[libcxx] Add a dependency on unwind in cxx_static, if LIBCXXABI_USE_LLVM_UNWINDER is set

[mstorsjo]

Summary

Even if building cxx_static in itself doesn't actually link in the
requested unwinder, add a synthetic dependency so that building
cxx_static makes sure that the unwinder that was requested to be used
also gets built.

[ldionne]

libcxx/src/CMakeLists.txt

289–294

Should we put that dependency on check-cxx instead? We could add it to cxx-test-depends.

[mstorsjo]

libcxx/src/CMakeLists.txt

289–294

That was my first idea, but in some sense, this felt more natural and symmetrical to adding the unwind dependency to cxx_shared right above here - so you get libunwind built if you have LIBCXXABI_USE_LLVM_UNWINDER set, when you do ninja cxx, even if you've only set up a static libcxx.

But I don't feel strongly, I can post that version of the patch as well.

[ldionne]

libcxx/src/CMakeLists.txt

289–294

Ok, I guess I am swayed by the symmetry argument with cxx_shared. Thanks for taking the time to explain. Let's go with this one and close D113481.

libcxx/src/CMakeLists.txt

PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
       POSITION_INDEPENDENT_CODE ON
   )
   cxx_add_common_build_flags(cxx_static)
   cxx_set_common_defines(cxx_static)
 
+  if (LIBCXXABI_USE_LLVM_UNWINDER)
+    # If we intend to use the just-built unwinder, add a dependency so that it
+    # gets built, even if we technically aren't going to link it in at this
+    # stage.
+    add_dependencies(cxx_static unwind)
+  endif()
+
   if (LIBCXX_HERMETIC_STATIC_LIBRARY)
     # If the hermetic library doesn't define the operator new/delete functions
     # then its code shouldn't declare them with hidden visibility.  They might
     # actually be provided by a shared library at link time.
     if (LIBCXX_ENABLE_NEW_DELETE_DEFINITIONS)
       append_flags_if_supported(CXX_STATIC_LIBRARY_FLAGS -fvisibility-global-new-delete-hidden)
     endif()
     target_compile_options(cxx_static PRIVATE ${CXX_STATIC_LIBRARY_FLAGS})