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1# This function merges multiple objects into a single relocatable object2# cc -r obj1.o obj2.o -o obj.o3# A relocatable object is an object file that is not fully linked into an4# executable or a shared library. It is an intermediate file format that can5# be passed into the linker.6# A crt object has arch-specific code and arch-agnostic code. To reduce code7# 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-linking10function(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 ${obj_list}26 )27 # Pass -r to the driver is much cleaner than passing -Wl,-r: the compiler knows it is28 # a relocatable linking and will not pass other irrelevant flags to the linker.29 set(link_opts -r -nostdlib)30 if (explicit_target_triple AND LLVM_ENABLE_LLD)31 list(APPEND link_opts --target=${explicit_target_triple})32 endif()33 target_link_options(${relocatable_target} PRIVATE ${link_opts})34 set_target_properties(35 ${relocatable_target}36 PROPERTIES37 RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}38 OUTPUT_NAME ${name}.o39 )40 add_library(${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 PROPERTIES46 LINKER_LANGUAGE CXX47 IMPORTED_OBJECTS ${CMAKE_CURRENT_BINARY_DIR}/${name}.o48 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 argument57 "SRC" # Single value arguments58 "DEPENDS;COMPILE_OPTIONS" # Multi value arguments59 ${ARGN}60 )61 62 get_fq_target_name(${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 PROPERTIES73 OUTPUT_NAME ${name}.o74 )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_start93 SRCS94 do_start.cpp95 HDRS96 do_start.h97 DEPENDS98 libc.config.app_h99 libc.hdr.stdint_proxy100 libc.include.sys_mman101 libc.include.sys_syscall102 libc.include.llvm-libc-macros.link_macros103 libc.src.__support.threads.thread104 libc.src.__support.OSUtil.osutil105 libc.src.stdlib.exit106 libc.src.stdlib.atexit107 libc.src.unistd.environ108 libc.src.__support.OSUtil.linux.auxv109 COMPILE_OPTIONS110 -ffreestanding # To avoid compiler warnings about calling the main function.111 -fno-builtin # avoid emit unexpected calls112 -fno-stack-protector # stack protect canary is not available yet.113)114 115# TODO: factor out crt1 into multiple objects116merge_relocatable_object(117 crt1118 .${LIBC_TARGET_ARCHITECTURE}.start119 .${LIBC_TARGET_ARCHITECTURE}.tls120 .do_start121)122 123add_startup_object(124 crti125 SRC126 crti.cpp127)128 129add_startup_object(130 crtn131 SRC132 crtn.cpp133)134 135add_custom_target(libc-startup)136set(startup_components crt1 crti crtn)137foreach(target IN LISTS startup_components)138 set(fq_target_name libc.startup.linux.${target})139 add_dependencies(libc-startup ${fq_target_name})140 install(FILES $<TARGET_OBJECTS:${fq_target_name}>141 DESTINATION ${LIBC_INSTALL_LIBRARY_DIR}142 RENAME $<TARGET_PROPERTY:${fq_target_name},OUTPUT_NAME>143 COMPONENT libc)144endforeach()145