Require GCC 5.1 and LLVM 3.5 at a minimum

[jfb]

Summary

As discussed in: http://lists.llvm.org/pipermail/llvm-dev/2018-May/123182.html

Let's start migrating to C++14:

1. Update the GCC and LLVM versions to the ones that first got full C++14 support
2. See if bots fall over and die, fix them
3. Update the rest of this file (MSVC and standard libraries)
4. Discuss next steps to move to -std=c++14 (it'll cause churn as we fix things)
5. Update various LLVM idioms one at a time to C++14

[lebedev.ri]

cmake/modules/CheckCompilerVersion.cmake

15–17

I think you want clang 3.5+
https://godbolt.org/g/yq25fe

[jfb]

cmake/modules/CheckCompilerVersion.cmake

15–17

I'm not sure: according to http://en.cppreference.com/w/cpp/compiler_support#cpp14 we're OK with 3.4. It's OK to use c++1y for 3.4 if it has all of C++14 and isn't buggy.

[echristo]

LGTM

[erichkeane]

Additionally, IMO, I'd think we would be better off specifying Clang 3.5. Being able to specify c++14 vs c++1y seems worth the minor inconvenience unless there is a platform we care about that ships 3.4. FWIW, 5.1 supports c++14.

cmake/modules/CheckCompilerVersion.cmake

11–12

If I recall correctly, 5.0 never existed as an official release. 5.0 would be GCC's development channel, so we likely want to specify 5.1 instead, right?

[Prazek]

I think you also need to change GettingStarted doc

[jfb]

GCC 5.1 / LLVM 3.5; update docs

[jfb]

docs/GettingStarted.rst

365

Can someone with easy Linux access confirm that this works? I got the URLs, but can't make sure it all works at the moment.

[eastig]

We might need to play with _GLIBCXX_USE_CXX11_ABI=0 and the -Wabi-tag option to mitigate libstdc++ ABI issues.

[jfb]

In D46723#1096025, @eastig wrote:

We might need to play with _GLIBCXX_USE_CXX11_ABI=0 and the -Wabi-tag option to mitigate libstdc++ ABI issues.

Agreed. Do you have a Linux machine to try it out? I don't have a VM set up with an older distro at the moment. It would be useful to see which libstdc++ features are used by LLVM when compiled with GCC 5.1 (as well as with LLVM 3.4 when using libstdc++).

[eastig]

In D46723#1096058, @jfb wrote:

In D46723#1096025, @eastig wrote:

We might need to play with _GLIBCXX_USE_CXX11_ABI=0 and the -Wabi-tag option to mitigate libstdc++ ABI issues.

Agreed. Do you have a Linux machine to try it out? I don't have a VM set up with an older distro at the moment. It would be useful to see which libstdc++ features are used by LLVM when compiled with GCC 5.1 (as well as with LLVM 3.4 when using libstdc++).

I have a machine with Ubuntu 14.04. I'll run experiments.

[eastig]

In D46723#1096542, @eastig wrote:

In D46723#1096058, @jfb wrote:

In D46723#1096025, @eastig wrote:

We might need to play with _GLIBCXX_USE_CXX11_ABI=0 and the -Wabi-tag option to mitigate libstdc++ ABI issues.

Agreed. Do you have a Linux machine to try it out? I don't have a VM set up with an older distro at the moment. It would be useful to see which libstdc++ features are used by LLVM when compiled with GCC 5.1 (as well as with LLVM 3.4 when using libstdc++).

I have a machine with Ubuntu 14.04. I'll run experiments.

I ran experiments. I installed gcc 5 from ppa:ubuntu-toolchain-r/test on Ubuntu 14.04. It was gcc 5.5. It updated the system libstdc++ to GLIBCXX_3.4.21. Clang/LLVM built with it won't run on clean Ubuntu 14.04. The library is built with the old library ABI.
The option '-Wabi-tag' did not provide any information because no ABI tags are used in the library header files. The option works for gcc on Ubuntu 16.04. With it enabled I managed to find 2069 places having library ABI issues (2041 - string; 28 - list).

[jfb]

I quite like the approach in D47073. I'll defer to it.

cmake/modules/CheckCompilerVersion.cmake

-# 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. LLVM requires at least GCC 5.1,
+# MSVC 2015 (Update 3), or Clang 3.5.
 
 include(CheckCXXSourceCompiles)
 
 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 5.1)
+        message(FATAL_ERROR "Host GCC version must be at least 5.1!")
       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 3.5)
+        message(FATAL_ERROR "Host Clang version must be at least 3.5!")
       endif()
 
       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++

docs/CodingStandards.rst

 working as expected until some build bot tells you otherwise. If you're in an
 uncertain area of one of the above points, but you cannot test on a Linux
 system, your best approach is to minimize your use of these features, and watch
 the Linux build bots to find out if your usage triggered a bug. For example, if
 you hit a type trait which doesn't work we can then add support to LLVM's
 traits header to emulate it.
 
 .. _the libstdc++ manual:
-  http://gcc.gnu.org/onlinedocs/gcc-4.8.0/libstdc++/manual/manual/status.html#status.iso.2011
+  https://gcc.gnu.org/onlinedocs/gcc-5.1.0/libstdc++/manual/manual/status.html#status.iso.2011
 
 Other Languages
 ---------------
 
 Any code written in the Go programming language is not subject to the
 formatting rules below. Instead, we adopt the formatting rules enforced by
 the `gofmt`_ tool.
 

docs/GettingStarted.rst

 for the software package that LLVM depends on. The Version column provides
 "known to work" versions of the package. The Notes column describes how LLVM
 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 <https://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`
 =========================================================== ============ ==========================================
 
 .. note::
 
    #. Only the C and C++ languages are needed so there's no need to build the
       other languages for LLVM's purposes. See `below` for specific version

 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:
 
-* Clang 3.1
-* GCC 4.8
+* Clang 3.5
+* GCC 5.1
 * 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 less widely used host toolchains such as ICC or xlC, be aware that a very

 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/271388/how-to-install-gcc-4-8-in-ubuntu-12-04-from-the-terminal
 
-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
+  % wget https://ftp.gnu.org/gnu/gcc/gcc-5.1.0/gcc-5.1.0.tar.bz2
+  % wget https://ftp.gnu.org/gnu/gcc/gcc-5.1.0/gcc-5.1.0.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-5.1.0.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-5.1.0.tar.bz2
+  % cd gcc-5.1.0
   % ./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-5.1.0-build
+  % cd gcc-5.1.0-build
+  % $PWD/../gcc-5.1.0/configure --prefix=$HOME/toolchains --enable-languages=c,c++
   % make -j$(nproc)
   % make install
 
 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
 version of libstdc++ is not on the system library search path, you need to pass
 extra linker flags so that it can be found at link time (``-L``) and at runtime
 (``-rpath``). If you are using CMake, this invocation should produce working
 binaries: