(GCC_MIN 4.8)
+set(GCC_SOFT_ERROR 5.1)
+set(CLANG_MIN 3.1)
+set(CLANG_SOFT_ERROR 3.5)
+set(APPLECLANG_MIN 3.1)
+set(APPLECLANG_SOFT_ERROR 6.0)
+set(MSVC_MIN 19.00.24213.1)
+set(MSVC_SOFT_ERROR 19.1)
- 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!")
- 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!")
- endif()
+# 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 (CMAKE_CXX_SIMULATE_ID MATCHES "MSVC")
- if (CMAKE_CXX_SIMULATE_VERSION VERSION_LESS 19.0)
- message(FATAL_ERROR "Host Clang must have at least -fms-compatibility-version=19.0")
- endif()
- set(CLANG_CL 1)
- elseif(NOT LLVM_ENABLE_LIBCXX)
- # Otherwise, 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.
- 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!")
- endif()
- set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
- set(CMAKE_REQUIRED_LIBRARIES ${OLD_CMAKE_REQUIRED_LIBRARIES})
- endif()
- elseif(CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
- if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.0)
- message(FATAL_ERROR "Host Visual Studio must be at least 2015")
- elseif(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.00.24213.1)
- message(WARNING "Host Visual Studio should at least be 2015 Update 3 (MSVC 19.00.24213.1)"
- " due to miscompiles from earlier versions")
+
+if(DEFINED LLVM_COMPILER_CHECKED)
+ return()
+endif()
+set(LLVM_COMPILER_CHECKED ON)
+
+if(LLVM_FORCE_USE_OLD_TOOLCHAIN)
+ return()
+endif()
+
+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 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_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} ${MSVC_SOFT_ERROR})
+
+if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+ if (CMAKE_CXX_SIMULATE_ID MATCHES "MSVC")
+ if (CMAKE_CXX_SIMULATE_VERSION VERSION_LESS MSVC_MIN)
+ message(FATAL_ERROR "Host Clang must have at least -fms-compatibility-version=${MSVC_MIN}, your version is ${CMAKE_CXX_COMPILER_VERSION}.")
+ endif()
+ set(CLANG_CL 1)
+ elseif(NOT LLVM_ENABLE_LIBCXX)
+ # 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 <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})
endif()
endif()
Modified: llvm/branches/release_80/docs/CMake.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/branches/release_80/docs/CMake.rst?rev=353512&r1=353511&r2=353512&view=diff
==============================================================================
--- llvm/branches/release_80/docs/CMake.rst (original)
+++ llvm/branches/release_80/docs/CMake.rst Fri Feb 8 03:06:27 2019
@@ -573,6 +573,15 @@ LLVM-specific variables
options, which are passed to the CCACHE_MAXSIZE and CCACHE_DIR environment
variables, respectively.
+**LLVM_FORCE_USE_OLD_TOOLCHAIN**:BOOL
+ If enabled, the compiler and standard library versions won't be checked. LLVM
+ 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
============
Modified: llvm/branches/release_80/docs/DeveloperPolicy.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/branches/release_80/docs/DeveloperPolicy.rst?rev=353512&r1=353511&r2=353512&view=diff
==============================================================================
--- llvm/branches/release_80/docs/DeveloperPolicy.rst (original)
+++ llvm/branches/release_80/docs/DeveloperPolicy.rst Fri Feb 8 03:06:27 2019
@@ -22,7 +22,7 @@ This policy is also designed to accompli
#. Make life as simple and easy for contributors as possible.
-#. Keep the tip of tree as stable as possible.
+#. Keep the top of tree as stable as possible.
#. Establish awareness of the project's :ref:`copyright, license, and patent
policies <copyright-license-patents>` with contributors to the project.
@@ -638,6 +638,47 @@ In essences, these rules are necessary f
status, but also markers to define bit-rot, and will be used to clean up the
tree from unmaintained targets.
+.. _toolchain:
+
+Updating Toolchain Requirements
+-------------------------------
+
+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; therefore, it
+will 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. which newer C++ language or
+ library features LLVM should use; 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 as
+ well as the :doc:`getting started<GettingStarted>` guide. 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 top-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 allow the new
+ features we've explicitly approved in the RFC.
+
+ * Start using the new features in LLVM's codebase.
+
+
.. _copyright-license-patents:
Copyright, License, and Patents
Modified: llvm/branches/release_80/docs/GettingStarted.rst
URL: http://llvm.org/viewvc/llvm-project/llvm/branches/release_80/docs/GettingStarted.rst?rev=353512&r1=353511&r2=353512&view=diff
==============================================================================
--- llvm/branches/release_80/docs/GettingStarted.rst (original)
+++ llvm/branches/release_80/docs/GettingStarted.rst Fri Feb 8 03:06:27 2019
@@ -170,7 +170,7 @@ uses the package and provides other deta
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`
=========================================================== ============ ==========================================
@@ -220,15 +220,25 @@ Host C++ Toolchain, both Compiler and St
------------------------------------------------------
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.
-
-For the most popular host toolchains we check for specific minimum versions in
-our build systems:
+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.
+
+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
* Visual Studio 2015 (Update 3)
@@ -282,33 +292,36 @@ The first step is to get a recent GCC to
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
@@ -316,7 +329,7 @@ For more details, check out the excellen
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
@@ -336,7 +349,7 @@ If you fail to set rpath, most LLVM bina
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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