[libc++] add linker option "-Wl,-z,defs" in standalone build

[zlei]

Summary

This option is used to avoid underlinking. It's available in LLVM's main tree, but not in a standalone build of libc++. This patch ensures the option is passed in whether libc++ is built in-tree or out-of-tree.

[EricWF]

LGTM. Thanks for the patch.

[rmaprath]

libcxx/trunk/CMakeLists.txt

329 ↗

(On Diff #72632)

Perhaps we should exclude LIBCXX_HAS_EXTERNAL_THREAD_API as well? Because there we explicitly strip off these flags.

[zlei]

libcxx/trunk/CMakeLists.txt

329 ↗

(On Diff #72632)

I agree. Could you prepare a patch for it?

BTW, I really don't like duplicating code snippet from llvm. Is there any better solution for that?

[rmaprath]

libcxx/trunk/CMakeLists.txt

329 ↗

(On Diff #72632)

Sure, I'll do a patch.

Btw, I think this configuration is better placed in HandleOutOfTreeLLVM.cmake module.

I'll see if I can find a better way to do this.

[mgorny]

I'm starting to regret that I've committed this. It breaks horribly any pure-LLVM build, i.e. without linking to libgcc_s. It seems that the build system is completely unprepared to link to compiler-rt or libunwind when linking the shared library, and with -DLIBCXX_HAS_GCC_S_LIB=OFF, it fails due to a lot of missing symbols. I am going to revert this until we get the build system working completely.

[rmaprath]

In D24119#554234, @mgorny wrote:

I'm starting to regret that I've committed this. It breaks horribly any pure-LLVM build, i.e. without linking to libgcc_s. It seems that the build system is completely unprepared to link to compiler-rt or libunwind when linking the shared library, and with -DLIBCXX_HAS_GCC_S_LIB=OFF, it fails due to a lot of missing symbols. I am going to revert this until we get the build system working completely.

Doesn't -DLIBCXXABI_USE_LLVM_UNWINDER=ON make it possible to build without libgcc_s? Or is this something else?

[mgorny]

In D24119#554251, @rmaprath wrote:

Doesn't -DLIBCXXABI_USE_LLVM_UNWINDER=ON make it possible to build without libgcc_s? Or is this something else?

It's supposed to but it doesn't add the necessary libraries when linking libc++.so.

libcxx/CMakeLists.txt

 endif()
 remove_flags(-stdlib=libc++ -stdlib=libstdc++)
 
 # FIXME(EricWF): See the FIXME on LIBCXX_ENABLE_PEDANTIC.
 # Remove the -pedantic flag and -Wno-pedantic and -pedantic-errors
 # so they don't get transformed into -Wno and -errors respectivly.
 remove_flags(-Wno-pedantic -pedantic-errors -pedantic)
 
+# FIXME: this is cribbed from HandleLLVMOptions.cmake.
+if(LIBCXX_STANDALONE_BUILD)
+  # Pass -Wl,-z,defs. This makes sure all symbols are defined. Otherwise a DSO
+  # build might work on ELF but fail on MachO/COFF.
+  if(NOT (${CMAKE_SYSTEM_NAME} MATCHES "Darwin" OR WIN32 OR CYGWIN OR
+          ${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD" OR
+          ${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD") AND
+     NOT LLVM_USE_SANITIZER)
+    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,-z,defs")
+  endif()
+endif()
+
 # Required flags ==============================================================
 set(LIBCXX_STANDARD_VER c++11 CACHE INTERNAL "internal option to change build dialect")
 add_compile_flags_if_supported(-std=${LIBCXX_STANDARD_VER})
 mangle_name("LIBCXX_SUPPORTS_STD_EQ_${LIBCXX_STANDARD_VER}_FLAG" SUPPORTS_DIALECT_NAME)
 if (NOT MSVC AND NOT ${SUPPORTS_DIALECT_NAME})
   message(FATAL_ERROR "C++11 or greater is required but the compiler does not support ${LIBCXX_STANDARD_VER}")
 endif()