[CMake] Warn when the version is older than 3.20.0.

[Mordante]

Summary

This is a preparation to require CMake 3.20.0 after LLVM 16 has been
released.

This change has been discussed on discourse
https://discourse.llvm.org/t/rfc-upgrading-llvms-minimum-required-cmake-version/66193

[thieta]

I think this is fine as we have discussed before. But I really dislike the code duplication for the check. We could put it in a include() I guess - but maybe it's not worth it.

[Mordante]

In D137724#3917616, @thieta wrote:

I think this is fine as we have discussed before. But I really dislike the code duplication for the check. We could put it in a include() I guess - but maybe it's not worth it.

I wanted to make sure it shows up in different build scenarios. The code is temporary, once we switch to CMake 3.20.0 we can just change the cmake_minimum_required and remove these verbose blocks.

[MaskRay]

I think if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR) checks for standalone builds is not necessary. The check in llvm/CMakeLists.txt suffices.
It's unlikely the users will use different cmake versions to configure llvm and a subproject like clang so that: when the cmake used for llvm satisfies the check, the cmake used for clang does not satisfy the check.

[Mordante]

In D137724#3917644, @MaskRay wrote:

I think if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR) checks for standalone builds is not necessary. The check in llvm/CMakeLists.txt suffices.
It's unlikely the users will use different cmake versions to configure llvm and a subproject like clang.

I can remove the others, but we need to keep the one in runtimes/CMakeLists.txt too. The runtimes (libc++, libc++abi, and libunwind) can be build without building or configuring LLVM.

[ldionne]

LGTM (sorry, it looks like I approved on behalf of all libc++ vendors but that wasn't my intention).

[mgorny]

clang/CMakeLists.txt

14

I wonder if we could move this to CMakePolicy.cmake, though admittedly you'd have to somehow avoid warning multiple times in in-tree builds.

[ChuanqiXu]

LGTM. Thanks.

[phosek]

LGTM

[to268]

LGTM

[thieta]

I think this is ready to land @Mordante or is there anything else missing?

[Mordante]

Thanks for all reviews!

In D137724#3974764, @thieta wrote:

I think this is ready to land @Mordante or is there anything else missing?

No but I've been quite busy. I needed some time to find the list of buildbot owners to mail after landing this.

clang/CMakeLists.txt

14

Yes I prefer to keep it this way. The warning will be removed once 3.20.0 or newer is mandated.

clang/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)
   set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 endif()
 include(${LLVM_COMMON_CMAKE_UTILS}/Modules/CMakePolicy.cmake
   NO_POLICY_SCOPE)
 
 # If we are not building as a part of LLVM, build Clang as an
 # standalone project, using LLVM as an external library:
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(Clang)
   set(CLANG_BUILT_STANDALONE TRUE)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 if(CLANG_BUILT_STANDALONE)
   set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
   set(CMAKE_CXX_STANDARD_REQUIRED YES)

compiler-rt/CMakeLists.txt

 # CMake build for CompilerRT.
 #
 # An important constraint of the build is that it only produces libraries
 # based on the ability of the host toolchain to target various platforms.
 
 cmake_minimum_required(VERSION 3.13.4)
 
 # Check if compiler-rt is built as a standalone project.
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR COMPILER_RT_STANDALONE_BUILD)
   project(CompilerRT C CXX ASM)
   set(COMPILER_RT_STANDALONE_BUILD TRUE)
   set_property(GLOBAL PROPERTY USE_FOLDERS ON)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 endif()
 
 set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")
 
 # Add path for custom compiler-rt modules.
 list(INSERT CMAKE_MODULE_PATH 0
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules"

compiler-rt/lib/builtins/CMakeLists.txt

 # This directory contains a large amount of C code which provides
 # generic implementations of the core runtime library along with optimized
 # architecture-specific code in various subdirectories.
 
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   cmake_minimum_required(VERSION 3.13.4)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 
   set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
   project(CompilerRTBuiltins C ASM)
   set(COMPILER_RT_STANDALONE_BUILD TRUE)
   set(COMPILER_RT_BUILTINS_STANDALONE_BUILD TRUE)
 
   set(COMPILER_RT_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../..")
 

compiler-rt/lib/crt/CMakeLists.txt

 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   cmake_minimum_required(VERSION 3.13.4)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 
   set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
   project(CompilerRTCRT C)
   set(COMPILER_RT_STANDALONE_BUILD TRUE)
   set(COMPILER_RT_CRT_STANDALONE_BUILD TRUE)
 
   set(COMPILER_RT_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../..")
 

flang/CMakeLists.txt

 option(FLANG_ENABLE_WERROR "Fail and stop building flang if a warning is triggered." OFF)
 
 # Check for a standalone build and configure as appropriate from
 # there.
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   message("Building Flang as a standalone project.")
   project(Flang)
   set(FLANG_STANDALONE_BUILD ON)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 else()
   set(FLANG_STANDALONE_BUILD OFF)
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 # MSVC + clang-cl build requires clang_rt.builtin.${target} library

flang/runtime/CMakeLists.txt

 #===-- runtime/CMakeLists.txt ----------------------------------------------===#
 #
 # Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 # See https://llvm.org/LICENSE.txt for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 #
 #===------------------------------------------------------------------------===#
 
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   cmake_minimum_required(VERSION 3.13.4)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 
   project(FlangRuntime C CXX)
 
   set(CMAKE_CXX_STANDARD 17)
   set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
   set(CMAKE_CXX_EXTENSIONS OFF)
 
   set(FLANG_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/..")

