Bump minimum toolchain version

[thieta]

Summary

RFC: https://discourse.llvm.org/t/rfc-increasing-the-gcc-and-clang-requirements-to-support-c-17-in-llvm

Following the policy here: https://llvm.org/docs/DeveloperPolicy.html#toolchain

This forum post here will be updated with the timeline and status: https://discourse.llvm.org/t/important-new-toolchain-requirements-to-build-llvm-will-most-likely-be-landing-within-a-week-prepare-your-buildbots/61447

[xbolva00]

GCC X.1 / 2 releases tend to be quite buggy. Maybe 7.5 is better option?

[cor3ntin]

LGTM

[tschuett]

Do you want to say something about buildbots, C++ features, and that you are not committing until foo.

[efriedma]

Did anyone actually try building LLVM with the "minimum required" compilers? I recall having problems with an old build config using clang 5 not too long ago...

[mehdi_amini]

I have a bot building with clang-5, and one building with gcc 5.x, but I’ll happily upgrade them to whatever is the new minimum

[thieta]

In D122976#3424609, @xbolva00 wrote:

GCC X.1 / 2 releases tend to be quite buggy. Maybe 7.5 is better option?

While that sounds nice - the versions in this change is the ones we have discussed in the RFC and the higher we bump versions the more people we are going to lock out - so I think we'll keep 7.1 for now and if we encounter specific breakages / bugs in that version we might update the document and suggest people use the latest 7.x version instead.

In D122976#3424764, @tschuett wrote:

Do you want to say something about buildbots, C++ features, and that you are not committing until foo.

Here is the rough plan I am thinking right now and I will post in the forums soon:

• Announcement in the forums that this is happening today or tomorrow.
• Let's allow a week for buildbot owners to upgrade before we merge this commit. Maybe merge it end of this week or maybe beginning of next.
• The soft error will live in main until we branch for LLVM 15.x
• When that has been branched - we will convert it to a hard error in main and up the CXX_VERSION to 17.
• LLVM 16.x will require the new toolchain and you will be allowed to use C++17 features.

[thieta]

Added note about toolchain requirements to release notes

[jhenderson]

LGTM, but:

Announcement in the forums that this is happening today or tomorrow.

I'd give it maybe a few more days than that (perhaps the end of the week or start of next), to ensure opportunity for those who have a bit of time off to chime in, in case they didn't see the original RFC/realise it had reached a conclusion.

[thieta]

In D122976#3425649, @jhenderson wrote:

LGTM, but:

Announcement in the forums that this is happening today or tomorrow.

I'd give it maybe a few more days than that (perhaps the end of the week or start of next), to ensure opportunity for those who have a bit of time off to chime in, in case they didn't see the original RFC/realise it had reached a conclusion.

Yeah I have no idea why I wrote today or tomorrow here when I was thinking end of this week or start of next one - as I wrote in the post in the forum as well. Must have been before my second cup of coffee. Good spot though!

[jyknight]

llvm/cmake/modules/CheckCompilerVersion.cmake

22

Replace this with set(LIBSTDCXX_SOFT_ERROR 7) and use per comment below.

99–100

As this change increments the minimum GCC version to 7, we should switch to using _GLIBCXX_RELEASE to check the libstdc++ version. This is better than using __GLIBCXX__ dates, because the dates are updated for every patch release on every branch, so it cannot actually distinguish between branches properly.

The hack of checking iostream_category can go away, as well (it was just a workaround for the unreliability of GLIBCXX).

So, the whole check below should be able to be simplified to:

#if defined(__GLIBCXX__)
#if !defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE < ${LIBSTDCXX_SOFT_ERROR}
#error Unsupported libstdc++ version
#endif
#endif
int main() { return 0; }

[thieta]

Simpilify the libstdc++ check - as suggested by jyknight

[thieta]

@jyknight Addressed your comments - I did test it on my system to fudge the required versions a bit. But if someone have access to a GCC < 7.1 system it might be good to test it there. Otherwise I'll try to figure out how to test it in a docker container maybe.

[thieta]

Gentle Ping - I am planning to land this tomorrow, unless I get feedback to the opposite.

[h-vetinari]

llvm/cmake/modules/CheckCompilerVersion.cmake

108

Was this additional indent intentional? Shouldn't then the whole content of check_cxx_sources_compiles be indented?

[thieta]

Fixed un-intentional indent.

[thieta]

llvm/cmake/modules/CheckCompilerVersion.cmake

108

that was not intentional no - probably my editor doing something funny. Thanks for spotting that.

[jyknight]

LG, just some minor comments.

llvm/cmake/modules/CheckCompilerVersion.cmake

99–100

Delete the comment; it's now incorrect, and no longer necessary since the test is now obvious.
Delete unnecessary #include <iosfwd>.

[thieta]

llvm/cmake/modules/CheckCompilerVersion.cmake

99–100

iosfwd was needed in order for _GLIBCXX_RELEASE to be defined - maybe we could include a smaller header to get that define - but I couldn't delete it.

