[libc] Add a new rule `add_object`.

[sivachandra]

Summary

This rule can help add targets to generate special object files like the
crt1.o on linux. Also, it can be used to add specially compiled object
stubs which are to be linked into the entrypoint objects.

[abrachet]

Could you show how you are using this? I have tried like:

add_object(
  special
  SRC
    special.cpp
  COMPILE_OPTIONS
    -Wframe-larger-than=0
)

add_entrypoint_object(
  entrypoint
  SRCS
    entrypoint.cpp
  HDRS
     entrypoint.h
  DEPENDS
     special
)

Doesn't work, entrypoint doesn't link against special.o

libc/cmake/modules/LLVMLibCRules.cmake

112

Won't add_library compile using CMAKE_CXX_FLAGS?

[sivachandra]

In D76271#1926018, @abrachet wrote:

Could you show how you are using this? I have tried like:

add_object(
  special
  SRC
    special.cpp
  COMPILE_OPTIONS
    -Wframe-larger-than=0
)

add_entrypoint_object(
  entrypoint
  SRCS
    entrypoint.cpp
  HDRS
     entrypoint.h
  DEPENDS
     special
)

Doesn't work, entrypoint doesn't link against special.o

I have explained how to use it on the sigaction review page.

libc/cmake/modules/LLVMLibCRules.cmake

112

Yes, but any new compile options you add get added at the end and hence will override (if required) the options earlier in the line.

[sivachandra]

libc/cmake/modules/LLVMLibCRules.cmake

134

Looks like object libraries don't give us the flexibility to customize the location and name of the generated object files. I will submit after removing lines 134 to 146 which seem to be doing nothing for an object library.

libc/cmake/modules/LLVMLibCRules.cmake

function(add_gen_header target_name)
   )
 
   add_custom_target(
     ${target_name}
     DEPENDS ${out_file} ${ADD_GEN_HDR_DEPENDS}
   )
 endfunction(add_gen_header)
 
+set(SINGLE_OBJECT_TARGET_TYPE "LIBC_SINGLE_OBJECT")
+
+# Function to generate single object file.
+# Usage:
+#     add_object(
+#       <target_name>
+#       SRC <source file to compile>
+#       DEPENDS <list of dependencies>
+#       COMPILE_OPTIONS <optional list of special compile options for this target>
+function(add_object target_name)
+  cmake_parse_arguments(
+    "ADD_OBJECT"
+    "" # No option arguments
+    "SRC" # Single value arguments
+    "COMPILE_OPTIONS;DEPENDS" # Multivalue arguments
+    ${ARGN}
+  )
+
+  if(NOT ADD_OBJECT_SRC)
+    message(FATAL_ERROR "'add_object' rules requires a SRC to be specified.")
+  endif()
+
+  add_library(
+    ${target_name}
+    OBJECT
+    ${ADD_OBJECT_SRC}
+  )
+  target_include_directories(
+    ${target_name}
+    PRIVATE
+      "${LIBC_BUILD_DIR}/include;${LIBC_SOURCE_DIR};${LIBC_BUILD_DIR}"
+  )
+  if(ADD_OBJECT_COMPILE_OPTIONS)
+    target_compile_options(
+      ${target_name}
+      PRIVATE ${ADD_OBJECT_COMPILE_OPTIONS}
+    )
+  endif()
+  if(ADD_OBJECT_DEPENDS)
+    add_dependencies(
+      ${target_name}
+      ${ADD_OBJECT_DEPENDS}
+    )
+  endif()
+  set_target_properties(
+    ${target_name}
+    PROPERTIES
+      "TARGET_TYPE" ${SINGLE_OBJECT_TARGET_TYPE}
+      "OBJECT_FILE" $<TARGET_OBJECTS:${target_name}>
+  )
+endfunction(add_object)
+
 set(ENTRYPOINT_OBJ_TARGET_TYPE "ENTRYPOINT_OBJ")
 
 # A rule for entrypoint object targets.
 # Usage:
 #     add_entrypoint_object(
 #       <target_name>
 #       [REDIRECTED] # Specified if the entrypoint is redirected.
 #       [NAME] <the C name of the entrypoint if different from target_name>
 #       SRCS <list of .cpp files>
 #       HDRS <list of .h files>
 #       DEPENDS <list of dependencies>
-#       COMPILE_OPTIONS <list of special compile options for this target>
+#       COMPILE_OPTIONS <optional list of special compile options for this target>
+#       SPECIAL_OBJECTS <optional list of special object targets added by the rule `add_object`>
 #     )
 function(add_entrypoint_object target_name)
   cmake_parse_arguments(
     "ADD_ENTRYPOINT_OBJ"
     "REDIRECTED" # Optional argument
     "NAME" # Single value arguments
-    "SRCS;HDRS;DEPENDS;COMPILE_OPTIONS"  # Multi value arguments
+    "SRCS;HDRS;SPECIAL_OBJECTS;DEPENDS;COMPILE_OPTIONS"  # Multi value arguments
     ${ARGN}
   )
   if(NOT ADD_ENTRYPOINT_OBJ_SRCS)
     message(FATAL_ERROR "`add_entrypoint_object` rule requires SRCS to be specified.")
   endif()
   if(NOT ADD_ENTRYPOINT_OBJ_HDRS)
     message(FATAL_ERROR "`add_entrypoint_object` rule requires HDRS to be specified.")
   endif()
target_compile_options(
       ${target_name}_objects
       PRIVATE ${ADD_ENTRYPOINT_OBJ_COMPILE_OPTIONS}
     )
   endif()
 
   set(object_file_raw "${CMAKE_CURRENT_BINARY_DIR}/${target_name}_raw.o")
   set(object_file "${CMAKE_CURRENT_BINARY_DIR}/${target_name}.o")
 
+  set(input_objects $<TARGET_OBJECTS:${target_name}_objects>)
+  if(ADD_ENTRYPOINT_OBJ_SPECIAL_OBJECTS)
+    foreach(obj_target IN LISTS ADD_ENTRYPOINT_OBJ_SPECIAL_OBJECTS)
+      get_target_property(obj_type ${obj_target} "TARGET_TYPE")
+      if((NOT obj_type) OR (NOT (${obj_type} STREQUAL ${SINGLE_OBJECT_TARGET_TYPE})))
+        message(FATAL_ERROR "Unexpected target type for 'SPECIAL_OBJECT' - should be a target introduced by the `add_object` rule.")
+      endif()
+      list(APPEND input_objects $<TARGET_OBJECTS:${obj_target}>)
+    endforeach(obj_target)
+  endif()
+
   add_custom_command(
     OUTPUT ${object_file_raw}
-    DEPENDS $<TARGET_OBJECTS:${target_name}_objects>
-    COMMAND ${CMAKE_LINKER} -r $<TARGET_OBJECTS:${target_name}_objects> -o ${object_file_raw}
+    DEPENDS ${input_objects}
+    COMMAND ${CMAKE_LINKER} -r ${input_objects} -o ${object_file_raw}
   )
 
   set(alias_attributes "0,function,global")
   if(ADD_ENTRYPOINT_OBJ_REDIRECTED)
     set(alias_attributes "${alias_attributes},hidden")
   endif()
 
   add_custom_command(