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# 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 has arch-specific code and arch-agnostic code. To reduce code
# duplication, the implementation is split into multiple units. As a result,
# we need to merge them into a single relocatable object.
# See also: https://maskray.me/blog/2022-11-21-relocatable-linking
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()
list(REMOVE_DUPLICATES obj_list)
list(REMOVE_DUPLICATES fq_link_libraries)
get_fq_target_name(${name} fq_name)
set(relocatable_target "${fq_name}.__relocatable__")
add_executable(
${relocatable_target}
${obj_list}
)
# Pass -r to the driver is much cleaner than passing -Wl,-r: the compiler knows it is
# a relocatable linking and will not pass other irrelevant flags to the linker.
target_link_options(${relocatable_target} PRIVATE -r -nostdlib)
set_target_properties(
${relocatable_target}
PROPERTIES
RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
OUTPUT_NAME ${name}.o
)
add_library(${fq_name} OBJECT IMPORTED GLOBAL)
add_dependencies(${fq_name} ${relocatable_target})
target_link_libraries(${fq_name} INTERFACE ${fq_link_libraries})
set_target_properties(
${fq_name}
PROPERTIES
LINKER_LANGUAGE CXX
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"
"" # Option argument
"SRC" # Single value arguments
"DEPENDS;COMPILE_OPTIONS" # Multi value arguments
${ARGN}
)
get_fq_target_name(${name} fq_target_name)
add_object_library(
${name}
SRCS ${ADD_STARTUP_OBJECT_SRC}
DEPENDS ${ADD_STARTUP_OBJECT_DEPENDS}
COMPILE_OPTIONS ${ADD_STARTUP_OBJECT_COMPILE_OPTIONS}
)
set_target_properties(
${fq_target_name}
PROPERTIES
OUTPUT_NAME ${name}.o
)
endfunction()
check_cxx_compiler_flag("-r" LIBC_LINKER_SUPPORTS_RELOCATABLE)
if(NOT LIBC_LINKER_SUPPORTS_RELOCATABLE)
message(STATUS "Skipping startup for target architecture ${LIBC_TARGET_ARCHITECTURE}: linker does not support -r")
return()
endif()
if(NOT (EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_ARCHITECTURE}))
message(STATUS "Skipping startup for target architecture ${LIBC_TARGET_ARCHITECTURE}")
return()
endif()
add_subdirectory(${LIBC_TARGET_ARCHITECTURE})
add_object_library(
do_start
SRCS
do_start.cpp
HDRS
do_start.h
DEPENDS
libc.config.linux.app_h
libc.include.sys_mman
libc.include.sys_syscall
libc.src.__support.threads.thread
libc.src.__support.OSUtil.osutil
libc.src.stdlib.exit
libc.src.stdlib.atexit
libc.src.unistd.environ
COMPILE_OPTIONS
-ffreestanding # To avoid compiler warnings about calling the main function.
-fno-builtin # avoid emit unexpected calls
-fno-stack-protector # stack protect canary is not available yet.
)
# TODO: factor out crt1 into multiple objects
merge_relocatable_object(
crt1
.${LIBC_TARGET_ARCHITECTURE}.start
.${LIBC_TARGET_ARCHITECTURE}.tls
.do_start
)
add_startup_object(
crti
SRC
crti.cpp
)
add_startup_object(
crtn
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})
install(FILES $<TARGET_OBJECTS:${fq_target_name}>
DESTINATION ${LIBC_INSTALL_LIBRARY_DIR}
RENAME $<TARGET_PROPERTY:${fq_target_name},OUTPUT_NAME>
EXCLUDE_FROM_ALL
COMPONENT libc)
endforeach()
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