[cmake] Make check_linker_flags operate via linker flags

[smeenai]

Summary

check_linker_flags currently sets the *compiler* flags (via
CMAKE_REQUIRED_FLAGS), and thus implicitly relies on cmake's default
behavior of passing the compiler flags to the linker. This breaks when
cmake's build rules have been altered to not pollute the link line with
compiler flags (which can be desirable for build cleanliness). Instead,
set CMAKE_EXE_LINKER_FLAGS explicitly and use CMP0056 to ensure the
linker flags are passed along. Additionally, since we're inside a
function, we can just alter the variable directly (as the alteration
will be limited to the scope of the function) rather than saving and
restoring the old value.

[compnerd]

LG if none of the subtree projects are negatively impacted by this.

CMakeLists.txt

20 ↗

(On Diff #120884)

Since this impacts all try_compile, can you ensure that libc++ in tree builds are not impacted by this change?

[smeenai]

CMakeLists.txt

20 ↗

(On Diff #120884)

I confirmed that clang, LLD and libc++ all build correctly after this change.

llvm/trunk/CMakeLists.txt

if (POLICY CMP0051)
   # CMake 3.1 and higher include generator expressions of the form
   # $<TARGETLIB:obj> in the SOURCES property.  These need to be
   # stripped everywhere that access the SOURCES property, so we just
   # defer to the OLD behavior of not including generator expressions
   # in the output for now.
   cmake_policy(SET CMP0051 OLD)
 endif()
 
+if(POLICY CMP0056)
+  cmake_policy(SET CMP0056 NEW)
+endif()
+
 if(POLICY CMP0057)
   cmake_policy(SET CMP0057 NEW)
 endif()
 
 if(NOT DEFINED LLVM_VERSION_MAJOR)
   set(LLVM_VERSION_MAJOR 6)
 endif()
 if(NOT DEFINED LLVM_VERSION_MINOR)

llvm/trunk/cmake/modules/CheckLinkerFlag.cmake

 include(CheckCXXCompilerFlag)
 
 function(check_linker_flag flag out_var)
-  set(OLD_CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS}")
-  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${flag}")
+  set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${flag}")
   check_cxx_compiler_flag("" ${out_var})
-  set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
 endfunction()