[libunwind] Support building hermetic static library

[phosek]

Summary

This is useful when the static libunwind library is being linked into
shared libraries that may be used in with other shared libraries that
use different unwinder. We want to avoid avoid exporting libunwind
symbols in those cases. This achieved by a new CMake option which can be
enabled by libunwind vendors as needed.

The same CMake option has already been added to libc++ and libc++abi in
D55404 and D56026.

[ldionne]

libunwind/src/CMakeLists.txt

134

We need to define this when building libunwind? This circular dependency looks like a smell.

[phosek]

libunwind/src/CMakeLists.txt

134

Unfortunately yes because libunwind uses operator new (https://github.com/llvm/llvm-project/blob/master/libunwind/src/libunwind.cpp#L125) which definition comes from libc++ and without this define, we get a conflicting declaration error (the implicit declaration provided by the compiler has hidden visibility due to -fvisibility-global-new-delete-hidden but that one that comes from libc++ has default visibility).

[mstorsjo]

libunwind/src/CMakeLists.txt

134

I was under the impression that even if libunwind requires C++ headers to be available, it shouldn't actually require linking to any C++ library (or that this was the design/intention at least); I regularly build it in this setup where my libunwind doesn't link against libc++, as that indeed is a nasty circular dependency. (I only have the checked out libcxx available for libunwind to find headers from.)

This seems to be one of few cases where it's actually invoked as a normal operator new. The other cases that do exist use placement new for invoking the constructor, but avoiding a call to any Standard C++ library function. Would it be better to just adjust this call as well, e.g. into malloc + placement new, to avoid the libcxx linking dependency altogether?

This code seems to be within #ifdef UNW_REMOTE, and I don't see anything within libunwind actually even setting that define, so how are you ending up running into it? (That explains why I haven't noticed.)

[mstorsjo]

libunwind/src/CMakeLists.txt

134

That particular function you linked, unw_create_addr_space_for_task, seems to even lack a return statement, so I'm rather sure that this function isn't in use. IIRC the remote unwinding part of libunwind isn't fully implemented, not built by default, and probably bitrotting.

[phosek]

libunwind/src/CMakeLists.txt

134

I looked into the remote unwinding recently and the current implementation is unfinished and doesn't work (and I don't think it ever worked).

I'd be fine changing this to placement new to eliminate this dependency, but maybe it'd be better to remove this code altogether? Do you have any preference?

[mstorsjo]

libunwind/src/CMakeLists.txt

134

I don't mind removing the unused and incomplete remote unwinding code, but that's up to the code owners, @mclow.lists, @EricWF and @ldionne.

[phosek]

libunwind/src/CMakeLists.txt

134

I've created D57251 to switch to using placement new, and D57252 to remove the remote unwinding support altogether. I'd prefer the latter, but either should unblock this change.

[phosek]

Is this acceptable now that we got rid of the C++ library dependency?

[phosek]

Now that D57262 has landed, would it be fine to land this as well?

libunwind/CMakeLists.txt

 if (CMAKE_SIZEOF_VOID_P EQUAL 8 AND NOT WIN32)
   if (LIBUNWIND_BUILD_32_BITS AND NOT LLVM_BUILD_32_BITS) # Don't duplicate the output from LLVM
     message(STATUS "Building 32 bits executables and libraries.")
   endif()
 elseif(LIBUNWIND_BUILD_32_BITS)
   message(FATAL_ERROR "LIBUNWIND_BUILD_32_BITS=ON is not supported on this platform.")
 endif()
 
+option(LIBUNWIND_HERMETIC_STATIC_LIBRARY
+  "Do not export any symbols from the static library." OFF)
+
 #===============================================================================
 # Configure System
 #===============================================================================
 
