[CMake] Fix libc++abi arm build w/o libunwind.

[logan]

Summary

This commit fixes libc++abi build when LLVM unwinder (libunwind_llvm) is
not enabled.

This commit fixes the problem by removing "LLVM_NATIVE_ARCH MATCHES ARM"
from CMakeLists.txt so that LIBCXXABI_USE_LLVM_UNWINDER will only be
defined when LLVM unwinder is enabled.

We need LIBCXXABI_USE_LLVM_UNWINDER becase there is a subtle difference
between the unwinder from libgcc and the one from libunwind_llvm. For
the unwinder from libgcc, we have to initialize register r12 with the
address of _Unwind_Control_Block; otherwise,
_Unwind_GetLanguageSpecificData() and _Unwind_GetRegionStart() won't
work properly. Consequently, there is an extra _Unwind_SetGR() when
LLVM unwinder is disabled. Check cxa_personality.cpp for details.

[rengolin]

That match would only make sense if LLVM's unwinder was forced on ARM targets, and they're most certainly not. Expecting both unwindwers to behave identically makes no sense.

This fix looks good to me, but I'll let Marshall or Eric have a look first.

[logan]

Ping? Any other comments?

[rmaprath]

LGTM too.

@EricWF or @mclow.lists need to approve.

[logan]

Hi @EricWF and @mclow.lists:

Do you have any comments?

[EricWF]

Is ON the right default for ARM? If so please apply the inline comment to fix the default settings.. Otherwise this LGTM.

CMakeLists.txt

117

cmake_dependent_option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder" OFF
                                           "LLVM_NATIVE_ARCH MATCHES ARM" ON)

And don't forget to add the include for cmake_dependent_option somewhere above!

[EricWF]

CMakeLists.txt

117

The above code isn't correct as-is. You'll have to escape the strings for MATCHES.

[logan]

Hi @EricWF:

Thanks for your comment.

However, I think OFF is a better default for ARM (just like other platforms.) I usually use libc++abi without libunwind since libgcc is quite stable and usually linked by default. Besides, it is much more tricky to get the libc++/libc++abi/libunwind/compiler-rt combo work. The situation goes even worse when you are linking libgcc together.

Logan

[logan]

Hi @EricWF:

Would you mind if I commit this patch as-is? I believe OFF will is the better default value for ARM as well. Thanks.

Logan

[EricWF]

Nope LGTM.

[logan]

Thanks. Committed as rL286759.

CMakeLists.txt

 # Setup CMake Options
 #===============================================================================
 
 # Define options.
 option(LIBCXXABI_ENABLE_EXCEPTIONS "Use exceptions." ON)
 option(LIBCXXABI_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." 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_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_BUILD_32_BITS "Build 32 bit libc++abi." ${LLVM_BUILD_32_BITS})
 set(LIBCXXABI_TARGET_TRIPLE "" CACHE STRING "Target triple for cross compiling.")
 set(LIBCXXABI_GCC_TOOLCHAIN "" CACHE PATH "GCC toolchain for cross compiling.")
 set(LIBCXXABI_SYSROOT "" CACHE PATH "Sysroot for cross compiling.")
 set(LIBCXXABI_LIBCXX_LIBRARY_PATH "" CACHE PATH "The path to libc++ library.")

 endif()
 
 # This is the _ONLY_ place where add_definitions is called.
 if (MSVC)
   add_definitions(-D_CRT_SECURE_NO_WARNINGS)
 endif()
 
 # Define LIBCXXABI_USE_LLVM_UNWINDER for conditional compilation.
-if (LIBCXXABI_USE_LLVM_UNWINDER OR LLVM_NATIVE_ARCH MATCHES ARM)
+if (LIBCXXABI_USE_LLVM_UNWINDER)
   add_definitions(-DLIBCXXABI_USE_LLVM_UNWINDER=1)
 endif()
 
 string(REPLACE ";" " " LIBCXXABI_CXX_FLAGS "${LIBCXXABI_CXX_FLAGS}")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${LIBCXXABI_CXX_FLAGS}")
 set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${LIBCXXABI_C_FLAGS}")
 
 #===============================================================================