Revert "Reland "[CMake] Bumps minimum version to 3.20.0.""

This reverts commit 65429b9af6a2c99d340ab2dcddd41dab201f399c.

Broke several projects, see https://reviews.llvm.org/D144509#4347562 onwards.

Also reverts follow-up commit "[OpenMP] Compile assembly files as ASM, not C"

This reverts commit 4072c8aee4c89c4457f4f30d01dc9bb4dfa52559.

Also reverts fix attempt  "[cmake] Set CMP0091 to fix Windows builds after the cmake_minimum_required bump"

This reverts commit 7d47dac5f828efd1d378ba44a97559114f00fb64.

8 files changed, 19 insertions, 10 deletions

diff --git a/openmp/CMakeLists.txt b/openmp/CMakeLists.txt
index 04678cbd4f33..e7403e5f60f6 100644
--- a/openmp/CMakeLists.txt
+++ b/openmp/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.20.0)
+cmake_minimum_required(VERSION 3.13.4)
 
 set(LLVM_COMMON_CMAKE_UTILS ${CMAKE_CURRENT_SOURCE_DIR}/../cmake)
 
@@ -11,7 +11,14 @@ list(INSERT CMAKE_MODULE_PATH 0
 # 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 ASM)
+  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(..)
diff --git a/openmp/cmake/DetectTestCompiler/CMakeLists.txt b/openmp/cmake/DetectTestCompiler/CMakeLists.txt
index 8ea7ab8d45ba..bc2aa52fbc1c 100644
--- a/openmp/cmake/DetectTestCompiler/CMakeLists.txt
+++ b/openmp/cmake/DetectTestCompiler/CMakeLists.txt
@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.20.0)
+cmake_minimum_required(VERSION 3.13.4)
 project(DetectTestCompiler C CXX)
 
 include(CheckCCompilerFlag)
diff --git a/openmp/docs/SupportAndFAQ.rst b/openmp/docs/SupportAndFAQ.rst
index c50433bb5b6e..249eb18b82a2 100644
--- a/openmp/docs/SupportAndFAQ.rst
+++ b/openmp/docs/SupportAndFAQ.rst
@@ -307,7 +307,7 @@ require a few additions.
 
 .. code-block:: cmake
 
-  cmake_minimum_required(VERSION 3.20.0)
+  cmake_minimum_required(VERSION 3.13.4)
   project(offloadTest VERSION 1.0 LANGUAGES CXX)
 
   list(APPEND CMAKE_MODULE_PATH "${PATH_TO_OPENMP_INSTALL}/lib/cmake/openmp")
@@ -318,7 +318,7 @@ require a few additions.
   target_link_libraries(offload PRIVATE OpenMPTarget::OpenMPTarget_NVPTX)
   target_sources(offload PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/src/Main.cpp)
 
-Using this module requires at least CMake version 3.20.0. Supported languages
+Using this module requires at least CMake version 3.13.4. Supported languages
 are C and C++ with Fortran support planned in the future. Compiler support is
 best for Clang but this module should work for other compiler vendors such as
 IBM, GNU.
diff --git a/openmp/libompd/src/CMakeLists.txt b/openmp/libompd/src/CMakeLists.txt
index 0402a0177201..f361fcf35a27 100644
--- a/openmp/libompd/src/CMakeLists.txt
+++ b/openmp/libompd/src/CMakeLists.txt
@@ -9,7 +9,7 @@
 #
 
 project (libompd)
-cmake_minimum_required(VERSION 3.20.0)
+cmake_minimum_required(VERSION 3.13.4)
 
 add_library (ompd SHARED TargetValue.cpp omp-debug.cpp omp-state.cpp omp-icv.cpp)
 
diff --git a/openmp/libomptarget/plugins/remote/src/CMakeLists.txt b/openmp/libomptarget/plugins/remote/src/CMakeLists.txt
index 8a16963c913a..6299fb38ee8b 100644
--- a/openmp/libomptarget/plugins/remote/src/CMakeLists.txt
+++ b/openmp/libomptarget/plugins/remote/src/CMakeLists.txt
@@ -10,7 +10,7 @@
 #
 ##===----------------------------------------------------------------------===##
 
-cmake_minimum_required(VERSION 3.20.0)
+cmake_minimum_required(VERSION 3.13.4)
 
 # Define the suffix for the runtime messaging dumps.
 add_definitions(-DTARGET_NAME=RPC)
diff --git a/openmp/runtime/src/CMakeLists.txt b/openmp/runtime/src/CMakeLists.txt
index bb5822264514..0e778ec14536 100644
--- a/openmp/runtime/src/CMakeLists.txt
+++ b/openmp/runtime/src/CMakeLists.txt
@@ -133,6 +133,8 @@ libomp_get_asmflags(LIBOMP_CONFIGURED_ASMFLAGS)
 # Set the compiler flags for each type of source
 set_source_files_properties(${LIBOMP_CXXFILES} PROPERTIES COMPILE_FLAGS "${LIBOMP_CONFIGURED_CXXFLAGS}")
 set_source_files_properties(${LIBOMP_ASMFILES} ${LIBOMP_GNUASMFILES} PROPERTIES COMPILE_FLAGS "${LIBOMP_CONFIGURED_ASMFLAGS}")
+# Let the compiler handle the GNU assembly files
+set_source_files_properties(${LIBOMP_GNUASMFILES} PROPERTIES LANGUAGE C)
 
 # Remove any cmake-automatic linking of the standard C++ library.
 # We neither need (nor want) the standard C++ library dependency even though we compile c++ files.
diff --git a/openmp/tools/Modules/FindOpenMPTarget.cmake b/openmp/tools/Modules/FindOpenMPTarget.cmake
index 424294090d5d..3591a49f685b 100644
--- a/openmp/tools/Modules/FindOpenMPTarget.cmake
+++ b/openmp/tools/Modules/FindOpenMPTarget.cmake
@@ -79,7 +79,7 @@ be used to override the standard flag searching for a given compiler.
 # TODO: Test more compilers
 
 cmake_policy(PUSH)
-cmake_policy(VERSION 3.20.0)
+cmake_policy(VERSION 3.13.4)
 
 find_package(OpenMP ${OpenMPTarget_FIND_VERSION} REQUIRED)
 
diff --git a/openmp/tools/Modules/README.rst b/openmp/tools/Modules/README.rst
index b8cc5f08b390..f19619e389b3 100644
--- a/openmp/tools/Modules/README.rst
+++ b/openmp/tools/Modules/README.rst
@@ -26,7 +26,7 @@ require a few additions.
 
 .. code-block:: cmake
 
-  cmake_minimum_required(VERSION 3.20.0)
+  cmake_minimum_required(VERSION 3.13.4)
   project(offloadTest VERSION 1.0 LANGUAGES CXX)
 
   list(APPEND CMAKE_MODULE_PATH "${PATH_TO_OPENMP_INSTALL}/lib/cmake/openmp")
@@ -37,7 +37,7 @@ require a few additions.
   target_link_libraries(offload PRIVATE OpenMPTarget::OpenMPTarget_NVPTX)
   target_sources(offload PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/src/Main.cpp)
 
-Using this module requires at least CMake version 3.20.0. Supported languages
+Using this module requires at least CMake version 3.13.4. Supported languages
 are C and C++ with Fortran support planned in the future. If your application
 requires building for a specific device architecture you can set the
 ``OpenMPTarget_<device>_ARCH=<flag>`` variable. Compiler support is best for