Bump minimum toolchain version

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

Reviewed By: mehdi_amini, jyknight, jhenderson, cor3ntin, MaskRay

Differential Revision: https://reviews.llvm.org/D122976

llvm/cmake/modules/CheckCompilerVersion.cmake

@@ -5,11 +5,11 @@
 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
@@ -19,7 +19,7 @@ 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)
@@ -97,27 +97,21 @@ int main() { ++chk; return 0; }
     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})

llvm/docs/GettingStarted.rst

@@ -175,7 +175,7 @@ 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`
@@ -235,10 +235,10 @@ 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.
@@ -303,11 +303,11 @@ GCC from source. It is also quite easy to do these days.
 .. _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

llvm/docs/ReleaseNotes.rst

@@ -42,18 +42,26 @@ Non-comprehensive list of changes in this release
    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).
+Update on required toolchains to build LLVM
+-------------------
 
-   Special New Feature
-   -------------------
+With LLVM 15.x we will raise the version requirements of the toolchain used
+to build LLVM. The new requirements are as follows:
 
-   Makes programs 10x faster by doing Special New Thing.
+* 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
 ----------------------