[libc++abi] Remove the LIBCXXABI_ENABLE_PIC option

[ldionne]

Summary

Instead, people should be using CMAKE_POSITION_INDEPENDENT_CODE to control
whether they want to use PIC or not. We should try to avoid reinventing
the wheel whenever CMake natively supports something.

This makes libc++abi consistent with libc++ and libunwind.

[ldionne]

Note that this option was added in https://reviews.llvm.org/D60049.

I was at most lukewarm about adding it but we still did, because there seemed to be a use case. @phosek just asked me why libc++abi was defining its own knob for enabling PIC, which led me to reconsider D60049 and to this patch.

@sbc100 Is there no way you can get by with setting CMAKE_POSITION_INDEPENDENT_CODE instead? In D60049 you mention that it's going to impact all the other LLVM components you're building, but perhaps that is reasonable? Also, if you're building a toolchain, you should be using the Runtimes build. I don't know whether you are, however if so, it should be possible to specify that option only when building the runtimes, not the tools themselves (and if that's not possible, then we should make it possible cause I think it's a valid and useful thing to have).

[phosek]

LGTM for consistency with other runtimes.

[ldionne]

Remove forgotten mention of LIBCXXABI_ENABLE_PIC

[sbc100]

wasm-ld has since been improved so its now possible to link PIC objects into static binaries. This means this is way less of an issue for WebAssembly these days. Plus I no longer maintain the toolchain that was using these CMake files to build libc++abi.

So I think I fine with removing this option, especially in light the "Runtimes" build which IIUC any modern toolchain builders should be using anyway.

[ldionne]

In D103973#2808513, @sbc100 wrote:

wasm-ld has since been improved so its now possible to link PIC objects into static binaries. This means this is way less of an issue for WebAssembly these days. Plus I no longer maintain the toolchain that was using these CMake files to build libc++abi.

So I think I fine with removing this option, especially in light the "Runtimes" build which IIUC any modern toolchain builders should be using anyway.

Thanks for your input!

libcxx/cmake/caches/Apple.cmake

 set(LIBCXX_CXX_ABI libcxxabi CACHE STRING "")
 set(LIBCXX_HIDE_FROM_ABI_PER_TU_BY_DEFAULT ON CACHE BOOL "")
 set(LIBCXX_ENABLE_DEBUG_MODE_SUPPORT OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS ON CACHE BOOL "")
 
 set(LIBCXX_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")
 set(LIBCXXABI_HERMETIC_STATIC_LIBRARY ON CACHE BOOL "")
 
-set(LIBCXXABI_ENABLE_PIC OFF CACHE BOOL "")
 set(LIBCXXABI_ENABLE_ASSERTIONS OFF CACHE BOOL "")
 set(LIBCXXABI_ENABLE_FORGIVING_DYNAMIC_CAST ON CACHE BOOL "")

libcxx/docs/ReleaseNotes.rst

- The ``std::pointer_safety`` utility and related functions are not available
   in C++03 anymore. Furthermore, in other standard modes, it has changed from
   a struct to a scoped enumeration, which is an ABI break. Finally, the
   ``std::get_pointer_safety`` function was previously in the dylib, but it
   is now defined as inline in the headers.
 
   While this is technically both an API and an ABI break, we do not expect
   ``std::pointer_safety`` to have been used at all in real code, since we
   never implemented the underlying support for garbage collection.
+
+- The `LIBCXXABI_ENABLE_PIC` CMake option was removed. If you are building your
+  own libc++abi from source and were using `LIBCXXABI_ENABLE_PIC`, please use
+  `CMAKE_POSITION_INDEPENDENT_CODE=ON` instead.

libcxxabi/CMakeLists.txt

 include(HandleCompilerRT)
 
 # Define options.
 option(LIBCXXABI_ENABLE_EXCEPTIONS
   "Provide support for exceptions in the runtime.
   When disabled, libc++abi does not support stack unwinding and other exceptions-related features." ON)
 option(LIBCXXABI_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
-option(LIBCXXABI_ENABLE_PIC "Build Position-Independent Code, even in static library" ON)
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
 option(LIBCXXABI_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBCXXABI_ENABLE_THREADS "Build with threads enabled" ON)
 option(LIBCXXABI_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 option(LIBCXXABI_HAS_WIN32_THREAD_API "Ignore auto-detection and force use of win32 thread API" OFF)
 option(LIBCXXABI_HAS_EXTERNAL_THREAD_API

libcxxabi/src/CMakeLists.txt

set_target_properties(cxxabi_shared
                             "c++abi"
                           SOVERSION
                             "1"
                           VERSION
                             "${LIBCXXABI_LIBRARY_VERSION}"
                           DEFINE_SYMBOL
                             "")
 
-  if(LIBCXXABI_ENABLE_PIC)
-    set_target_properties(cxxabi_shared PROPERTIES POSITION_INDEPENDENT_CODE ON)
-  endif()
-
   list(APPEND LIBCXXABI_BUILD_TARGETS "cxxabi_shared")
   if (LIBCXXABI_INSTALL_SHARED_LIBRARY)
     list(APPEND LIBCXXABI_INSTALL_TARGETS "cxxabi_shared")
   endif()
 
   add_library(cxxabi-reexports INTERFACE)
 
   # -exported_symbols_list is only available on Apple platforms
set_target_properties(cxxabi_static
                             OFF
                           COMPILE_FLAGS
                             "${LIBCXXABI_COMPILE_FLAGS}"
                           LINK_FLAGS
                             "${LIBCXXABI_LINK_FLAGS}"
                           OUTPUT_NAME
                             "c++abi")
 
-  if(LIBCXXABI_ENABLE_PIC)
-    set_target_properties(cxxabi_static PROPERTIES POSITION_INDEPENDENT_CODE ON)
-  endif()
-
   if(LIBCXXABI_HERMETIC_STATIC_LIBRARY)
     append_flags_if_supported(CXXABI_STATIC_LIBRARY_FLAGS -fvisibility=hidden)
     # 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 (LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS)
       append_flags_if_supported(CXXABI_STATIC_LIBRARY_FLAGS -fvisibility-global-new-delete-hidden)
     endif()