lld/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)
   set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 endif()
 include(${LLVM_COMMON_CMAKE_UTILS}/Modules/CMakePolicy.cmake
   NO_POLICY_SCOPE)
 
 # If we are not building as a part of LLVM, build LLD as an
 # standalone project, using LLVM as an external library:
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(lld)
   set(LLD_BUILT_STANDALONE TRUE)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 if(LLD_BUILT_STANDALONE)
   set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
   set(CMAKE_CXX_STANDARD_REQUIRED YES)

lldb/CMakeLists.txt

set(CMAKE_MODULE_PATH
   )
 
 # If we are not building as part of LLVM, build LLDB as a standalone project,
 # using LLVM as an external library.
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(lldb)
   set(LLDB_BUILT_STANDALONE TRUE)
   set(LLVM_INCLUDE_TESTS ON CACHE INTERNAL "")
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 option(LLDB_INCLUDE_TESTS "Generate build targets for the LLDB unit tests." ${LLVM_INCLUDE_TESTS})
 
 if(LLDB_BUILT_STANDALONE)

lldb/tools/debugserver/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 
 project(Debugserver LANGUAGES C CXX ASM-ATT)
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   set(LLDB_BUILT_STANDALONE TRUE)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 
   set(CMAKE_MODULE_PATH
     ${CMAKE_MODULE_PATH}
     "${CMAKE_SOURCE_DIR}/../../cmake"
     "${CMAKE_SOURCE_DIR}/../../cmake/modules"
     )
 
   include(LLDBStandalone)

llvm/CMakeLists.txt

 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 cmake_minimum_required(VERSION 3.13.4)
+if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+    "minimum version of CMake required to build LLVM will become 3.20.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.20.0 now to avoid issues in the future!")
+endif()
 
 set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 include(${LLVM_COMMON_CMAKE_UTILS}/Modules/CMakePolicy.cmake
   NO_POLICY_SCOPE)
 
 set(CMAKE_BUILD_WITH_INSTALL_NAME_DIR ON)
 
 if(NOT DEFINED LLVM_VERSION_MAJOR)

llvm/docs/ReleaseNotes.rst

 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 >= 10.0
 * Visual Studio 2019 >= 16.7
 
+With LLVM 16.x we will raise the version requirement of CMake used to build
+LLVM. The new requirements are as follows:
+
+* CMake >= 3.20.0
+
+In LLVM 16.x this requirement will be "soft", there will only be a diagnostic.
+
+With the release of LLVM 17.x this requirement will be hard and LLVM developers
+can start using CMake 3.20.0 features, making it impossible to build with older
+versions of CMake.
+
 Changes to the LLVM IR
 ----------------------
 
 * The ``readnone``, ``readonly``, ``writeonly``, ``argmemonly``,
   ``inaccessiblememonly`` and ``inaccessiblemem_or_argmemonly`` function
   attributes have been replaced by a single ``memory(...)`` attribute. The
   old attributes may be mapped to the new one as follows:
 

mlir/CMakeLists.txt

 # MLIR project.
 cmake_minimum_required(VERSION 3.13.4)
 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)
   set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 endif()
 include(${LLVM_COMMON_CMAKE_UTILS}/Modules/CMakePolicy.cmake
   NO_POLICY_SCOPE)
 
 # Check if MLIR is built as a standalone project.
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(mlir)
   set(MLIR_STANDALONE_BUILD TRUE)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 if(MLIR_STANDALONE_BUILD)
   find_package(LLVM CONFIG REQUIRED)
   set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${LLVM_CMAKE_DIR})

openmp/CMakeLists.txt

 cmake_minimum_required(VERSION 3.13.4)
 
 set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 
 # Add path for custom modules
 list(INSERT CMAKE_MODULE_PATH 0
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
   "${LLVM_COMMON_CMAKE_UTILS}/Modules"
   )
 
 # llvm/runtimes/ will set OPENMP_STANDALONE_BUILD.
 if (OPENMP_STANDALONE_BUILD OR "${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_SOURCE_DIR}")
   set(OPENMP_STANDALONE_BUILD TRUE)
   project(openmp C CXX)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 if (OPENMP_STANDALONE_BUILD)
   # CMAKE_BUILD_TYPE was not set, default to Release.
   if (NOT CMAKE_BUILD_TYPE)

polly/CMakeLists.txt

 # Check if this is a in tree build.
 if (NOT DEFINED LLVM_MAIN_SRC_DIR)
   project(Polly)
   cmake_minimum_required(VERSION 3.13.4)
+  if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+    message(WARNING
+      "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+      "minimum version of CMake required to build LLVM will become 3.20.0, and "
+      "using an older CMake will become an error. Please upgrade your CMake to "
+      "at least 3.20.0 now to avoid issues in the future!")
+  endif()
   set(POLLY_STANDALONE_BUILD TRUE)
 endif()
 
 # Must go below project(..)
 include(GNUInstallDirs)
 
 if(POLLY_STANDALONE_BUILD)
   # Where is LLVM installed?

runtimes/CMakeLists.txt

 # This file handles building LLVM runtime sub-projects.
 cmake_minimum_required(VERSION 3.13.4)
+if ("${CMAKE_VERSION}" VERSION_LESS "3.20.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 17.0.0, the "
+    "minimum version of CMake required to build LLVM will become 3.20.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.20.0 now to avoid issues in the future!")
+endif()
 project(Runtimes C CXX ASM)
 
 # Add path for custom and the LLVM build's modules to the CMake module path.
 set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")
 list(INSERT CMAKE_MODULE_PATH 0
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules"
   "${LLVM_COMMON_CMAKE_UTILS}"