Allow libc++abi to be built without unwinder.

[logan]

Summary

The efforts to build LLVM ARM EHABI unwinder breaks the possibility
to build libcxxabi with libgcc. This commit fix the problem by
introducing the LIBCXXABI_USE_LLVM_UNWINDER conditional compilation
flag and guard the unwinder-specific changes in libc++abi.

(p.s. This is the 2/6 patch to fix the backtrace_test and libgcc support.)

[rengolin]

LGTM, Thanks!

[logan]

Thanks. Committed as rL226819.

CMakeLists.txt

if (NOT LIBCXXABI_ENABLE_THREADS)
   add_definitions(-DLIBCXXABI_HAS_NO_THREADS=1)
 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)
+  add_definitions(-DLIBCXXABI_USE_LLVM_UNWINDER=1)
+endif()
+
 append_if(LIBCXXABI_COMPILE_FLAGS LIBCXXABI_TARGET_TRIPLE
           "-target ${LIBCXXABI_TARGET_TRIPLE}")
 append_if(LIBCXXABI_COMPILE_FLAGS LIBCXXABI_GCC_TOOLCHAIN
          "-gcc-toolchain ${LIBCXXABI_GCC_TOOLCHAIN}")
 append_if(LIBCXXABI_COMPILE_FLAGS LIBCXXABI_SYSROOT
           "--sysroot ${LIBCXXABI_SYSROOT}")
 string(REPLACE ";" " " LIBCXXABI_CXX_FLAGS "${LIBCXXABI_CXX_FLAGS}")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${LIBCXXABI_CXX_FLAGS}")

src/cxa_personality.cpp

if (actions & _UA_CLEANUP_PHASE)
         //     if we were called improperly).
         return results.reason;
     }
     // We were called improperly: neither a phase 1 or phase 2 search
     return _URC_FATAL_PHASE1_ERROR;
 }
 #else
 
+#if !LIBCXXABI_USE_LLVM_UNWINDER
+extern "C" _Unwind_Reason_Code __gnu_unwind_frame(_Unwind_Exception*, _Unwind_Context*);
+#endif
+
 // Helper function to unwind one frame.
 // ARM EHABI 7.3 and 7.4: If the personality function returns _URC_CONTINUE_UNWIND, the
 // personality routine should update the virtual register set (VRS) according to the
 // corresponding frame unwinding instructions (ARM EHABI 9.3.)
-static _Unwind_Reason_Code continue_unwind(_Unwind_Context* context,
-                                           uint32_t* unwind_opcodes,
-                                           size_t opcode_words)
+static _Unwind_Reason_Code continue_unwind(_Unwind_Exception* unwind_exception,
+                                           _Unwind_Context* context)
 {
+#if LIBCXXABI_USE_LLVM_UNWINDER
+    // ARM EHABI # 6.2, # 9.2
+    //
+    //  +---- ehtp
+    //  v
+    // +--------------------------------------+
+    // | +--------+--------+--------+-------+ |
+    // | |0| prel31 to __gxx_personality_v0 | |
+    // | +--------+--------+--------+-------+ |
+    // | |      N |      unwind opcodes     | |  <-- unwind_opcodes
+    // | +--------+--------+--------+-------+ |
+    // | | Word 2        unwind opcodes     | |
+    // | +--------+--------+--------+-------+ |
+    // | ...                                  |
+    // | +--------+--------+--------+-------+ |
+    // | | Word N        unwind opcodes     | |
+    // | +--------+--------+--------+-------+ |
+    // | | LSDA                             | |  <-- lsda
+    // | | ...                              | |
+    // | +--------+--------+--------+-------+ |
+    // +--------------------------------------+
+
+    uint32_t *unwind_opcodes = unwind_exception->pr_cache.ehtp + 1;
+    size_t opcode_words = ((*unwind_opcodes >> 24) & 0xff) + 1;
     if (_Unwind_VRS_Interpret(context, unwind_opcodes, 1, opcode_words * 4) !=
         _URC_CONTINUE_UNWIND)
         return _URC_FAILURE;
+#else
+    if (__gnu_unwind_frame(unwind_exception, context) != _URC_OK)
+        return _URC_FAILURE;
+#endif
     return _URC_CONTINUE_UNWIND;
 }
 
