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[libc] Basic implementation of crt0 (#146863)
In order to run hermetic tests downstream (#145349), we need startup code. This is based on the crt0 that is present in Arm Toolchain: https://github.com/arm/arm-toolchain/tree/5446a3cbf4ef73196ccce490fc9fbf87bee093aa/arm-software/embedded/llvmlibc-support/crt0. Note that some tests do fail due to lack of hardware setup, which will be implemented at a later time. Most of the CMake/structure is a copy from the existing code in both `libc/startup/linux` and `libc/startup/gpu`. It is required to build an object file to be linked with. Also add header files for init/fini such that crt0 can use the provided symbols. --------- Co-authored-by: Michael Jones <[email protected]>
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‎libc/startup/baremetal/CMakeLists.txt

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# TODO: Use generic "add_startup_object" https://github.com/llvm/llvm-project/issues/133156
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function(add_startup_object name)
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cmake_parse_arguments(
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"ADD_STARTUP_OBJECT"
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"ALIAS" # Option argument
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"SRC" # Single value arguments
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"DEPENDS;COMPILE_OPTIONS" # Multi value arguments
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${ARGN}
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)
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get_fq_target_name(${name} fq_target_name)
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if(ADD_STARTUP_OBJECT_ALIAS)
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get_fq_deps_list(fq_dep_list ${ADD_STARTUP_OBJECT_DEPENDS})
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add_library(${fq_target_name} ALIAS ${fq_dep_list})
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return()
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endif()
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add_object_library(
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${name}
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SRCS ${ADD_STARTUP_OBJECT_SRC}
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COMPILE_OPTIONS ${ADD_STARTUP_OBJECT_COMPILE_OPTIONS}
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${ADD_STARTUP_OBJECT_UNPARSED_ARGUMENTS}
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DEPENDS ${ADD_STARTUP_OBJECT_DEPENDS}
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)
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set_target_properties(
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${fq_target_name}
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PROPERTIES
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OUTPUT_NAME ${name}.o
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)
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endfunction()
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add_entrypoint_object(
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init
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SRCS
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init.cpp
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DEPENDS
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libc.hdr.stdint_proxy
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libc.src.__support.common
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HDRS
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init.h
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)
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add_entrypoint_object(
@@ -14,4 +47,29 @@ add_entrypoint_object(
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DEPENDS
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libc.hdr.stdint_proxy
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libc.src.__support.common
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HDRS
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fini.h
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)
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if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_ARCHITECTURE})
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add_subdirectory(${LIBC_TARGET_ARCHITECTURE})
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else()
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message(WARNING "Cannot build 'crt1.o' for ${LIBC_TARGET_ARCHITECTURE} yet.")
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return()
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endif()
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add_startup_object(
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crt1
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ALIAS
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DEPENDS
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.${LIBC_TARGET_ARCHITECTURE}.crt1
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)
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add_custom_target(libc-startup)
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set(fq_target_name libc.startup.baremetal.${LIBC_TARGET_ARCHITECTURE}.crt1)
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add_dependencies(libc-startup ${fq_target_name})
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install(FILES $<TARGET_OBJECTS:${fq_target_name}>
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DESTINATION ${LIBC_INSTALL_LIBRARY_DIR}
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RENAME $<TARGET_PROPERTY:${fq_target_name},OUTPUT_NAME>
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COMPONENT libc)

‎libc/startup/baremetal/arm/CMakeLists.txt

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add_startup_object(
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crt1
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SRC
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start.cpp
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DEPENDS
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libc.src.stdlib.atexit
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libc.src.stdlib.exit
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libc.src.string.memcpy
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libc.src.string.memset
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libc.startup.baremetal.init
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libc.startup.baremetal.fini
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COMPILE_OPTIONS
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-ffreestanding # To avoid compiler warnings about calling the main function.
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-fno-builtin
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-Wno-global-constructors # To allow vector table initialization
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)

