[CMake] Cleanup libunwind lookup code.

[logan]

Summary

This commit refines the code for libunwind lookup.

First, this commit will always search for libunwind no matter LLVM
unwinder is enabled or not. We have to do so because we need the
modified "unwind.h" from libunwind. (Note: <unwind.h> which is bundled
with clang/gcc is not sufficient.)

Second, this commit removes LIBCXXABI_LIBUNWIND_INCLUDE_INTERNAL and
refines the find_path() function. Now, it looks similar to the one for
libc++.

Third, this commit removes LIBCXXABI_LIBUNWIND_SOURCES since it haven't
been used for a while. (Note: The reference to "libunwind_ext.h" has
been removed since r241993.)

[rengolin]

We have to do so because we need the modified "unwind.h" from libunwind. (Note: <unwind.h> which is bundled with clang/gcc is not sufficient.)

So, you need libunwind's sources, even if you're not building or using them, to build libc++abi?

[logan]

Yes. This is what we have today.

We can either (1) specify the path to libunwind or (2) manually download unwind.h header from libunwind project and place it to include directory.

[rengolin]

In D24084#530427, @logan wrote:

We can either (1) specify the path to libunwind or (2) manually download unwind.h header from libunwind project and place it to include directory.

That is a horrible dependency... I never hit it because I always test libc++ with libunwind.

Marshall (@mclow.lists), isn't there a better way of doing this? It sounds terrible...

cheers,
--renato

[logan]

In D24084#530433, @rengolin wrote:

That is a horrible dependency... I never hit it because I always test libc++ with libunwind.

I think the best solution is to move/merge libunwind/include/unwind.h to/with clang/lib/Headers/unwind.h. And, modify the libc++abi code base, so that libc++abi can accept the <unwind.h> bundled with clang or gcc. However, it takes time to reach that point.

[asl]

I totally agree with Renato. What part of libc++abi requires exactly libunwind's unwind.h, what is the dependency?

[logan]

In D24084#532724, @asl wrote:

I totally agree with Renato. What part of libc++abi requires exactly libunwind's unwind.h, what is the dependency?

To be specific, we need:

• Structure Definition: _Unwind_Control_Block (missing from <unwind.h> bundled with clang)
• Function Declarations: _Unwind_GetLanguageSpecificData(), _Unwind_GetRegionStart(), _Unwind_{Get,Set}{IP,GR}(), _Unwind_{Get,Set}_VRS(), _Unwind_RaiseException()

This is the status quo after libunwind was spun off (or splitted) last year.

[logan]

Ping. Any further comments? Or, should we duplicate <unwind.h> in multiple repositories?

[rengolin]

@EricWF any comments?

[rengolin]

So, we found this issue with the different unwinds between Clang and libunwind:

https://llvm.org/bugs/show_bug.cgi?id=31035

Is this related?

[logan]

Hi @rengolin:

Yeah. It is a kind of similar. Another route is to move the code from <libunwind>/include/unwind.h to <clang>/lib/Headers and then ship a shim layer with libc++abi and/or libunwind for old clang.

CMakeLists.txt

 # system over a static libc++abi from the output directory.
 option(LIBCXXABI_ENABLE_SHARED "Build libc++abi as a shared library." ON)
 option(LIBCXXABI_ENABLE_STATIC "Build libc++abi as a static library." ON)
 
 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()
 
+# Find libc++ include path and source path.
 find_path(
   LIBCXXABI_LIBCXX_INCLUDES
   vector
   PATHS ${LIBCXXABI_LIBCXX_INCLUDES}
         ${LIBCXXABI_LIBCXX_PATH}/include
         ${CMAKE_BINARY_DIR}/${LIBCXXABI_LIBCXX_INCLUDES}
         ${LLVM_MAIN_SRC_DIR}/projects/libcxx/include
         ${LLVM_INCLUDE_DIR}/c++/v1

 if (LIBCXXABI_LIBCXX_PATH STREQUAL "LIBCXXABI_LIBCXX_PATH-NOTFOUND")
   message(WARNING "LIBCXXABI_LIBCXX_PATH was not specified and couldn't be infered.")
   set(LIBCXXABI_LIBCXX_PATH "")
 endif()
 
 set(LIBCXXABI_LIBCXX_PATH "${LIBCXXABI_LIBCXX_PATH}" CACHE PATH
     "Specify path to libc++ source." FORCE)
 
