[LLVM] Removes CMake work-arounds.

[Mordante]

Summary

CMake older than 3.20.0 is no longer supported.
This removes work-arounds for no longer supported versions.

[h-vetinari]

Is this aiming to be exhaustive? For example, there's some still left in libunwind/src/CMakeLists.txt, as came up in https://reviews.llvm.org/D151344

[Mordante]

In D152103#4394617, @h-vetinari wrote:

Is this aiming to be exhaustive? For example, there's some still left in libunwind/src/CMakeLists.txt, as came up in https://reviews.llvm.org/D151344

This should be everything in the LLVM directory, the range [D152099, D152103) contains the other directories.

[h-vetinari]

Old CMake mentioned in llvm-owned files:

• https://github.com/llvm/llvm-project/blob/main/llvm/cmake/modules/LLVMCheckLinkerFlag.cmake#L8
• https://github.com/llvm/llvm-project/blob/main/llvm/cmake/modules/AddLLVM.cmake#L2199

There's also a bunch in the docs (primarily https://github.com/llvm/llvm-project/blob/main/llvm/docs/CMakePrimer.rst, but also here & here) though that's probably more work.

llvm/CMakeLists.txt

101–113

Marked as up for simplification if CMake >=3.17

[Mordante]

In D152103#4397757, @h-vetinari wrote:

Old CMake mentioned in llvm-owned files:

• https://github.com/llvm/llvm-project/blob/main/llvm/cmake/modules/LLVMCheckLinkerFlag.cmake#L8
• https://github.com/llvm/llvm-project/blob/main/llvm/cmake/modules/AddLLVM.cmake#L2199

I feel changing this is out of scope. I agree it would be nice to fix this, but I will not pursue it. Feel free to provide patches for these improvements.

There's also a bunch in the docs (primarily https://github.com/llvm/llvm-project/blob/main/llvm/docs/CMakePrimer.rst, but also here & here) though that's probably more work.

Thanks I'll fix this.

[kwk]

LGTM

llvm/CMakeLists.txt

Available options are:
   * -DCMAKE_BUILD_TYPE=Release - For an optimized build with no assertions or debug info.
   * -DCMAKE_BUILD_TYPE=Debug - For an unoptimized build with assertions and debug info.
   * -DCMAKE_BUILD_TYPE=RelWithDebInfo - For an optimized build with no assertions but with debug info.
   * -DCMAKE_BUILD_TYPE=MinSizeRel - For a build optimized for size instead of speed.
 Learn more about these options in our documentation at https://llvm.org/docs/CMake.html#cmake-build-type
 ")
 endif()
 
 # Set default build type for cmake's try_compile module.
 # CMake 3.17 or newer sets CMAKE_DEFAULT_BUILD_TYPE to one of the
 # items from CMAKE_CONFIGURATION_TYPES. Logic below can be further
 # simplified once LLVM's minimum CMake version is updated to 3.17.
 if(CMAKE_DEFAULT_BUILD_TYPE)
   set(CMAKE_TRY_COMPILE_CONFIGURATION ${CMAKE_DEFAULT_BUILD_TYPE})
 else()
   if(CMAKE_CONFIGURATION_TYPES)
     list(GET CMAKE_CONFIGURATION_TYPES 0 CMAKE_TRY_COMPILE_CONFIGURATION)
   elseif(CMAKE_BUILD_TYPE)
     set(CMAKE_TRY_COMPILE_CONFIGURATION ${CMAKE_BUILD_TYPE})
   endif()
 endif()
 
 # Side-by-side subprojects layout: automatically set the
 # LLVM_EXTERNAL_${project}_SOURCE_DIR using LLVM_ALL_PROJECTS
 # This allows an easy way of setting up a build directory for llvm and another
 # one for llvm+clang+... using the same sources.
 set(LLVM_ALL_PROJECTS "bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;lld;lldb;mlir;openmp;polly;pstl")
 # The flang project is not yet part of "all" projects (see C++ requirements)
 set(LLVM_EXTRA_PROJECTS "flang")

 endif()
 
 # Build with _XOPEN_SOURCE on AIX, as stray macros in _ALL_SOURCE mode tend to
 # break things. In this case we need to enable the large-file API as well.
 if (UNIX AND ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
           add_compile_definitions(_XOPEN_SOURCE=700)
           add_compile_definitions(_LARGE_FILE_API)
 
-  # CMake versions less than 3.16 set default linker flags to include -brtl, as
-  # well as setting -G when building libraries, so clear them out. Note we only
-  # try to clear the form that CMake will set as part of its initial
-  # configuration, it is still possible the user may force it as part of a
-  # compound option.
-  if(CMAKE_VERSION VERSION_LESS 3.16)
-    string(REGEX REPLACE "(^|[ \t]+)-Wl,-brtl([ \t]+|$)" " " CMAKE_EXE_LINKER_FLAGS  "${CMAKE_EXE_LINKER_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)-Wl,-brtl([ \t]+|$)" " " CMAKE_SHARED_LINKER_FLAGS  "${CMAKE_SHARED_LINKER_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)-Wl,-brtl([ \t]+|$)" " " CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)(-Wl,)?-G([ \t]+|$)" " " CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS
-      "${CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)(-Wl,)?-G([ \t]+|$)" " " CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS
-      "${CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)(-Wl,)?-G([ \t]+|$)" " " CMAKE_SHARED_LIBRARY_CREATE_ASM_FLAGS
-      "${CMAKE_SHARED_LIBRARY_CREATE_ASM_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)-Wl,-G," " -Wl," CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS
-      "${CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)-Wl,-G," " -Wl," CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS
-      "${CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS}")
-    string(REGEX REPLACE "(^|[ \t]+)-Wl,-G," " -Wl," CMAKE_SHARED_LIBRARY_CREATE_ASM_FLAGS
-      "${CMAKE_SHARED_LIBRARY_CREATE_ASM_FLAGS}")
-  endif()
-
   # Modules should be built with -shared -Wl,-G, so we can use runtime linking
   # with plugins.
   string(APPEND CMAKE_MODULE_LINKER_FLAGS " -shared -Wl,-G")
 
   # Also set the correct flags for building shared libraries.
   string(APPEND CMAKE_SHARED_LINKER_FLAGS " -shared")
 endif()
 

llvm/docs/CMakePrimer.rst

 functions, and defined macros. The CMake distribution also contains a suite of
 CMake modules that contain definitions for useful functionality.
 
 The example below is the full CMake build for building a C++ "Hello World"
 program. The example uses only CMake language-defined functions.
 
 .. code-block:: cmake
 
-   cmake_minimum_required(VERSION 3.15)
+   cmake_minimum_required(VERSION 3.20.0)
    project(HelloWorld)
    add_executable(HelloWorld HelloWorld.cpp)
 
 The CMake language provides control flow constructs in the form of foreach loops
 and if blocks. To make the example above more complicated you could add an if
 block to define "APPLE" when targeting Apple platforms:
 
 .. code-block:: cmake
 
-   cmake_minimum_required(VERSION 3.15)
+   cmake_minimum_required(VERSION 3.20.0)
    project(HelloWorld)
    add_executable(HelloWorld HelloWorld.cpp)
    if(APPLE)
      target_compile_definitions(HelloWorld PUBLIC APPLE)
    endif()
 
 Variables, Types, and Scope
 ===========================