xref: /llvm-project/libc/startup/linux/CMakeLists.txt (revision 2353f484a51b9aab992402400010bbb450fea1b2) 
1# This function merges multiple objects into a single relocatable object
2#                     cc -r obj1.o obj2.o -o obj.o
3# A relocatable object is an object file that is not fully linked into an
4# executable or a shared library. It is an intermediate file format that can
5# be passed into the linker.
6# A crt object has arch-specific code and arch-agnostic code. To reduce code
7# duplication, the implementation is split into multiple units. As a result,
8# we need to merge them into a single relocatable object.
9# See also:  https://maskray.me/blog/2022-11-21-relocatable-linking
10function(merge_relocatable_object name)
11  set(obj_list "")
12  set(fq_link_libraries "")
13  get_fq_deps_list(fq_dep_list ${ARGN})
14  foreach(target IN LISTS fq_dep_list)
15    list(APPEND obj_list "$<TARGET_OBJECTS:${target}>")
16    get_target_property(libs ${target} DEPS)
17    list(APPEND fq_link_libraries "${libs}")
18  endforeach()
19  list(REMOVE_DUPLICATES obj_list)
20  list(REMOVE_DUPLICATES fq_link_libraries)
21  get_fq_target_name(${name} fq_name)
22  set(relocatable_target "${fq_name}.__relocatable__")
23  add_executable(
24    ${relocatable_target}
25    ${obj_list}
26  )
27  # Pass -r to the driver is much cleaner than passing -Wl,-r: the compiler knows it is
28  # a relocatable linking and will not pass other irrelevant flags to the linker.
29  set(link_opts -r -nostdlib)
30  if (explicit_target_triple AND LLVM_ENABLE_LLD)
31    list(APPEND link_opts --target=${explicit_target_triple})
32  endif()
33  target_link_options(${relocatable_target} PRIVATE ${link_opts})
34  set_target_properties(
35    ${relocatable_target}
36    PROPERTIES
37      RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
38      OUTPUT_NAME ${name}.o
39  )
40  add_library(${fq_name} OBJECT IMPORTED GLOBAL)
41  add_dependencies(${fq_name} ${relocatable_target})
42  target_link_libraries(${fq_name} INTERFACE ${fq_link_libraries})
43  set_target_properties(
44    ${fq_name}
45    PROPERTIES
46      LINKER_LANGUAGE CXX
47      IMPORTED_OBJECTS ${CMAKE_CURRENT_BINARY_DIR}/${name}.o
48      TARGET_TYPE ${OBJECT_LIBRARY_TARGET_TYPE}
49      DEPS "${fq_link_libraries}"
50  )
51endfunction()
52
53function(add_startup_object name)
54  cmake_parse_arguments(
55    "ADD_STARTUP_OBJECT"
56    "" # Option argument
57    "SRC"   # Single value arguments
58    "DEPENDS;COMPILE_OPTIONS" # Multi value arguments
59    ${ARGN}
60  )
61
62  get_fq_target_name(${name} fq_target_name)
63
64  add_object_library(
65    ${name}
66    SRCS ${ADD_STARTUP_OBJECT_SRC}
67    DEPENDS ${ADD_STARTUP_OBJECT_DEPENDS}
68    COMPILE_OPTIONS ${ADD_STARTUP_OBJECT_COMPILE_OPTIONS}
69  )
70  set_target_properties(
71    ${fq_target_name}
72    PROPERTIES
73      OUTPUT_NAME ${name}.o
74  )
75endfunction()
76
77check_cxx_compiler_flag("-r" LIBC_LINKER_SUPPORTS_RELOCATABLE)
78
79if(NOT LIBC_LINKER_SUPPORTS_RELOCATABLE)
80  message(STATUS "Skipping startup for target architecture ${LIBC_TARGET_ARCHITECTURE}: linker does not support -r")
81  return()
82endif()
83
84if(NOT (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_ARCHITECTURE}))
85  message(STATUS "Skipping startup for target architecture ${LIBC_TARGET_ARCHITECTURE}")
86  return()
87endif()
88
89add_subdirectory(${LIBC_TARGET_ARCHITECTURE})
90
91add_object_library(
92  do_start
93  SRCS
94    do_start.cpp
95  HDRS
96    do_start.h
97  DEPENDS
98    libc.config.app_h
99    libc.include.sys_mman
100    libc.include.sys_syscall
101    libc.include.llvm-libc-macros.link_macros
102    libc.src.__support.threads.thread
103    libc.src.__support.OSUtil.osutil
104    libc.src.stdlib.exit
105    libc.src.stdlib.atexit
106    libc.src.unistd.environ
107  COMPILE_OPTIONS
108    -ffreestanding       # To avoid compiler warnings about calling the main function.
109    -fno-builtin         # avoid emit unexpected calls
110    -fno-stack-protector # stack protect canary is not available yet.
111)
112
113# TODO: factor out crt1 into multiple objects
114merge_relocatable_object(
115  crt1
116  .${LIBC_TARGET_ARCHITECTURE}.start
117  .${LIBC_TARGET_ARCHITECTURE}.tls
118  .do_start
119)
120
121add_startup_object(
122  crti
123  SRC
124    crti.cpp
125)
126
127add_startup_object(
128  crtn
129  SRC
130    crtn.cpp
131)
132
133add_custom_target(libc-startup)
134set(startup_components crt1 crti crtn)
135foreach(target IN LISTS startup_components)
136  set(fq_target_name libc.startup.linux.${target})
137  add_dependencies(libc-startup ${fq_target_name})
138  install(FILES $<TARGET_OBJECTS:${fq_target_name}>
139          DESTINATION ${LIBC_INSTALL_LIBRARY_DIR}
140          RENAME $<TARGET_PROPERTY:${fq_target_name},OUTPUT_NAME>
141          COMPONENT libc)
142endforeach()
143