+# Find libunwind include path (for unwind.h)
+find_path(
+  LIBCXXABI_LIBUNWIND_INCLUDES
+  libunwind.h
+  PATHS ${LIBCXXABI_LIBUNWIND_INCLUDES}
+        ${LIBCXXABI_LIBUNWIND_PATH}/include
+        ${CMAKE_BINARY_DIR}/${LIBCXXABI_LIBUNWIND_INCLUDES}
+        ${LLVM_MAIN_SRC_DIR}/projects/libunwind/include
+  NO_DEFAULT_PATH
+)
+
+if (LIBCXXABI_LIBUNWIND_INCLUDES STREQUAL "LIBCXXABI_LIBUNWIND_INCLUDES-NOTFOUND")
+  message(WARNING "LIBCXXABI_LIBUNWIND_INCLUDES was not specified and couldn't be infered.")
+  set(LIBCXXABI_LIBUNWIND_INCLUDES "")
+endif()
+
+set(LIBCXXABI_LIBUNWIND_INCLUDES "${LIBCXXABI_LIBUNWIND_INCLUDES}" CACHE PATH
+    "Specify path to libunwind includes." FORCE)
+
 #===============================================================================
 # Configure System
 #===============================================================================
 
 # Add path for custom modules
 set(CMAKE_MODULE_PATH
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake"
   "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules"

 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}")
 
 #===============================================================================
 # Setup Source Code
 #===============================================================================
 
-set(LIBCXXABI_LIBUNWIND_INCLUDES "${LIBCXXABI_LIBUNWIND_INCLUDES}" CACHE PATH
-    "Specify path to libunwind includes." FORCE)
-set(LIBCXXABI_LIBUNWIND_PATH "${LIBCXXABI_LIBUNWIND_PATH}" CACHE PATH
-    "Specify path to libunwind source." FORCE)
-
-include_directories(include)
-if (LIBCXXABI_USE_LLVM_UNWINDER OR LLVM_NATIVE_ARCH MATCHES ARM)
-  find_path(
-    LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL
-    libunwind.h
-    PATHS ${LIBCXXABI_LIBUNWIND_INCLUDES}
-          ${LIBCXXABI_LIBUNWIND_PATH}/include
-          ${CMAKE_BINARY_DIR}/${LIBCXXABI_LIBUNWIND_INCLUDES}
-          ${LLVM_MAIN_SRC_DIR}/projects/libunwind/include
-    NO_DEFAULT_PATH
-  )
-
-  find_path(
-    LIBCXXABI_LIBUNWIND_SOURCES
-    libunwind_ext.h
-    PATHS ${LIBCXXABI_LIBUNWIND_PATH}/src/
-          ${LIBCXXABI_LIBUNWIND_INCLUDES}/../src/
-          ${LLVM_MAIN_SRC_DIR}/projects/libunwind/src/
-    NO_DEFAULT_PATH
-  )
-
-  if (LIBCXXABI_LIBUNWIND_SOURCES STREQUAL "LIBCXXABI_LIBUNWIND_SOURCES-NOTFOUND")
-    message(WARNING "LIBCXXABI_LIBUNWIND_SOURCES was not specified and couldn't be infered.")
-    set(LIBCXXABI_LIBUNWIND_SOURCES "")
-  endif()
+# Add libunwind include to header search path (for unwind.h)
+if (LIBCXXABI_LIBUNWIND_INCLUDES)
+  include_directories("${LIBCXXABI_LIBUNWIND_INCLUDES}")
 
-  include_directories("${LIBCXXABI_LIBUNWIND_INCLUDES_INTERNAL}")
-  include_directories("${LIBCXXABI_LIBUNWIND_SOURCES}")
-endif()
+# Add libc++abi include to header search path.
+include_directories(include)
 
 # Add source code. This also contains all of the logic for deciding linker flags
 # soname, etc...
 add_subdirectory(src)
 
 if (LIBCXXABI_STANDALONE_BUILD AND NOT LIBCXXABI_ENABLE_SHARED)
   # We can't reasonably test the system C++ library with a static libc++abi.
   # We either need to be able to replace libc++abi at run time (with a shared