[libc++abi] Optionally move __cxa_demangle into libc++demangle.a

[rprichard]

Summary

The motivation is to reduce the size of libc++.so on Android. The flag
is always used with LIBCXXABI_NON_DEMANGLING_TERMINATE.

[rprichard]

This change was applied to Android's build of libc++abi here:
https://android-review.git.corp.google.com/c/platform/external/libcxxabi/+/1089752

[compnerd]

libcxxabi/CMakeLists.txt

135

I think that we should at least emit a warning when this option is enabled. This makes libc++abi non-conforming as __cxa_demangle is part of the ABI definition as per https://itanium-cxx-abi.github.io/cxx-abi/abi.html.

libcxxabi/CMakeLists.txt

 
 option(LIBCXXABI_BAREMETAL "Build libc++abi for baremetal targets." OFF)
 # The default terminate handler attempts to demangle uncaught exceptions, which
 # causes extra I/O and demangling code to be pulled in.
 option(LIBCXXABI_SILENT_TERMINATE "Set this to make the terminate handler default to a silent alternative" OFF)
 option(LIBCXXABI_NON_DEMANGLING_TERMINATE "Set this to make the terminate handler
 avoid demangling" OFF)
 
+# Omit __cxa_demangle to reduce the size of libc++[abi].so. Package it into a
+# separate static library for when it is actually needed.
+option(LIBCXXABI_STATIC_DEMANGLE_LIBRARY "Put __cxa_demangle into a separate static library" OFF)
+
+if (LIBCXXABI_STATIC_DEMANGLE_LIBRARY AND NOT LIBCXXABI_NON_DEMANGLING_TERMINATE)
+  message(FATAL_ERROR "libc++demangle is only supported when libc++abi doesn't use demangling")
+endif()
+
 if (NOT LIBCXXABI_ENABLE_SHARED AND NOT LIBCXXABI_ENABLE_STATIC)
   message(FATAL_ERROR "libc++abi must be built as either a shared or static library.")
 endif()
 
 # TODO: Remove this, which shouldn't be necessary since we know we're being built
 #       side-by-side with libc++.
 set(LIBCXXABI_LIBCXX_INCLUDES "" CACHE PATH
     "Specify path to libc++ includes.")

libcxxabi/src/CMakeLists.txt

 # Get sources
 set(LIBCXXABI_SOURCES
   # C++ABI files
   cxa_aux_runtime.cpp
   cxa_default_handlers.cpp
-  cxa_demangle.cpp
   cxa_exception_storage.cpp
   cxa_guard.cpp
   cxa_handlers.cpp
   cxa_vector.cpp
   cxa_virtual.cpp
   # C++ STL files
   stdlib_exception.cpp
   stdlib_stdexcept.cpp
   stdlib_typeinfo.cpp
   # Internal files
   abort_message.cpp
   fallback_malloc.cpp
   private_typeinfo.cpp
 )
 
+if (NOT LIBCXXABI_STATIC_DEMANGLE_LIBRARY)
+  list(APPEND LIBCXXABI_SOURCES
+    cxa_demangle.cpp
+  )
+endif()
+
 if (LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS)
   list(APPEND LIBCXXABI_SOURCES
     stdlib_new_delete.cpp
   )
 endif()
 
 if (LIBCXXABI_ENABLE_EXCEPTIONS)
   list(APPEND LIBCXXABI_SOURCES

 if (LLVM_ENABLE_MODULES)
   string(REPLACE "-Wl,-z,defs" "" CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS}")
 endif()
 
 if (NOT TARGET pstl::ParallelSTL)
   message(STATUS "Could not find ParallelSTL, libc++abi will not attempt to use it but the build may fail if the libc++ in use needs it to be available.")
 endif()
 
+if (LIBCXXABI_STATIC_DEMANGLE_LIBRARY)
+  add_library(cxxabi_demangle_static STATIC cxa_demangle.cpp ${LIBCXXABI_HEADERS})
+  target_link_libraries(cxxabi_demangle_static PRIVATE cxx-headers cxxabi-headers)
+  set_target_properties(cxxabi_demangle_static
+    PROPERTIES
+      CXX_EXTENSIONS OFF
+      CXX_STANDARD 20
+      CXX_STANDARD_REQUIRED OFF
+      COMPILE_FLAGS "${LIBCXXABI_COMPILE_FLAGS}"
+      LINK_FLAGS "${LIBCXXABI_LINK_FLAGS}"
+      OUTPUT_NAME "c++demangle"
+    )
+  list(APPEND LIBCXXABI_BUILD_TARGETS "cxxabi_demangle_static")
+  if (LIBCXXABI_INSTALL_STATIC_LIBRARY)
+    list(APPEND LIBCXXABI_INSTALL_TARGETS "cxxabi_demangle_static")
+  endif()
+endif()
+
 # Build the shared library.
 add_library(cxxabi_shared_objects OBJECT EXCLUDE_FROM_ALL ${LIBCXXABI_SOURCES} ${LIBCXXABI_HEADERS})
 if (LIBCXXABI_USE_LLVM_UNWINDER)
   if (LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY)
     target_link_libraries(cxxabi_shared_objects PUBLIC unwind_shared_objects) # propagate usage requirements
     target_sources(cxxabi_shared_objects PUBLIC $<TARGET_OBJECTS:unwind_shared_objects>)
   else()
     target_link_libraries(cxxabi_shared_objects PUBLIC unwind_shared)