[docs] Increase minimum supported GCC version for building LLVM to 4.8

[tejohnson]

Summary

The RFC proposal sent to increase the minimum required GCC version
to 4.8 received a lot of support. See the following thread:
http://lists.llvm.org/pipermail/llvm-dev/2016-October/105955.html,

This patch implements that by updating the docs. I believe the
references to libstdc++ 4.7 issues can be removed as well, please
let me know if that is not the case or if they should be updated
a different way.

[Meinersbur]

Should there be a check in CMakeLists.txt that concretely says that their version of GCC is not supported? This might avoid confusion for people that do not read GettingStarted document first, or are upgrading from an older version with gcc 4.7 still installed. Eg.

if (CMAKE_COMPILER_IS_GNUCC AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
  status(FATAL_ERROR "This version of LLVM is known to not compile with GCC before version 4.8")
endif ()

[rengolin]

docs/CodingStandards.rst

192

This should be 4.8.0

docs/GettingStarted.rst

278

Chop this line, too... :)

297

And this, we specifically don't support 3.0.

[rengolin]

In D25683#572892, @Meinersbur wrote:

Should there be a check in CMakeLists.txt that concretely says that their version of GCC is not supported? This might avoid confusion for people that do not read GettingStarted document first, or are upgrading from an older version with gcc 4.7 still installed. Eg.

if (CMAKE_COMPILER_IS_GNUCC AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8)
  status(FATAL_ERROR "This version of LLVM is known to not compile with GCC before version 4.8")
endif ()

Unsupported doesn't mean despised. :)

We could add a warning, though.

[jlebar]

In D25683#572923, @rengolin wrote:

status(FATAL_ERROR "This version of LLVM is known to not compile with GCC before version 4.8")

We could add a warning, though.

I am pretty sure you want a fatal error, otherwise people are going to try it, discover that it doesn't work, and complain on our list.

If someone really wants to use gcc 4.7, nothing is stopping them from going into cmake and commenting out this line. :)

[Meinersbur]

I just noticed this is already implemented in cmake/modules/CheckCompilerVersion.cmake, ie. the version increase is missing in this patch.

[tejohnson]

In D25683#573012, @Meinersbur wrote:

I just noticed this is already implemented in cmake/modules/CheckCompilerVersion.cmake, ie. the version increase is missing in this patch.

Will bump the min version there and add to this patch.

[tejohnson]

docs/CodingStandards.rst

192

Fixed

docs/GettingStarted.rst

278

Done

297

Done

[tejohnson]

Address review comments, including update of cmake check

[rengolin]

In D25683#573000, @jlebar wrote:

If someone really wants to use gcc 4.7, nothing is stopping them from going into cmake and commenting out this line. :)

Good point. :)

LGTM. Thanks!

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-4.7.3/libstdc++/manual/manual/status.html#status.iso.2011
+  http://gcc.gnu.org/onlinedocs/gcc-4.8.5/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/>`_                                >=4.7.0      C/C++ compiler\ :sup:`1`
+`GCC <http://gcc.gnu.org/>`_                                >=4.8.0      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

 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 4.7
+* GCC 4.8
 * Visual Studio 2013
 
 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
 recent version may be required to support all of the C++ features used in LLVM.
 
 We track certain versions of software that are *known* to fail when used as
 part of the host toolchain. These even include linkers at times.
 
 **GCC 4.6.3 on ARM**: Miscompiles ``llvm-readobj`` at ``-O3``. A test failure
 in ``test/Object/readobj-shared-object.test`` is one symptom of the problem.
 
 **GNU ld 2.16.X**. Some 2.16.X versions of the ld linker will produce very long
 warning messages complaining that some "``.gnu.linkonce.t.*``" symbol was
 defined in a discarded section. You can safely ignore these messages as they are
 erroneous and the linkage is correct.  These messages disappear using ld 2.17.
 
 **GNU binutils 2.17**: Binutils 2.17 contains `a bug
 <http://sourceware.org/bugzilla/show_bug.cgi?id=3111>`__ which causes huge link
 times (minutes instead of seconds) when building LLVM.  We recommend upgrading
 to a newer version (2.17.50.0.4 or later).
 
 **GNU Binutils 2.19.1 Gold**: This version of Gold contained `a bug
 <http://sourceware.org/bugzilla/show_bug.cgi?id=9836>`__ which causes
 intermittent failures when building LLVM with position independent code.  The
 symptom is an error about cyclic dependencies.  We recommend upgrading to a
 newer version of Gold.
 
-**Clang 3.0 with libstdc++ 4.7.x**: a few Linux distributions (Ubuntu 12.10,
-Fedora 17) have both Clang 3.0 and libstdc++ 4.7 in their repositories.  Clang
-3.0 does not implement a few builtins that are used in this library.  We
-recommend using the system GCC to compile LLVM and Clang in this case.
-
 **Clang 3.0 on Mageia 2**.  There's a packaging issue: Clang can not find at
 least some (``cxxabi.h``) libstdc++ headers.
 
-**Clang in C++11 mode and libstdc++ 4.7.2**.  This version of libstdc++
-contained `a bug <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53841>`__ which
-causes Clang to refuse to compile condition_variable header file.  At the time
-of writing, this breaks LLD build.
-
 Getting a Modern Host C++ Toolchain
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 This section mostly applies to Linux and older BSDs. On Mac OS X, you should
 have a sufficiently modern Xcode, or you will likely need to upgrade until you
 do. On Windows, just use Visual Studio 2013 as the host compiler, it is
 explicitly supported and widely available. FreeBSD 10.0 and newer have a modern
 Clang as the system compiler.