[cmake] Properly support target properties.

[stephenneuendorffer]

Summary

It's sometimes useful to use these directives when dealing with
external projects:
target_link_directories
target_link_libraries
target_include_directories

However, under certain circumstances,
llvm_add_library can generate multiple targets. We need to transfer
these properties to the new targets. Note that using a generator
expression is necessary because these properties will only be set
after llvm_add_library is called.

[stellaraccident]

Read my mind - was meaning to do this very change last week but ran out of time. How much further are you going? I'd ultimately like our targets to all have proper include/link directories defined so we don't need to do so much global munging.

I also frequently run into the problem that when object libraries are involved, you can only set compile flags on the object library, but that requires knowing whether an object library was used to define the target. It makes writing generic utilities brittle - and worse: in MLIR we often decide to use an object library or not based on what link mode is in use (and if not xcode). I haven't quite convinced myself that this forwarding trick is the right thing for such cases.

[bondhugula]

Thanks for fixing this!

llvm/cmake/modules/AddLLVM.cmake

489

Nit: terminate with a full stop - here and below.

533

Is there something to fix for target_link_libraries?
target_link_directories is to be rarely used per https://cmake.org/cmake/help/latest/command/target_link_libraries.html#command:target_link_libraries
Instead it's target_link_libraries is preferred.

[stephenneuendorffer]

In D108098#2945907, @stellaraccident wrote:

Read my mind - was meaning to do this very change last week but ran out of time. How much further are you going? I'd ultimately like our targets to all have proper include/link directories defined so we don't need to do so much global munging.

I'm not necessarily doing more with this.. mostly trying to help Uday :)

I also frequently run into the problem that when object libraries are involved, you can only set compile flags on the object library, but that requires knowing whether an object library was used to define the target. It makes writing generic utilities brittle - and worse: in MLIR we often decide to use an object library or not based on what link mode is in use (and if not xcode). I haven't quite convinced myself that this forwarding trick is the right thing for such cases.

Yeah, I definitely don't think it's the right thing to blindly forward *ALL* the myriad target properties. But I convinced myself that at least in the short term this was the right thing to do small... Let's start by expanding your handling of shared libraries to libMLIR.so, then we can see how it can be used to cleanup the bigger LLVM issues.

llvm/cmake/modules/AddLLVM.cmake

if(CMAKE_GENERATOR STREQUAL "Xcode")
       file(WRITE ${DUMMY_FILE} "// This file intentionally empty\n")
       set_property(SOURCE ${DUMMY_FILE} APPEND_STRING PROPERTY COMPILE_FLAGS "-Wno-empty-translation-unit")
     endif()
     set(ALL_FILES "$<TARGET_OBJECTS:${obj_name}>" ${DUMMY_FILE})
 
     # Do add_dependencies(obj) later due to CMake issue 14747.
     list(APPEND objlibs ${obj_name})
 
+    # Bring in the target include directories from our original target.
+    target_include_directories(${obj_name} PRIVATE $<TARGET_PROPERTY:${name},INCLUDE_DIRECTORIES>)
+
     set_target_properties(${obj_name} PROPERTIES FOLDER "Object Libraries")
     if(ARG_DEPENDS)
       add_dependencies(${obj_name} ${ARG_DEPENDS})
     endif()
     # Treat link libraries like PUBLIC dependencies.  LINK_LIBS might
     # result in generating header files.  Add a dependendency so that
     # the generated header is created before this object library.
     if(ARG_LINK_LIBS)
if(ARG_SHARED AND ARG_STATIC)
     endif()
     # DEPENDS has been appended to LLVM_COMMON_LIBS.
     llvm_add_library(${name_static} STATIC
       ${output_name}
       OBJLIBS ${ALL_FILES} # objlib
       LINK_LIBS ${ARG_LINK_LIBS}
       LINK_COMPONENTS ${ARG_LINK_COMPONENTS}
       )
+
+    # Bring in the target link info from our original target.
+    target_link_directories(${name_static} PRIVATE $<TARGET_PROPERTY:${name},LINK_DIRECTORIES>)
+    target_link_libraries(${name_static} PRIVATE $<TARGET_PROPERTY:${name},LINK_LIBRARIES>)
+
     # FIXME: Add name_static to anywhere in TARGET ${name}'s PROPERTY.
     set(ARG_STATIC)
   endif()
 
   if(ARG_MODULE)
     add_library(${name} MODULE ${ALL_FILES})
   elseif(ARG_SHARED)
     add_windows_version_resource_file(ALL_FILES ${ALL_FILES})