[CMake] Warn when the version of CMake is older than 3.31.0 (#204203)

This is a preparation to raise the minimum required CMake version to
3.31.0 in LLVM 24.

See
https://discourse.llvm.org/t/rfc-raising-minimum-required-cmake-version-to-3-31/91086.

bolt/CMakeLists.txt

@@ -1,4 +1,11 @@
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
 
 set(LLVM_SUBPROJECT_TITLE "BOLT")
 

clang/CMakeLists.txt

@@ -1,4 +1,12 @@
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "Clang")
 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)

compiler-rt/CMakeLists.txt

@@ -4,6 +4,14 @@
 # based on the ability of the host toolchain to target various platforms.
 
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "Compiler-RT")
 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)

flang/CMakeLists.txt

@@ -1,4 +1,12 @@
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "Flang")
 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)

libc/CMakeLists.txt

@@ -1,4 +1,12 @@
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "libc")
 
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)

libclc/CMakeLists.txt

@@ -1,4 +1,11 @@
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
 
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   project(libclc VERSION 0.2.0 LANGUAGES CXX C)

libcxx/CMakeLists.txt

@@ -2,6 +2,14 @@
 # Setup Project
 #===============================================================================
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "libc++")
 
 # Unset the C++ Standard: other projects sometimes override this global property, which

libcxxabi/CMakeLists.txt

@@ -5,6 +5,14 @@
 #===============================================================================
 
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "libc++abi")
 
 set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")

libsycl/CMakeLists.txt

@@ -2,6 +2,13 @@
 # Setup Project
 #===============================================================================
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
 
 set(LLVM_SUBPROJECT_TITLE "libsycl")
 

libunwind/CMakeLists.txt

@@ -3,6 +3,14 @@
 #===============================================================================
 
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "libunwind")
 
 set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")

lld/CMakeLists.txt

@@ -1,4 +1,12 @@
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "LLD")
 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)

lldb/CMakeLists.txt

@@ -1,4 +1,12 @@
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "LLDB")
 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)

llvm-libgcc/CMakeLists.txt

@@ -3,6 +3,14 @@
 #===============================================================================
 
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "LLVM libgcc")
 
 set(LLVM_COMMON_CMAKE_UTILS "${CMAKE_CURRENT_SOURCE_DIR}/../cmake")

llvm/CMakeLists.txt

@@ -1,5 +1,12 @@
 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
 
 include(CMakeDependentOption)
 

mlir/CMakeLists.txt

@@ -1,5 +1,13 @@
 # MLIR project.
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "MLIR")
 
 if(NOT DEFINED LLVM_COMMON_CMAKE_UTILS)

offload/CMakeLists.txt

@@ -2,6 +2,14 @@
 # to build offload with CMake.
 
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "liboffload")
 
 # The minimum versions required for dependencies.

openmp/CMakeLists.txt

@@ -1,4 +1,12 @@
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
 set(LLVM_SUBPROJECT_TITLE "OpenMP")
 
 set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)

orc-rt/CMakeLists.txt

@@ -5,6 +5,13 @@
 #===============================================================================
 
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 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

polly/CMakeLists.txt

@@ -2,6 +2,14 @@
 if (NOT DEFINED LLVM_MAIN_SRC_DIR)
   project(Polly)
   cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
+
   set(POLLY_STANDALONE_BUILD TRUE)
 endif()
 set(LLVM_SUBPROJECT_TITLE "Polly")

runtimes/CMakeLists.txt

@@ -1,5 +1,12 @@
 # This file handles building LLVM runtime sub-projects.
 cmake_minimum_required(VERSION 3.20.0)
+if("${CMAKE_VERSION}" VERSION_LESS "3.31.0")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 24, the "
+    "minimum version of CMake required to build LLVM will become 3.31.0, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.31.0 now to avoid issues in the future.")
+endif()
 
 # This file can be used in two ways: the bootstrapping build calls it from
 # llvm/runtimes/CMakeLists.txt where we reuse the build tree of the top-level