[libc] Provide __libc_{init,fini}_array for baremetal (llvm#90828)

These are provided by newlib and many baremetal projects assume they're
available rather than providing their own implementation.

libc/cmake/modules/LLVMLibCObjectRules.cmake

@@ -246,9 +246,6 @@ function(create_entrypoint_object fq_target_name)
   if(NOT ADD_ENTRYPOINT_OBJ_SRCS)
     message(FATAL_ERROR "`add_entrypoint_object` rule requires SRCS to be specified.")
   endif()
-  if(NOT ADD_ENTRYPOINT_OBJ_HDRS)
-    message(FATAL_ERROR "`add_entrypoint_object` rule requires HDRS to be specified.")
-  endif()
   if(NOT ADD_ENTRYPOINT_OBJ_CXX_STANDARD)
     set(ADD_ENTRYPOINT_OBJ_CXX_STANDARD ${CMAKE_CXX_STANDARD})
   endif()

libc/config/baremetal/arm/entrypoints.txt

@@ -183,6 +183,10 @@ set(TARGET_LIBC_ENTRYPOINTS
 
     # time.h entrypoints
     libc.src.time.difftime
+
+    # internal entrypoints
+    libc.startup.baremetal.init
+    libc.startup.baremetal.fini
 )
 
 set(TARGET_LIBM_ENTRYPOINTS

libc/config/baremetal/riscv/entrypoints.txt

@@ -183,6 +183,10 @@ set(TARGET_LIBC_ENTRYPOINTS
 
     # time.h entrypoints
     libc.src.time.difftime
+
+    # internal entrypoints
+    libc.startup.baremetal.init
+    libc.startup.baremetal.fini
 )
 
 set(TARGET_LIBM_ENTRYPOINTS

libc/startup/baremetal/CMakeLists.txt

@@ -0,0 +1,11 @@
+add_entrypoint_object(
+  init
+  SRCS
+    init.cpp
+)
+
+add_entrypoint_object(
+  fini
+  SRCS
+    fini.cpp
+)

libc/startup/baremetal/fini.cpp

@@ -0,0 +1,27 @@
+//===-- Implementation file of __libc_fini_array --------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <stddef.h>
+#include <stdint.h>
+
+extern "C" {
+extern uintptr_t __fini_array_start[];
+extern uintptr_t __fini_array_end[];
+}
+
+namespace LIBC_NAMESPACE {
+
+using FiniCallback = void(void);
+
+extern "C" void __libc_fini_array(void) {
+  size_t fini_array_size = __fini_array_end - __fini_array_start;
+  for (size_t i = fini_array_size; i > 0; --i)
+    reinterpret_cast<FiniCallback *>(__fini_array_start[i - 1])();
+}
+
+} // namespace LIBC_NAMESPACE

libc/startup/baremetal/init.cpp

@@ -0,0 +1,32 @@
+//===-- Implementation file of __libc_init_array --------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <stddef.h>
+#include <stdint.h>
+
+extern "C" {
+extern uintptr_t __preinit_array_start[];
+extern uintptr_t __preinit_array_end[];
+extern uintptr_t __init_array_start[];
+extern uintptr_t __init_array_end[];
+}
+
+namespace LIBC_NAMESPACE {
+
+using InitCallback = void(void);
+
+extern "C" void __libc_init_array(void) {
+  size_t preinit_array_size = __preinit_array_end - __preinit_array_start;
+  for (size_t i = 0; i < preinit_array_size; ++i)
+    reinterpret_cast<InitCallback *>(__preinit_array_start[i])();
+  size_t init_array_size = __init_array_end - __init_array_start;
+  for (size_t i = 0; i < init_array_size; ++i)
+    reinterpret_cast<InitCallback *>(__init_array_start[i])();
+}
+
+} // namespace LIBC_NAMESPACE