Use object library if cmake supports it

[tatyana-krasnukha]

Summary

Using $<TARGET_OBJECTS:...> in anything other than add_library and add_executable was not supported until cmake 3.9.
The issue was added with https://reviews.llvm.org/D55653.

[labath]

why not just change lldb-mi-utils into a conventional (STATIC) library, at which point it can just be linked in using regular target_link_libraries, which has worked since forever?

[tatyana-krasnukha]

It will cause some time overhead for linking a library. Linking is already formidable for the project and I'd avoid it when possible.

[hintonda]

tools/lldb/unittests/tools/lldb-mi/utils/CMakeLists.txt

16 ↗

(On Diff #205556)

I'm assuming from your comment, that this is a performance issue. If so, could you add a comment and also update the commit message so people can appreciate the need for the added complexity?

Btw, just tested with 3.4.3, and your change made cmake happy. Thanks!

[hintonda]

Otherwise, LGTM, thanks!

[hintonda]

I just reviewed all the release notes, and while 3.9 relaxed the usage of TARGET_OBJECTS, cmake doesn't explicitly mention allowing them in target_link_libraries until release 3.15.

I don't have any of those versions, but it looks like 3.15 is probably the version you need to use instead of 3.9. Please see: https://cmake.org/cmake/help/latest/release/3.15.html

[tatyana-krasnukha]

As I figured out, cmake allows to use $<TARGET_OBJECTS:...> anywhere since version 3.9 (commit).

After the version 3.12 (commit) object libraries may be used as a right-hand side of target_link_libraries.

Version 3.15 allows to use $<TARGET_OBJECTS:...> expression for other types of libraries, but this change doesn't affect the patch.

[tatyana-krasnukha]

Finally updated versions - target_sources supports using $<TARGET_OBJECTS:...> since CMake 3.5.0.

[hintonda]

In D63544#1551939, @tatyana-krasnukha wrote:

Finally updated versions - target_sources supports using $<TARGET_OBJECTS:...> since CMake 3.5.0.

Great, thanks for narrowing that down,

Still LGTM!

[beanz]

I really don't think this is the right solution.

$<TARGET_OBJECTS:lldb-mi-utils> can be passed in as source files in the unit test target, which should work just fine on CMake 3.4.x.

[tatyana-krasnukha]

@beanz you're absolutely right, thank you.

lldb/trunk/unittests/tools/lldb-mi/utils/CMakeLists.txt

-add_library(lldb-mi-utils OBJECT
+# Create object library to avoid unnecessary linking. If CMake version
+# doesn't support it, just create an ordinary library.
+if(NOT (${CMAKE_VERSION} VERSION_LESS "3.5.0"))
+  set (LIBRARY_TYPE "OBJECT")
+endif()
+
+add_library(lldb-mi-utils ${LIBRARY_TYPE}
   ${LLDB_SOURCE_DIR}/tools/lldb-mi/MIUtilString.cpp
   )
 
 add_lldb_unittest(LLDBMiUtilTests
   StringTest.cpp
 
   LINK_COMPONENTS
     Support
   )
 
+if((${CMAKE_VERSION} VERSION_LESS "3.5.0") OR
+    NOT (${CMAKE_VERSION} VERSION_LESS "3.12.0"))
+  # Link to either usual (cmake version < 3.5) or
+  # object (cmake version >= 3.12) library with lldm-mi sources.
+  target_link_libraries(LLDBMiUtilTests PRIVATE lldb-mi-utils)
+else()
+  # Object libraries still cannot be on the right-hand side of
+  # `target_link_libraries` but `$<TARGET_OBJECTS:objlib>` expression is
+  # already supported in `target_sources`.
   target_sources(LLDBMiUtilTests PRIVATE $<TARGET_OBJECTS:lldb-mi-utils>)
+endif()
+
 set_target_properties(lldb-mi-utils PROPERTIES FOLDER "lldb libraries")