Reland "Enable `-funwind-tables` flag when building libunwind"

Summary:
Relands https://reviews.llvm.org/D70815.

The original commit set `CMAKE_TRY_COMPILE_TARGET_TYPE` to
`STATIC_LIBRARY` globally in libunwind/CMakeLists.txt, which effectively
disabled the linking step in CMake checks.

This broke some builds (see 938c70b86c).

Here we set CMAKE_TRY_COMPILE_TARGET_TYPE to
STATIC_LIBRARY only when checking for presence of the `-funwind-tables`
flag, and then set it back to the original value so it doesn't affect
other checks.

Reviewers: mstorsjo, jfb

Subscribers: mgorny, christof, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D71117

libunwind/CMakeLists.txt

@@ -291,7 +291,29 @@ endif()
 add_cxx_compile_flags_if_supported(-fstrict-aliasing)
 add_cxx_compile_flags_if_supported(-EHsc)
 
+# Don't run the linker in this CMake check.
+#
+# The reason why this was added is that when building libunwind for
+# ARM Linux, we need to pass the -funwind-tables flag in order for it to
+# work properly with ARM EHABI.
+#
+# However, when performing CMake checks, adding this flag causes the check
+# to produce a false negative, because the compiler generates calls
+# to __aeabi_unwind_cpp_pr0, which is defined in libunwind itself,
+# which isn't built yet, so the linker complains about undefined symbols.
+#
+# This leads to libunwind not being built with this flag, which makes
+# libunwind quite useless in this setup.
+set(_previous_CMAKE_TRY_COMPILE_TARGET_TYPE ${CMAKE_TRY_COMPILE_TARGET_TYPE})
+set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
 add_compile_flags_if_supported(-funwind-tables)
+set(CMAKE_TRY_COMPILE_TARGET_TYPE ${_previous_CMAKE_TRY_COMPILE_TARGET_TYPE})
+
+if (LIBUNWIND_USES_ARM_EHABI AND NOT LIBUNWIND_SUPPORTS_FUNWIND_TABLES_FLAG)
+  message(SEND_ERROR "The -funwind-tables flag must be supported "
+                     "because this target uses ARM Exception Handling ABI")
+endif()
+
 add_cxx_compile_flags_if_supported(-fno-exceptions)
 add_cxx_compile_flags_if_supported(-fno-rtti)
 

libunwind/cmake/config-ix.cmake

@@ -2,6 +2,7 @@ include(CMakePushCheckState)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckLibraryExists)
+include(CheckSymbolExists)
 include(CheckCSourceCompiles)
 
 check_library_exists(c fopen "" LIBUNWIND_HAS_C_LIB)
@@ -73,3 +74,13 @@ check_cxx_compiler_flag(-nostdinc++ LIBUNWIND_HAS_NOSTDINCXX_FLAG)
 # Check libraries
 check_library_exists(dl dladdr "" LIBUNWIND_HAS_DL_LIB)
 check_library_exists(pthread pthread_once "" LIBUNWIND_HAS_PTHREAD_LIB)
+
+# Check symbols
+check_symbol_exists(__arm__ "" LIBUNWIND_TARGET_ARM)
+check_symbol_exists(__USING_SJLJ_EXCEPTIONS__ "" LIBUNWIND_USES_SJLJ_EXCEPTIONS)
+check_symbol_exists(__ARM_DWARF_EH__ "" LIBUNWIND_USES_DWARF_EH)
+
+if(LIBUNWIND_TARGET_ARM AND NOT LIBUNWIND_USES_SJLJ_EXCEPTIONS AND NOT LIBUNWIND_USES_DWARF_EH)
+  # This condition is copied from __libunwind_config.h
+  set(LIBUNWIND_USES_ARM_EHABI ON)
+endif()

libunwind/test/CMakeLists.txt

@@ -16,6 +16,7 @@ pythonize_bool(LIBCXX_ENABLE_SHARED)
 pythonize_bool(LIBUNWIND_ENABLE_SHARED)
 pythonize_bool(LIBUNWIND_ENABLE_THREADS)
 pythonize_bool(LIBUNWIND_ENABLE_EXCEPTIONS)
+pythonize_bool(LIBUNWIND_USES_ARM_EHABI)
 pythonize_bool(LIBUNWIND_USE_COMPILER_RT)
 pythonize_bool(LIBUNWIND_BUILD_EXTERNAL_THREAD_LIBRARY)
 set(LIBUNWIND_TARGET_INFO "libcxx.test.target_info.LocalTI" CACHE STRING

libunwind/test/libunwind/test/config.py

@@ -37,6 +37,8 @@ class Configuration(LibcxxConfiguration):
         super(Configuration, self).configure_features()
         if not self.get_lit_bool('enable_exceptions', True):
             self.config.available_features.add('libcxxabi-no-exceptions')
+        if self.get_lit_bool('arm_ehabi', False):
+            self.config.available_features.add('libunwind-arm-ehabi')
 
     def configure_compile_flags(self):
         self.cxx.compile_flags += ['-DLIBUNWIND_NO_TIMER']
@@ -66,3 +68,11 @@ class Configuration(LibcxxConfiguration):
 
     def configure_compile_flags_rtti(self):
         pass
+
+    def configure_link_flags_cxx_library(self):
+        # libunwind tests should not link with libc++
+        pass
+
+    def configure_link_flags_abi_library(self):
+        # libunwind tests should not link with libc++abi
+        pass

libunwind/test/lit.site.cfg.in

@@ -19,6 +19,7 @@ config.executor                 = "@LIBUNWIND_EXECUTOR@"
 config.libunwind_shared         = @LIBUNWIND_ENABLE_SHARED@
 config.enable_shared            = @LIBCXX_ENABLE_SHARED@
 config.enable_exceptions        = @LIBUNWIND_ENABLE_EXCEPTIONS@
+config.arm_ehabi                = @LIBUNWIND_USES_ARM_EHABI@
 config.host_triple              = "@LLVM_HOST_TRIPLE@"
 config.target_triple            = "@TARGET_TRIPLE@"
 config.use_target               = bool("@LIBUNWIND_TARGET_TRIPLE@")

libunwind/test/signal_frame.pass.cpp

@@ -9,6 +9,8 @@
 
 // Ensure that functions marked as signal frames are reported as such.
 
+// UNSUPPORTED: libunwind-arm-ehabi
+
 #include <assert.h>
 #include <stdlib.h>
 #include <libunwind.h>
@@ -20,9 +22,7 @@ void test() {
   unw_getcontext(&uc);
   unw_init_local(&cursor, &uc);
   assert(unw_step(&cursor) > 0);
-#if !defined(_LIBUNWIND_ARM_EHABI)
   assert(unw_is_signal_frame(&cursor));
-#endif
 }
 
 int main() {