 // ARM register names
+#if !LIBCXXABI_USE_LLVM_UNWINDER
 static const uint32_t REG_UCB = 12;  // Register to save _Unwind_Control_Block
+#endif
 static const uint32_t REG_SP = 13;
 
 static void save_results_to_barrier_cache(_Unwind_Exception* unwind_exception,
                                           const scan_results& results)
 {
     unwind_exception->barrier_cache.bitpattern[0] = (uint32_t)results.adjustedPtr;
     unwind_exception->barrier_cache.bitpattern[1] = (uint32_t)results.actionRecord;
     unwind_exception->barrier_cache.bitpattern[2] = (uint32_t)results.languageSpecificData;
__gxx_personality_v0(_Unwind_State state,
                      _Unwind_Context* context)
 {
     if (unwind_exception == 0 || context == 0)
         return _URC_FATAL_PHASE1_ERROR;
 
     bool native_exception = (unwind_exception->exception_class & get_vendor_and_language) ==
                             (kOurExceptionClass & get_vendor_and_language);
 
-#if LIBCXXABI_ARM_EHABI
-    // ARM EHABI # 6.2, # 9.2
-    //
-    //  +---- ehtp
-    //  v
-    // +--------------------------------------+
-    // | +--------+--------+--------+-------+ |
-    // | |0| prel31 to __gxx_personality_v0 | |
-    // | +--------+--------+--------+-------+ |
-    // | |      N |      unwind opcodes     | |  <-- UnwindData
-    // | +--------+--------+--------+-------+ |
-    // | | Word 2        unwind opcodes     | |
-    // | +--------+--------+--------+-------+ |
-    // | ...                                  |
-    // | +--------+--------+--------+-------+ |
-    // | | Word N        unwind opcodes     | |
-    // | +--------+--------+--------+-------+ |
-    // | | LSDA                             | |  <-- lsda
-    // | | ...                              | |
-    // | +--------+--------+--------+-------+ |
-    // +--------------------------------------+
-
-    uint32_t *UnwindData = unwind_exception->pr_cache.ehtp + 1;
-    uint32_t FirstDataWord = *UnwindData;
-    size_t N = ((FirstDataWord >> 24) & 0xff);
-    size_t NDataWords = N + 1;
-#endif
-
+#if !LIBCXXABI_USE_LLVM_UNWINDER
     // Copy the address of _Unwind_Control_Block to r12 so that
     // _Unwind_GetLanguageSpecificData() and _Unwind_GetRegionStart() can
     // return correct address.
     _Unwind_SetGR(context, REG_UCB, reinterpret_cast<uint32_t>(unwind_exception));
+#endif
 
     scan_results results;
     switch (state) {
     case _US_VIRTUAL_UNWIND_FRAME:
         // Phase 1 search:  All we're looking for in phase 1 is a handler that halts unwinding
         scan_eh_tab(results, _UA_SEARCH_PHASE, native_exception, unwind_exception, context);
         if (results.reason == _URC_HANDLER_FOUND)
         {
             unwind_exception->barrier_cache.sp = _Unwind_GetGR(context, REG_SP);
             if (native_exception)
                 save_results_to_barrier_cache(unwind_exception, results);
             return _URC_HANDLER_FOUND;
         }
         // Did not find the catch handler
         if (results.reason == _URC_CONTINUE_UNWIND)
-            return continue_unwind(context, UnwindData, NDataWords);
+            return continue_unwind(unwind_exception, context);
         return results.reason;
 
     case _US_UNWIND_FRAME_STARTING:
         // Phase 2 search
         if (unwind_exception->barrier_cache.sp == _Unwind_GetGR(context, REG_SP))
         {
             // Found a catching handler in phase 1
             if (native_exception)
case _US_UNWIND_FRAME_STARTING:
 
             // Install the context for the cleanup handler
             set_registers(unwind_exception, context, results);
             return _URC_INSTALL_CONTEXT;
         }
 
         // Did not find any handler
         if (results.reason == _URC_CONTINUE_UNWIND)
-            return continue_unwind(context, UnwindData, NDataWords);
+            return continue_unwind(unwind_exception, context);
         return results.reason;
 
     case _US_UNWIND_FRAME_RESUME:
-        return continue_unwind(context, UnwindData, NDataWords);
+        return continue_unwind(unwind_exception, context);
     }
 
     // We were called improperly: neither a phase 1 or phase 2 search
     return _URC_FATAL_PHASE1_ERROR;
 }
 #endif