[CMake][compiler-rt] Provide a dedicated option for LLVM unwinder

[phosek]

Summary

This allows configuring LLVM unwinder separately from the C++ library
matching how we configure it in libcxx.

[mstorsjo]

This LGTM (although it would probably be easier to reason about if merged with the other patch).

Libcxx seems to maybe be moving to a different style of option for requesting linking e.g. libcxxabi statically, but I'm not sure if it's worth (or even feasible) to try to align this option with the suggested new libcxx option format.

[phosek]

Merged into D115674.

compiler-rt/CMakeLists.txt

if(CMAKE_CONFIGURATION_TYPES)
   set(CMAKE_CFG_RESOLVED_INTDIR "${CMAKE_CFG_INTDIR}/")
 else()
   set(CMAKE_CFG_RESOLVED_INTDIR "")
 endif()
 
 include(SetPlatformToolchainTools)
 include(base-config-ix)
 include(CompilerRTUtils)
+include(CMakeDependentOption)
 
 option(COMPILER_RT_BUILD_BUILTINS "Build builtins" ON)
 mark_as_advanced(COMPILER_RT_BUILD_BUILTINS)
 option(COMPILER_RT_BUILD_CRT "Build crtbegin.o/crtend.o" ON)
 mark_as_advanced(COMPILER_RT_BUILD_CRT)
 option(COMPILER_RT_CRT_USE_EH_FRAME_REGISTRY "Use eh_frame in crtbegin.o/crtend.o" ON)
 mark_as_advanced(COMPILER_RT_CRT_USE_EH_FRAME_REGISTRY)
 option(COMPILER_RT_BUILD_SANITIZERS "Build sanitizers" ON)

 
 # This needs to be a full C++ library for linking gtest and unit tests.
 set(SANITIZER_TEST_CXX "default" CACHE STRING
     "Specify C++ library to use for tests.")
 set(CXXLIBS none default libstdc++ libc++)
 set_property(CACHE SANITIZER_TEST_CXX PROPERTY STRINGS ;${CXXLIBS})
 handle_default_cxx_lib(SANITIZER_TEST_CXX)
 
+option(COMPILER_RT_USE_LLVM_UNWINDER "Use the LLVM unwinder." OFF)
+cmake_dependent_option(COMPILER_RT_ENABLE_STATIC_UNWINDER
+  "Statically link the LLVM unwinder." OFF
+  "COMPILER_RT_USE_LLVM_UNWINDER" OFF)
+
 set(DEFAULT_SANITIZER_USE_STATIC_LLVM_UNWINDER OFF)
 if (FUCHSIA)
   set(DEFAULT_SANITIZER_USE_STATIC_LLVM_UNWINDER ON)
 elseif (DEFINED LIBUNWIND_ENABLE_SHARED AND NOT LIBUNWIND_ENABLE_SHARED)
   set(DEFAULT_SANITIZER_USE_STATIC_LLVM_UNWINDER ON)
 endif()
 
 option(SANITIZER_USE_STATIC_LLVM_UNWINDER

 # Remove -stdlib= which is unused when passing -nostdinc++...
 string(REGEX MATCHALL "-stdlib=[a-zA-Z+]*" stdlib_flag "${CMAKE_CXX_FLAGS}")
 string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
 
 # ...we need it to build some runtimes and tests so readd it where appropriate.
 list(APPEND COMPILER_RT_COMMON_CFLAGS ${stdlib_flag})
 list(APPEND COMPILER_RT_COMMON_LINK_FLAGS ${stdlib_flag})
 
+if(COMPILER_RT_USE_LLVM_UNWINDER)
+  # TODO: set --unwindlib=none after https://reviews.llvm.org/D115674 lands.
+  if (COMPILER_RT_ENABLE_STATIC_UNWINDER)
+    set(unwinder_target unwind_static)
+  else()
+    set(unwinder_target unwind_shared)
+  endif()
+  list(APPEND SANITIZER_CXX_ABI_LIBRARIES "$<$<TARGET_EXISTS:${unwind_target}>:$<TARGET_LINKER_FILE:${unwind_target}>>")
+endif()
+
 macro(append_libcxx_libs var)
   if (${var}_INTREE)
     if (SANITIZER_USE_STATIC_LLVM_UNWINDER AND (TARGET unwind_static OR HAVE_LIBUNWIND))
       list(APPEND ${var}_LIBRARIES unwind_static)
     elseif (TARGET unwind_shared OR HAVE_LIBUNWIND)
       list(APPEND ${var}_LIBRARIES unwind_shared)
     endif()