VERSION_GREATER_EQUAL not supported in llvm's minimum required cmake version

[rs]

Summary

Patch https://reviews.llvm.org/D56329 caused build failures for me when building on Windows because of the use of cmake operator 'VERSION_GREATER_EQUAL' which isn't supported in older versions of cmake. The llvm website states that minimum required version of cmake for building llvm is 3.4.3 https://llvm.org/docs/CMake.html

[aganea]

Thank you for fixing this! Looks like VERSION_GREATER_EQUAL is there only since cmake 3.7. However the minimum required cmake for LLVM is about three years old. @rnk is it the cmake policy the same as for the compiler, that is, keep compatibility for at least three years?

[rnk]

In D57326#1373588, @aganea wrote:

Thank you for fixing this! Looks like VERSION_GREATER_EQUAL is there only since cmake 3.7. However the minimum required cmake for LLVM is about three years old. @rnk is it the cmake policy the same as for the compiler, that is, keep compatibility for at least three years?

I think the answer is yes, we want about three years of cmake backwards compatibility. The goal of that is to make sure anyone can check out Linux in a random Linux distro from the last few years and they'll have a cmake version that works. Amusingly, developers on Windows and Mac tend to have more up-to-date developer tools, since they don't rely on a system package manager.

llvm/trunk/unittests/Support/CMakeLists.txt

 # disabling all warnings for all the compilers by using pragmas.
 # Don't disable on MSVC, because all incriminated warnings are already disabled
 # in source; and because we would otherwise see this warning:
 #   cl : Command line warning D9025: overriding '/W4' with '/w'
 if(NOT MSVC)
   set_source_files_properties(AlignOfTest.cpp PROPERTIES COMPILE_FLAGS -w)
 endif()
 if(MSVC)
-  if( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 19.15 )
+  if( CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 19.14 )
     # Since VS2017 15.8, the following snippet: Failed<CustomSubError>()
     # generates a warning:
     # \svn\llvm\utils\unittest\googlemock\include\gmock\gmock-matchers.h(186):
     #   warning C5046: 'testing::MatcherInterface<T>::~MatcherInterface': Symbol involving type with internal linkage not defined
     set_source_files_properties(ErrorTest.cpp PROPERTIES COMPILE_FLAGS -wd5046)
   endif()
 endif()
 
 # ManagedStatic.cpp uses <pthread>.
 target_link_libraries(SupportTests PRIVATE LLVMTestingSupport ${LLVM_PTHREAD_LIB})
 
 add_subdirectory(DynamicLibrary)