[LLVM] Update C++ standard to 17

Also make the soft toolchain requirements hard. This allows
us to use C++17 features in LLVM now.

If we find patterns with C++17 that improve readability
it should be recommended in the coding standards.

Reviewed By: jhenderson, cor3ntin, MaskRay

Differential Revision: https://reviews.llvm.org/D130689

bolt/runtime/CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 3.13.4)
 include(CheckIncludeFiles)
 
 set(CMAKE_CXX_EXTENSIONS OFF)
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 17)
 
 project(libbolt_rt_project)
 

clang/CMakeLists.txt

@@ -11,7 +11,7 @@ endif()
 include(GNUInstallDirs)
 
 if(CLANG_BUILT_STANDALONE)
-  set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
+  set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
   set(CMAKE_CXX_STANDARD_REQUIRED YES)
   set(CMAKE_CXX_EXTENSIONS NO)
 

lld/CMakeLists.txt

@@ -11,7 +11,7 @@ endif()
 include(GNUInstallDirs)
 
 if(LLD_BUILT_STANDALONE)
-  set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
+  set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
   set(CMAKE_CXX_STANDARD_REQUIRED YES)
   set(CMAKE_CXX_EXTENSIONS NO)
 

lldb/CMakeLists.txt

@@ -20,7 +20,7 @@ include(GNUInstallDirs)
 if(LLDB_BUILT_STANDALONE)
   include(LLDBStandalone)
 
-  set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
+  set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
   set(CMAKE_CXX_STANDARD_REQUIRED YES)
   set(CMAKE_CXX_EXTENSIONS NO)
 endif()

llvm/CMakeLists.txt

@@ -58,7 +58,7 @@ project(LLVM
 # Must go after project(..)
 include(GNUInstallDirs)
 
-set(CMAKE_CXX_STANDARD 14 CACHE STRING "C++ standard to conform to")
+set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
 set(CMAKE_CXX_STANDARD_REQUIRED YES)
 if (CYGWIN)
   # Cygwin is a bit stricter and lack things like 'strdup', 'stricmp', etc in

llvm/cmake/modules/CheckCompilerVersion.cmake

@@ -4,21 +4,19 @@
 
 include(CheckCXXSourceCompiles)
 
-set(GCC_MIN 5.1)
+set(GCC_MIN 7.1)
 set(GCC_SOFT_ERROR 7.1)
-set(CLANG_MIN 3.5)
+set(CLANG_MIN 5.0)
 set(CLANG_SOFT_ERROR 5.0)
-set(APPLECLANG_MIN 6.0)
+set(APPLECLANG_MIN 9.3)
 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_MIN 19.27)
 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(LIBSTDCXX_MIN 7)
 set(LIBSTDCXX_SOFT_ERROR 7)
 
 
@@ -76,22 +74,14 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
     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")
-    # Test for libstdc++ version of at least 4.8 by checking for _ZNKSt17bad_function_call4whatEv.
-    # 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_MIN_DATE}
+#if !defined(_GLIBCXX_RELEASE) || _GLIBCXX_RELEASE < ${LIBSTDCXX_MIN}
 #error Unsupported libstdc++ version
 #endif
 #endif
-#if defined(__GLIBCXX__)
-extern const char _ZNKSt17bad_function_call4whatEv[];
-const char *chk = _ZNKSt17bad_function_call4whatEv;
-#else
-const char *chk = \"\";
-#endif
-int main() { ++chk; return 0; }
+int main() { return 0; }
 "
       LLVM_LIBSTDCXX_MIN)
     if(NOT LLVM_LIBSTDCXX_MIN)

llvm/docs/CodingStandards.rst

@@ -53,7 +53,7 @@ choice.
 C++ Standard Versions
 ---------------------
 
-Unless otherwise documented, LLVM subprojects are written using standard C++14
+Unless otherwise documented, LLVM subprojects are written using standard C++17
 code and avoid unnecessary vendor-specific extensions.
 
 Nevertheless, we restrict ourselves to features which are available in the
@@ -63,7 +63,13 @@ section `Software`).
 Each toolchain provides a good reference for what it accepts:
 
 * Clang: https://clang.llvm.org/cxx_status.html
-* GCC: https://gcc.gnu.org/projects/cxx-status.html#cxx14
+
+  * libc++: https://libcxx.llvm.org/Status/Cxx17.html
+
+* GCC: https://gcc.gnu.org/projects/cxx-status.html#cxx17
+
+  * libstdc++: https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017
+
 * MSVC: https://msdn.microsoft.com/en-us/library/hh567368.aspx
 
 

llvm/docs/ReleaseNotes.rst

@@ -47,6 +47,18 @@ Non-comprehensive list of changes in this release
 Update on required toolchains to build LLVM
 -------------------------------------------
 
+LLVM is now built with C++17 by default. This means C++17 can be used in
+the code base.
+
+The previous "soft" toolchain requirements have now been changed to "hard".
+This means that the the following versions are now required to build LLVM
+and there is no way to suppress this error.
+
+* GCC >= 7.1
+* Clang >= 5.0
+* Apple Clang >= 9.3
+* Visual Studio 2019 >= 16.7
+
 Changes to the LLVM IR
 ----------------------