[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.

This also applies changes made to libunwind+libcxxabi+libcxx in D113253
to compiler-rt.

[mstorsjo]

LGTM

(Why don't the sanitizers default to any of the *-commits mailing lists as subscribers?)

[mstorsjo]

compiler-rt/CMakeLists.txt

491

I presume we should only do this if we actually know we're linking against a libunwind that is built in the same cmake invocation, i.e. either of the TARGET unwind_* OR HAVE_LIBUNWIND cases below, otherwise this does break things.

[phosek]

compiler-rt/CMakeLists.txt

491

Yes, I hope this could be more cleanly addressed by D126909.

[mstorsjo]

compiler-rt/CMakeLists.txt

487

This todo refers to this change itself.

491

Hmm, this feels a bit nonobvious, when this here isn't directly related to the COMPILER_RT_USE_LLVM_UNWINDER variable. Is it the case that this only gets called when COMPILER_RT_USE_LLVM_UNWINDER is true, or can you end up with other kinds of mismatches?

[phosek]

compiler-rt/CMakeLists.txt

491

I removed it from this change. Prior to this change, C++ ABI and unwinder in compiler-rt is controlled by a single option, but that's causing issues. This change introduces a separate option for controlling the unwinder. I'm also working on another change that will introduce a new option for controlling C++ library. When those land, I think we should deprecate and remove the original flag.

[phosek]

@mstorsjo let me know if this looks good to you, I tested this locally and it seems to be working fine.

[mstorsjo]

The changes left in this patch seem fine to me now!

compiler-rt/CMakeLists.txt

 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})
 
 macro(append_libcxx_libs var)
   if (${var}_INTREE)
+    # If we're linking directly against the libunwind that we're building
+    # in the same invocation, don't try to link in the toolchain's
+    # default libunwind (which may be missing still).
+    append_list_if(COMPILER_RT_SUPPORTS_UNWINDLIB_NONE_FLAG --unwindlib=none SANITIZER_COMMON_LINK_FLAGS)
     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()
 
     if (SANITIZER_USE_STATIC_CXX_ABI AND (TARGET cxxabi_static OR HAVE_LIBCXXABI))
       list(APPEND ${var}_LIBRARIES cxxabi_static)

compiler-rt/cmake/config-ix.cmake

 include(CMakePushCheckState)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckIncludeFiles)
 include(CheckLibraryExists)
+include(CheckLinkerFlag)
 include(CheckSymbolExists)
 include(TestBigEndian)
 
+# The compiler driver may be implicitly trying to link against libunwind.
+# This is normally ok (libcxx relies on an unwinder), but if libunwind is
+# built in the same cmake invocation as compiler-rt and we're using the
+# in tree version of runtimes, we'd be linking against the just-built
+# libunwind (and the compiler implicit -lunwind wouldn't succeed as the newly
+# built libunwind isn't installed yet). For those cases, it'd be good to
+# link with --uwnindlib=none. Check if that option works.
+llvm_check_linker_flag("--unwindlib=none" COMPILER_RT_SUPPORTS_UNWINDLIB_NONE_FLAG)
+if (COMPILER_RT_SUPPORTS_UNWINDLIB_NONE_FLAG)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} --unwindlib=none")
+endif()
+
 function(compiler_rt_check_linker_flag flag out_var)
   cmake_push_check_state()
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} ${flag}")
   check_cxx_compiler_flag("" ${out_var})
   cmake_pop_check_state()
 endfunction()
 
 check_library_exists(c fopen "" COMPILER_RT_HAS_LIBC)