‎libc/startup/baremetal/arm/start.cpp

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//===-- Implementation of crt for arm -------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "src/__support/macros/config.h"
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#include "src/stdlib/atexit.h"
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#include "src/stdlib/exit.h"
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#include "src/string/memcpy.h"
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#include "src/string/memset.h"
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#include "startup/baremetal/fini.h"
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#include "startup/baremetal/init.h"
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#include <stdint.h>
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extern "C" {
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int main(int argc, char **argv);
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void _start();
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// Semihosting library initialisation if applicable. Required for printf, etc.
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[[gnu::weak]] void _platform_init() {}
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// These symbols are provided by the linker. The exact names are not defined by
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// a standard.
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extern uintptr_t __stack;
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extern uintptr_t __data_source[];
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extern uintptr_t __data_start[];
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extern uintptr_t __data_size[];
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extern uintptr_t __bss_start[];
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extern uintptr_t __bss_size[];
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// Based on
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// https://developer.arm.com/documentation/107565/0101/Use-case-examples/Generic-Information/What-is-inside-a-program-image-/Vector-table
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void NMI_Handler() {}
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void HardFault_Handler() { LIBC_NAMESPACE::exit(1); }
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void MemManage_Handler() { LIBC_NAMESPACE::exit(1); }
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void BusFault_Handler() { LIBC_NAMESPACE::exit(1); }
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void UsageFault_Handler() { LIBC_NAMESPACE::exit(1); }
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void SVC_Handler() {}
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void DebugMon_Handler() {}
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void PendSV_Handler() {}
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void SysTick_Handler() {}
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// Architecturally the bottom 7 bits of VTOR are zero, meaning the vector table
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// has to be 128-byte aligned, however an implementation can require more bits
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// to be zero and Cortex-M23 can require up to 10, so 1024-byte align the vector
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// table.
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using HandlerType = void (*)(void);
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const HandlerType vector_table[]
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__attribute__((section(".vectors"), aligned(1024), used)) = {
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(HandlerType)&__stack, // SP
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_start, // Reset
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NMI_Handler, // NMI Handler
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HardFault_Handler, // Hard Fault Handlerß
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MemManage_Handler, // MPU Fault Han`dler
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BusFault_Handler, // Bus Fault Handler
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UsageFault_Handler, // Usage Fault Handler
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0, // Reserved
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0, // Reserved
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0, // Reserved
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0, // Reserved
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SVC_Handler, // SVC Handler
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DebugMon_Handler, // Debug Monitor Handler
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0, // Reserved
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PendSV_Handler, // PendSV Handler
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SysTick_Handler, // SysTick Handler
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// Unused
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};
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} // extern "C"
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namespace LIBC_NAMESPACE_DECL {
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[[noreturn]] void do_start() {
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// FIXME: set up the QEMU test environment
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// Perform the equivalent of scatterloading
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LIBC_NAMESPACE::memcpy(__data_start, __data_source, (uintptr_t)__data_size);
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LIBC_NAMESPACE::memset(__bss_start, '\0', (uintptr_t)__bss_size);
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__libc_init_array();
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_platform_init();
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LIBC_NAMESPACE::atexit(&__libc_fini_array);
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LIBC_NAMESPACE::exit(main(0, 0));
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}
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} // namespace LIBC_NAMESPACE_DECL
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extern "C" void _start() {
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asm volatile("mov sp, %0" : : "r"(&__stack));
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asm volatile("bl %0" : : "X"(LIBC_NAMESPACE::do_start));
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}

‎libc/startup/baremetal/fini.cpp

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//
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//===----------------------------------------------------------------------===//
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#include "hdr/stdint_proxy.h"
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#include "startup/baremetal/fini.h"
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#include "src/__support/macros/config.h"
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#include <stddef.h>
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namespace LIBC_NAMESPACE_DECL {
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extern "C" {
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extern uintptr_t __fini_array_start[];
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extern uintptr_t __fini_array_end[];
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}
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using FiniCallback = void(void);
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extern "C" void __libc_fini_array(void) {

‎libc/startup/baremetal/fini.h

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//===-- Implementation header of __libc_fini_array ------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "hdr/stdint_proxy.h"
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extern "C" {
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extern uintptr_t __fini_array_start[];
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extern uintptr_t __fini_array_end[];
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void __libc_fini_array(void);
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} // extern "C"

‎libc/startup/baremetal/init.cpp

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//
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//===----------------------------------------------------------------------===//
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#include "hdr/stdint_proxy.h"
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#include "startup/baremetal/init.h"
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#include "src/__support/macros/config.h"
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#include <stddef.h>
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namespace LIBC_NAMESPACE_DECL {
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extern "C" {
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extern uintptr_t __preinit_array_start[];
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extern uintptr_t __preinit_array_end[];
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extern uintptr_t __init_array_start[];
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extern uintptr_t __init_array_end[];
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}
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using InitCallback = void(void);
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extern "C" void __libc_init_array(void) {

‎libc/startup/baremetal/init.h

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//===-- Implementation header of __libc_init_array ------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "hdr/stdint_proxy.h"
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extern "C" {
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extern uintptr_t __preinit_array_start[];
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extern uintptr_t __preinit_array_end[];
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extern uintptr_t __init_array_start[];
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extern uintptr_t __init_array_end[];
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void __libc_init_array(void);
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} // extern "C"

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