 # Add path for custom modules
 set(CMAKE_MODULE_PATH
     "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
     ${CMAKE_MODULE_PATH})

libunwind/src/CMakeLists.txt

 string(REPLACE ";" " " LIBUNWIND_C_FLAGS "${LIBUNWIND_C_FLAGS}")
 string(REPLACE ";" " " LIBUNWIND_LINK_FLAGS "${LIBUNWIND_LINK_FLAGS}")
 
 set_property(SOURCE ${LIBUNWIND_CXX_SOURCES}
              APPEND_STRING PROPERTY COMPILE_FLAGS " ${LIBUNWIND_CXX_FLAGS}")
 set_property(SOURCE ${LIBUNWIND_C_SOURCES}
              APPEND_STRING PROPERTY COMPILE_FLAGS " ${LIBUNWIND_C_FLAGS}")
 
+macro(unwind_object_library name)
+  cmake_parse_arguments(ARGS "" "" "DEFINES;FLAGS" ${ARGN})
+
 # Add a object library that contains the compiled source files.
-add_library(unwind_objects OBJECT ${LIBUNWIND_SOURCES} ${LIBUNWIND_HEADERS})
+  add_library(${name} OBJECT ${LIBUNWIND_SOURCES} ${LIBUNWIND_HEADERS})
+
+  if(ARGS_DEFINES)
+    target_compile_definitions(${name} PRIVATE ${ARGS_DEFINES})
+  endif()
 
   set_target_properties(unwind_objects
                         PROPERTIES
                           COMPILE_FLAGS "${LIBUNWIND_COMPILE_FLAGS}"
                           POSITION_INDEPENDENT_CODE ON)
 
+  if(ARGS_FLAGS)
+    target_compile_options(${name} PRIVATE ${ARGS_FLAGS})
+  endif()
+endmacro()
+
+if(LIBUNWIND_HERMETIC_STATIC_LIBRARY)
+  append_flags_if_supported(UNWIND_STATIC_OBJECTS_FLAGS -fvisibility=hidden)
+  append_flags_if_supported(UNWIND_STATIC_OBJECTS_FLAGS -fvisibility-global-new-delete-hidden)
+  unwind_object_library(unwind_static_objects
+    DEFINES _LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS
+    FLAGS ${UNWIND_STATIC_OBJECTS_FLAGS})
+  unwind_object_library(unwind_shared_objects)
+  set(unwind_static_sources $<TARGET_OBJECTS:unwind_static_objects>)
+  set(unwind_shared_sources $<TARGET_OBJECTS:unwind_shared_objects>)
+else()
+  unwind_object_library(unwind_objects)
+  set(unwind_static_sources $<TARGET_OBJECTS:unwind_objects>)
+  set(unwind_shared_sources $<TARGET_OBJECTS:unwind_objects>)
+endif()
+
 # Build the shared library.
 if (LIBUNWIND_ENABLE_SHARED)
-  add_library(unwind_shared SHARED $<TARGET_OBJECTS:unwind_objects>)
+  add_library(unwind_shared SHARED ${unwind_shared_sources})
   if(COMMAND llvm_setup_rpath)
     llvm_setup_rpath(unwind_shared)
   endif()
   target_link_libraries(unwind_shared ${libraries})
   set_target_properties(unwind_shared
                         PROPERTIES
                           LINK_FLAGS    "${LIBUNWIND_LINK_FLAGS}"
                           OUTPUT_NAME   "unwind"
                           VERSION       "1.0"
                           SOVERSION     "1")
   list(APPEND LIBUNWIND_BUILD_TARGETS "unwind_shared")
   if (LIBUNWIND_INSTALL_SHARED_LIBRARY)
     list(APPEND LIBUNWIND_INSTALL_TARGETS "unwind_shared")
   endif()
 endif()
 
 # Build the static library.
 if (LIBUNWIND_ENABLE_STATIC)
-  add_library(unwind_static STATIC $<TARGET_OBJECTS:unwind_objects>)
+  add_library(unwind_static STATIC ${unwind_static_sources})
   target_link_libraries(unwind_static ${libraries})
   set_target_properties(unwind_static
                         PROPERTIES
                           LINK_FLAGS    "${LIBUNWIND_LINK_FLAGS}"
                           OUTPUT_NAME   "unwind")
   list(APPEND LIBUNWIND_BUILD_TARGETS "unwind_static")
   if (LIBUNWIND_INSTALL_STATIC_LIBRARY)
     list(APPEND LIBUNWIND_INSTALL_TARGETS "unwind_static")