[libc] Cleanup the CMake infrastructure to add startup objects.

[sivachandra]

Summary

Instead of using a custom target to copy the startup object file to a
file with the desired name, a normal object library with a special
property is used.

Follow up patches will do more cleanup wrt how the startup objects are
used in integration tests.

libc/cmake/modules/LLVMLibCTestRules.cmake

 #     HDRS [hdr1.cpp ...]
 #     STARTUP <fully qualified startup system target name>
 #     DEPENDS <list of entrypoint or other object targets>
 #     ARGS <list of command line arguments to be passed to the test>
 #     ENV <list of environment variables to set before running the test>
 #     COMPILE_OPTIONS <list of special compile options for this target>
 #   )
 #
-# The startup target should provide a property named STARTUP_OBJECT which is
-# the full path to the object file produced when the startup system is built.
-#
 # The DEPENDS list can be empty. If not empty, it should be a list of
 # targets added with add_entrypoint_object or add_object_library.
 function(add_integration_test test_name)
   get_fq_target_name(${test_name} fq_target_name)
   if(NOT (${LIBC_TARGET_OS} STREQUAL "linux"))
     message(STATUS "Skipping ${fq_target_name} as it is not available on ${LIBC_TARGET_OS}.")
     return()
   endif()
function(add_integration_test test_name)
   endif()
 
   # Create a sysroot structure
   set(sysroot ${CMAKE_CURRENT_BINARY_DIR}/${test_name}/sysroot)
   file(MAKE_DIRECTORY ${sysroot})
   file(MAKE_DIRECTORY ${sysroot}/include)
   set(sysroot_lib ${sysroot}/lib)
   file(MAKE_DIRECTORY ${sysroot_lib})
-  get_target_property(startup_object_file ${INTEGRATION_TEST_STARTUP} STARTUP_OBJECT)
-  get_target_property(crti_object_file libc.startup.linux.crti STARTUP_OBJECT)
-  get_target_property(crtn_object_file libc.startup.linux.crtn STARTUP_OBJECT)
+  set(startup_object_file $<TARGET_OBJECTS:${INTEGRATION_TEST_STARTUP}>)
+  set(crti_object_file $<TARGET_OBJECTS:libc.startup.linux.crti>)
+  set(crtn_object_file $<TARGET_OBJECTS:libc.startup.linux.crtn>)
   set(dummy_archive $<TARGET_PROPERTY:libc_integration_test_dummy,ARCHIVE_OUTPUT_DIRECTORY>/lib$<TARGET_PROPERTY:libc_integration_test_dummy,ARCHIVE_OUTPUT_NAME>.a)
-  if(NOT startup_object_file)
-    message(FATAL_ERROR "Missing STARTUP_OBJECT property of ${INTEGRATION_TEST_STARTUP}.")
-  endif()
-  set(startup_dst ${sysroot_lib}/${LIBC_TARGET_ARCHITECTURE}-linux-gnu/crt1.o)
+  # TODO: Copy the startup files to the correct target-triple directory instead
+  # of to a partly hard-coded directory.
+  set(startup_dst ${sysroot_lib}/${LIBC_TARGET_ARCHITECTURE}-linux-gnu)
   add_custom_command(
     OUTPUT ${startup_dst} ${sysroot}/lib/crti.o ${sysroot}/lib/crtn.o ${sysroot}/lib/libm.a ${sysroot}/lib/libc++.a
-    COMMAND cmake -E copy ${startup_object_file} ${startup_dst}
-    COMMAND cmake -E copy ${crti_object_file} ${sysroot}/lib
-    COMMAND cmake -E copy ${crtn_object_file} ${sysroot}/lib
+    COMMAND cmake -E copy ${startup_object_file} ${startup_dst}/$<TARGET_PROPERTY:${INTEGRATION_TEST_STARTUP},OUTPUT_NAME>
+    COMMAND cmake -E copy ${crti_object_file} ${sysroot_lib}/$<TARGET_PROPERTY:libc.startup.linux.crti,OUTPUT_NAME>
+    COMMAND cmake -E copy ${crtn_object_file} ${sysroot_lib}/$<TARGET_PROPERTY:libc.startup.linux.crtn,OUTPUT_NAME>
     # We copy the dummy archive as libm.a and libc++.a as the compiler drivers expect them.
-    COMMAND cmake -E copy ${dummy_archive} ${sysroot}/lib/libm.a
-    COMMAND cmake -E copy ${dummy_archive} ${sysroot}/lib/libc++.a
+    COMMAND cmake -E copy ${dummy_archive} ${sysroot_lib}/libm.a
+    COMMAND cmake -E copy ${dummy_archive} ${sysroot_lib}/libc++.a
     DEPENDS ${INTEGRATION_TEST_STARTUP} libc.startup.linux.crti libc.startup.linux.crtn libc_integration_test_dummy
   )
   add_custom_target(
     ${fq_target_name}.__copy_startup__
     DEPENDS ${startup_dst}
   )
 
