[OpenMP] Set cmake policies CMP0074 and CMP0075 to NEW

[protze.joachim]

Summary

Building with cmake 3.16, I get the following warning:

CMake Warning (dev) at /usr/share/cmake-3.16/Modules/CheckIncludeFiles.cmake:120 (message):
  Policy CMP0075 is not set: Include file check macros honor
  CMAKE_REQUIRED_LIBRARIES.  Run "cmake --help-policy CMP0075" for policy
  details.  Use the cmake_policy command to set the policy and suppress this
  warning.

  CMAKE_REQUIRED_LIBRARIES is set to:

    m

  For compatibility with CMake 3.11 and below this check is ignoring it.
Call Stack (most recent call first):
  /home/jprotze/OMPD/llvm-project/openmp/runtime/cmake/config-ix.cmake:223 (check_include_files)
  /home/jprotze/OMPD/llvm-project/openmp/runtime/CMakeLists.txt:223 (include)
This warning is for project developers.  Use -Wno-dev to suppress it.

The cmake_minimum_required command in the OpenMP directory overwrites the policy setting of llvm. Also, we want to support stand-alone builds, so we should set the policy.

lldb introduced the same patch in : https://reviews.llvm.org/D58279

[ye-luo]

A side question. Currently requirement is 2.8 but LLVM requires 3.4.3 and LLVM 12 will require 3.13.4. Should we align with LLVM?

[protze.joachim]

OpenMP can be built stand-alone. Some HPC centers have quite old OS on their systems, therefore we only bumped cmake requirement if really necessary.

[ye-luo]

In D82074#2101026, @protze.joachim wrote:

OpenMP can be built stand-alone. Some HPC centers have quite old OS on their systems, therefore we only bumped cmake requirement if really necessary.

"We only bumped cmake requirement if really necessary." I agree to this to some extend.
"Some HPC centers have quite old OS on their systems, therefore we only bumped cmake requirement if really necessary." I don't agree with this.
HPC centers got constant requests for new cmake from cmake users and they always provide quite new cmake (within 2 years). They never relies on system cmake.
If there is only the old system cmake, building cmake is fairly easy. If sb is not able to build cmake, that person probably doesn't have the need of building libomp standalone.

Comes to the need. CMake had many rounds of change for handling CUDA since 2.8 and I don't believe current 2.8 is still valid.
I also think re-trigger cmake when make is caused by some legacy way of *.cu handling in CMake.

LLVM cmake requirement is very conservative. Most people builds openmp with llvm and the cmake version is getting validated.
It is bad someone comes and say openmp cannot be built and we find it was caused by an old cmake between 2.8 and LLVM required version.

[protze.joachim]

On a different system, I also get warnings for CMP0074.
The two policies were introduced with cmake 3.12. As long as we don't bump the required cmake version to at least 3.12, we need to set the policies.

[protze.joachim]

The cmake version was bumped to 3.14

[ye-luo]

In D82074#2173271, @protze.joachim wrote:

The cmake version was bumped to 3.14

Shall we bump ​cmake_minimum_required here as well?

[protze.joachim]

afa1afd4108d changed the minimum required version in all subdirectories, including openmp

edit: the bump was to 3.13.4

openmp/CMakeLists.txt

 cmake_minimum_required(VERSION 2.8 FATAL_ERROR)
 
+if(POLICY CMP0074)
+  # Warning: Policy CMP0074 is not set: find_package uses <PackageName>_ROOT variables.
+  cmake_policy(SET CMP0074 NEW)
+endif()
+
+if(POLICY CMP0075)
+  # Warning: Policy CMP0075 is not set: Include file check macros honor
+  cmake_policy(SET CMP0075 NEW)
+endif()
+
 # Add cmake directory to search for custom cmake functions.
 set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake ${CMAKE_MODULE_PATH})
 
 # 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)