[thieta]

Removed unnecessary comment.

llvm/cmake/modules/CheckCompilerVersion.cmake

 # 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)
 
 set(GCC_MIN 5.1)
-set(GCC_SOFT_ERROR 5.1)
+set(GCC_SOFT_ERROR 7.1)
 set(CLANG_MIN 3.5)
-set(CLANG_SOFT_ERROR 3.5)
+set(CLANG_SOFT_ERROR 5.0)
 set(APPLECLANG_MIN 6.0)
-set(APPLECLANG_SOFT_ERROR 6.0)
+set(APPLECLANG_SOFT_ERROR 9.3)
 
 # https://en.wikipedia.org/wiki/Microsoft_Visual_C#Internal_version_numbering
 # _MSC_VER == 1920 MSVC++ 14.20 Visual Studio 2019 Version 16.0
 # _MSC_VER == 1927 MSVC++ 14.27 Visual Studio 2019 Version 16.7
 set(MSVC_MIN 19.20)
 set(MSVC_SOFT_ERROR 19.27)
 
 # Map the above GCC versions to dates: https://gcc.gnu.org/develop.html#timeline
 set(GCC_MIN_DATE 20150422)
-set(GCC_SOFT_ERROR_DATE 20150422)
+set(LIBSTDCXX_SOFT_ERROR 7)
 
 
 if(DEFINED LLVM_COMPILER_CHECKED)
   return()
 endif()
 set(LLVM_COMPILER_CHECKED ON)
 
 if(LLVM_FORCE_USE_OLD_TOOLCHAIN)

 #else
 const char *chk = \"\";
 #endif
 int main() { ++chk; return 0; }
 "
       LLVM_LIBSTDCXX_MIN)
     if(NOT LLVM_LIBSTDCXX_MIN)
       message(FATAL_ERROR "libstdc++ version must be at least ${GCC_MIN}.")
     endif()
-    # Test for libstdc++ version of at least 5.1 by checking for std::iostream_category().
-    # Note: We should check _GLIBCXX_RELEASE when possible (i.e., for GCC 7.1 and up).
     check_cxx_source_compiles("
 #include <iosfwd>
 #if defined(__GLIBCXX__)
-#if __GLIBCXX__ < ${GCC_SOFT_ERROR_DATE}
+#if !defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE < ${LIBSTDCXX_SOFT_ERROR}
 #error Unsupported libstdc++ version
 #endif
 #endif
-#if defined(__GLIBCXX__)
-#include <ios>
-void foo(void) { (void) std::iostream_category(); }
-#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.")
+        message(WARNING "libstdc++ version should be at least ${LIBSTDCXX_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.")
+        message(FATAL_ERROR "libstdc++ version should be at least ${LIBSTDCXX_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()

llvm/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
 =========================================================== ============ ==========================================
 `CMake <http://cmake.org/>`__                               >=3.13.4     Makefile/workspace generator
-`GCC <http://gcc.gnu.org/>`_                                >=5.1.0      C/C++ compiler\ :sup:`1`
+`GCC <http://gcc.gnu.org/>`_                                >=7.1.0      C/C++ compiler\ :sup:`1`
 `python <http://www.python.org/>`_                          >=3.6        Automated test suite\ :sup:`2`
 `zlib <http://zlib.net>`_                                   >=1.2.3.4    Compression library\ :sup:`3`
 `GNU Make <http://savannah.gnu.org/projects/make>`_         3.79, 3.79.1 Makefile/build processor\ :sup:`4`
 =========================================================== ============ ==========================================
 
 .. note::
 
    #. Only the C and C++ languages are needed so there's no need to build the

 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 2019
+* Clang 5.0
+* Apple Clang 9.3
+* GCC 7.1
+* Visual Studio 2019 16.7
 
 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.

 
 .. _toolchain testing PPA:
   https://launchpad.net/~ubuntu-toolchain-r/+archive/test
 .. _ask ubuntu stack exchange:
   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 5.1.0:
+Easy steps for installing GCC 7.1.0:
 
 .. code-block:: console
 
-  % gcc_version=5.1.0
+  % gcc_version=7.1.0
   % 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-${gcc_version}.tar.bz2.sig`
   % if [ $signature_invalid ]; then echo "Invalid signature" ; exit 1 ; fi
   % tar -xvjf gcc-${gcc_version}.tar.bz2
   % cd gcc-${gcc_version}
   % ./contrib/download_prerequisites

llvm/docs/ReleaseNotes.rst

 =================================================
 .. NOTE
    For small 1-3 sentence descriptions, just add an entry at the end of
    this list. If your description won't fit comfortably in one bullet
    point (e.g. maybe you would like to give an example of the
    functionality, or simply have a lot to talk about), see the `NOTE` below
    for adding a new subsection.
 
+* ...
 
-.. NOTE
-   If you would like to document a larger change, then you can add a
-   subsection about it right here. You can copy the following boilerplate
-   and un-indent it (the indentation causes it to be inside this comment).
-
-   Special New Feature
-   -------------------
+Update on required toolchains to build LLVM
 
-   Makes programs 10x faster by doing Special New Thing.
+With LLVM 15.x we will raise the version requirements of the toolchain used
+to build LLVM. The new requirements are as follows:
 
-* ...
+* GCC >= 7.1
+* Clang >= 5.0
+* Apple Clang >= 9.3
+* Visual Studio 2019 >= 16.7
+
+In LLVM 15.x these requirements will be "soft" requirements and the version
+check can be skipped by passing -DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=ON
+to CMake.
+
+With the release of LLVM 16.x these requirements will be hard and LLVM developers
+can start using C++17 features, making it impossible to build with older
+versions of these toolchains.
 
 Changes to the LLVM IR
 ----------------------
 
 Changes to building LLVM
 ------------------------
 
 Changes to TableGen