[libc-commits] [libc] [libc] [startup] add cmake function to merge separated crt1 objects (PR #75413)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libc

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

<details>
<summary>Changes</summary>

As part of startup refactoring, this patch adds a function to merge multiple objects into a single relocatable object:
                     cc -r obj1.o obj2.o -o obj.o

A relocatable object is an object file that is not fully linked into an executable or a shared library. It is an intermediate file format that can be passed into the linker.

A crt object can have arch-specific code and arch-agnostic code. To reduce code cohesion, the implementation is splitted into multiple units. As a result, we need to merge them into a single relocatable object.

---
Full diff: https://github.com/llvm/llvm-project/pull/75413.diff


1 Files Affected:

- (modified) libc/startup/linux/CMakeLists.txt (+42-4) 


``````````diff
diff --git a/libc/startup/linux/CMakeLists.txt b/libc/startup/linux/CMakeLists.txt
index 007aa30c17d6ab..5ca5010f6b1fe4 100644
--- a/libc/startup/linux/CMakeLists.txt
+++ b/libc/startup/linux/CMakeLists.txt
@@ -1,3 +1,42 @@
+# This function merges multiple objects into a single relocatable object
+#                     cc -r obj1.o obj2.o -o obj.o
+# A relocatable object is an object file that is not fully linked into an
+# executable or a shared library. It is an intermediate file format that can
+# be passed into the linker.
+# A crt object have arch-specific code and arch-agnostic code. To reduce code
+# cohesion, the implementation is splitted into multiple units. As a result,
+# we need to merge them into a single relocatable object.
+function(merge_relocatable_object name)
+  set(obj_list "")
+  set(fq_link_libraries "")
+  get_fq_deps_list(fq_dep_list ${ARGN})
+  foreach(target IN LISTS fq_dep_list)
+    list(APPEND obj_list "$<TARGET_OBJECTS:${target}>")
+    get_target_property(libs ${target} DEPS)
+    list(APPEND fq_link_libraries "${libs}")
+  endforeach()
+  get_fq_target_name(${name} fq_name)
+  add_custom_command(
+    OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${name}.o
+    COMMAND ${CMAKE_CXX_COMPILER} -r ${obj_list} -o ${CMAKE_CURRENT_BINARY_DIR}/${name}.o
+    DEPENDS ${obj_list}
+    COMMAND_EXPAND_LISTS
+  )
+  add_custom_target(${fq_name}.relocatable DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${name}.o)
+  add_library(${fq_name} OBJECT IMPORTED GLOBAL)
+  add_dependencies(${fq_name} ${fq_name}.relocatable)
+  target_link_libraries(${fq_name} INTERFACE ${fq_link_libraries})
+  set_target_properties(
+    ${fq_name} 
+    PROPERTIES
+      LINKER_LANGUAGE CXX
+      OBJECT_FILES ${CMAKE_CURRENT_BINARY_DIR}/${name}.o
+      IMPORTED_OBJECTS ${CMAKE_CURRENT_BINARY_DIR}/${name}.o
+      TARGET_TYPE ${OBJECT_LIBRARY_TARGET_TYPE}
+      DEPS "${fq_link_libraries}"
+  ) 
+endfunction()
+
 function(add_startup_object name)
   cmake_parse_arguments(
     "ADD_STARTUP_OBJECT"
@@ -34,11 +73,10 @@ endif()
 
 add_subdirectory(${LIBC_TARGET_ARCHITECTURE})
 
-add_startup_object(
+# TODO: factor out crt1 into multiple objects
+merge_relocatable_object(
   crt1
-  ALIAS
-  DEPENDS
-    .${LIBC_TARGET_ARCHITECTURE}.crt1
+  .${LIBC_TARGET_ARCHITECTURE}.crt1
 )
 
 add_startup_object(

``````````

</details>


https://github.com/llvm/llvm-project/pull/75413