   add_library(

libc/startup/linux/CMakeLists.txt

 function(add_startup_object name)
   cmake_parse_arguments(
     "ADD_STARTUP_OBJECT"
     "ALIAS" # Option argument
     "SRC"   # Single value arguments
     "DEPENDS;COMPILE_OPTIONS" # Multi value arguments
     ${ARGN}
   )
 
   get_fq_target_name(${name} fq_target_name)
-  get_fq_deps_list(fq_deps_list ${ADD_STARTUP_OBJECT_DEPENDS})
   if(ADD_STARTUP_OBJECT_ALIAS)
-    list(LENGTH ADD_STARTUP_OBJECT_DEPENDS deps_size)
-    if(NOT (${deps_size} EQUAL "1"))
-      message(FATAL_ERROR "A startup object alias should have exactly one dependency.")
-    endif()
-    list(GET ADD_STARTUP_OBJECT_DEPENDS 0 dep)
-    get_fq_dep_name(fq_dep_name ${dep})
-
-    add_custom_target(${fq_target_name})
-    add_dependencies(${fq_target_name} ${fq_dep_name})
-    get_target_property(startup_object ${fq_dep_name} STARTUP_OBJECT)
-    set_target_properties(
-      ${fq_target_name}
-      PROPERTIES
-        "TARGET_TYPE" "${OBJECT_LIBRARY_TARGET_TYPE}"
-        "STARTUP_OBJECT" "${startup_object}"
-        "OBJECT_FILES" ""
-        "DEPS" "${fq_dep_name}"
-    )
+    get_fq_deps_list(fq_dep_list ${ADD_STARTUP_OBJECT_DEPENDS})
+    add_library(${fq_target_name} ALIAS ${fq_dep_list})
     return()
   endif()
 
   add_object_library(
-    ${name}.__objects__
+    ${name}
     SRCS ${ADD_STARTUP_OBJECT_SRC}
     DEPENDS ${ADD_STARTUP_OBJECT_DEPENDS}
     COMPILE_OPTIONS ${ADD_STARTUP_OBJECT_COMPILE_OPTIONS}
   )
-
-  set(objfile ${LIBC_BUILD_DIR}/lib/${name}.o)
-  add_custom_command(
-    OUTPUT ${objfile}
-    COMMAND cp $<TARGET_OBJECTS:${fq_target_name}.__objects__> ${objfile}
-    DEPENDS $<TARGET_OBJECTS:${fq_target_name}.__objects__>
-  )
-  add_custom_target(
-    ${fq_target_name}
-    DEPENDS ${objfile}
-  )
   set_target_properties(
     ${fq_target_name}
     PROPERTIES
-      "TARGET_TYPE" "${OBJECT_LIBRARY_TARGET_TYPE}"
-      "STARTUP_OBJECT" "${objfile}"
-      "OBJECT_FILES" ""
-      "DEPS" "${fq_target_name}.__objects__"
+      OUTPUT_NAME ${name}.o
   )
 endfunction()
 
 if(NOT (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_ARCHITECTURE}))
   message(STATUS "Skipping startup for target architecture ${LIBC_TARGET_ARCHITECTURE}")
   return()
 endif()
 
SRC
     crtn.cpp
 )
 
 add_custom_target(libc-startup)
 set(startup_components crt1 crti crtn)
 foreach(target IN LISTS startup_components)
   set(fq_target_name libc.startup.linux.${target})
   add_dependencies(libc-startup ${fq_target_name})
-  get_target_property(startup_object ${fq_target_name} STARTUP_OBJECT)
-  install(FILES ${startup_object}
+  install(FILES $<TARGET_OBJECTS:${fq_target_name}>
           DESTINATION ${CMAKE_INSTALL_LIBDIR}
+          RENAME $<TARGET_PROPERTY:${fq_target_name},OUTPUT_NAME>
           COMPONENT libc)
 endforeach()