[libcxx] Option to disable overridden function detection

The overriden function detection requires the use of a custom section
which can break existing linker scripts that are commonly used in
embedded systems. This change introduces an option to disable this
feature.

Author: petrhosek

libcxx/CMakeLists.txt

@@ -127,6 +127,9 @@ option(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS
    to provide compile-time errors when using features unavailable on some version of
    the shared library they shipped should turn this on and see `include/__configuration/availability.h`
    for more details." OFF)
+option(LIBCXX_ENABLE_OVERRIDDEN_FUNCTION_DETECTION
+  "Whether to support detecting if an overridable function (typically a weak symbol)
+   has been overidden by a user or not." ON)
 
 if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   set(LIBCXX_DEFAULT_TEST_CONFIG "llvm-libc++-shared-gcc.cfg.in")
@@ -763,6 +766,7 @@ config_define_if_not(LIBCXX_ENABLE_UNICODE _LIBCPP_HAS_NO_UNICODE)
 config_define_if_not(LIBCXX_ENABLE_WIDE_CHARACTERS _LIBCPP_HAS_NO_WIDE_CHARACTERS)
 config_define_if_not(LIBCXX_ENABLE_TIME_ZONE_DATABASE _LIBCPP_HAS_NO_TIME_ZONE_DATABASE)
 config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
+config_define_if_not(LIBCXX_ENABLE_OVERRIDDEN_FUNCTION_DETECTION _LIBCPP_HAS_NO_OVERRIDDEN_FUNCTION_DETECTION)
 
 if (LIBCXX_ENABLE_ASSERTIONS)
   message(DEPRECATION "LIBCXX_ENABLE_ASSERTIONS is deprecated and will be removed in LLVM 20. Please use LIBCXX_HARDENING_MODE instead.")

libcxx/include/__config_site.in

@@ -22,6 +22,7 @@
 #cmakedefine _LIBCPP_HAS_THREAD_API_WIN32
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 #cmakedefine _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS
+#cmakedefine _LIBCPP_HAS_NO_OVERRIDDEN_FUNCTION_DETECTION
 #cmakedefine _LIBCPP_NO_VCRUNTIME
 #cmakedefine _LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION @_LIBCPP_TYPEINFO_COMPARISON_IMPLEMENTATION@
 #cmakedefine _LIBCPP_HAS_NO_FILESYSTEM

libcxx/src/include/overridable_function.h

@@ -63,7 +63,7 @@
 // want to be defining special sections inside user's executables which use our headers.
 //
 
-#if defined(_LIBCPP_OBJECT_FORMAT_MACHO)
+#if defined(_LIBCPP_OBJECT_FORMAT_MACHO) && !defined(_LIBCPP_HAS_NO_OVERRIDDEN_FUNCTION_DETECTION)
 
 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
 #  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE                                                                 \
@@ -97,7 +97,7 @@ _LIBCPP_HIDE_FROM_ABI bool __is_function_overridden(_Ret (*__fptr)(_Args...)) no
 _LIBCPP_END_NAMESPACE_STD
 
 // The NVPTX linker cannot create '__start/__stop' sections.
-#elif defined(_LIBCPP_OBJECT_FORMAT_ELF) && !defined(__NVPTX__)
+#elif defined(_LIBCPP_OBJECT_FORMAT_ELF) && !defined(__NVPTX__) && !defined(_LIBCPP_HAS_NO_OVERRIDDEN_FUNCTION_DETECTION)
 
 #  define _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION 1
 #  define _LIBCPP_MAKE_OVERRIDABLE_FUNCTION_DETECTABLE __attribute__((__section__("__lcxx_override")))