[libc++abi] Add LIBCXXABI_ENABLE_PIC cmake option

[sbc100]

Summary

This is on by default, since on some platforms and configurations
libc++abi.a gets statically linked into shared libraries and/or
PIE executables.

This change is a followup to https://reviews.llvm.org/D60005 which
allows us to default to PIC code, but disable this if needed (for
example on WebAssembly where PIC code its currently incompatible with
static linking).

[jiangyi]

Looks good to me.

[ldionne]

I would much rather let people use CMAKE_POSITION_INDEPENDENT_CODE instead. Why is that not a viable approach?

[sbc100]

In D60049#1449833, @ldionne wrote:

I would much rather let people use CMAKE_POSITION_INDEPENDENT_CODE instead. Why is that not a viable approach?

I think the reason is that we want more fine grained control here. My understanding is that CMAKE_POSITION_INDEPENDENT_CODE effects all the targets in the entire cmake build which in the case of llvm might include llvm, clang, lld, libc++, libc++abi, etc. i could be wrong though.

Also we already have LLVM_ENABLE_PIC option for the wider project so this naming made sense. @phosek might have more background here?

[ldionne]

In D60049#1450031, @sbc100 wrote:

In D60049#1449833, @ldionne wrote:

I would much rather let people use CMAKE_POSITION_INDEPENDENT_CODE instead. Why is that not a viable approach?

I think the reason is that we want more fine grained control here. My understanding is that CMAKE_POSITION_INDEPENDENT_CODE effects all the targets in the entire cmake build which in the case of llvm might include llvm, clang, lld, libc++, libc++abi, etc. i could be wrong though.

Yes, it does. I'm not a big fan of duplicating logic that CMake already has builtin, but OK. However, why doesn't this control whether -fPIC is being used for the shared library AND the static library?

[sbc100]

My understanding is that POSITION_INDEPENDENT_CODE should always be on when building shared libraries. What would it mean to build a non-PIC shared library?

The motivation for this change is allow me to disable PIC code in non-shared libraries. For WebAssembly we currently disable shared libraries so increasing the reach of this option wouldn't break us, but I'm not sure I see any reason to do that.

[brzycki]

Hello @sbc100, I'm seeing a build break caused by D60005 when I attempt to do the following:

1. compile llvm for aarch64 with a gnu toolchain sysroot
2. use the freshly built llvm aarch64 compiler to build libcxxabi aarch64 before building libcxx aarch64

I see the following ninja error when attempting to create the libc++abi.so.1.0. I do not see this issue with the immediately previous SHA commit of 5f0c4c67bbff53.

I also tried applying the patch in this ticket to tip and recompiled and I still see the same error.

Please let me know if you need further build details.

cmake -G Ninja -D CMAKE_INSTALL_PREFIX=/work/b.rzycki/4/aarch64-sarc-linux-gnu/sysroot -D LLVM_PATH=/tmp/tmp.N01uFPofsv/src/llvm -D LIBCXXABI_LIBCXX_PATH=/tmp/tmp.N01uFPofsv/src/libcxx -D CMAKE_C_FLAGS= -D CMAKE_CXX_FLAGS= /tmp/tmp.N01uFPofsv/src/libcxxabi
Re-run cmake no build system arguments
-- The CXX compiler identification is Clang 9.0.0
-- The C compiler identification is Clang 9.0.0
-- Check for working CXX compiler: /work/b.rzycki/4/bin/aarch64-linux-clang++
-- Check for working CXX compiler: /work/b.rzycki/4/bin/aarch64-linux-clang++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Check for working C compiler: /work/b.rzycki/4/bin/aarch64-linux-clang
-- Check for working C compiler: /work/b.rzycki/4/bin/aarch64-linux-clang -- works
...

ninja -v -j 64
...
FAILED: lib/libc++abi.so.1.0
: && /work/b.rzycki/4/bin/aarch64-linux-clang++ -fPIC   -nodefaultlibs -shared -Wl,-soname,libc++abi.so.1 -o lib/libc++abi.so.1.0 src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_demangle.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception_storage.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_guard.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_handlers.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_unexpected.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_vector.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_virtual.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_exception.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_stdexcept.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_typeinfo.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/abort_message.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/fallback_malloc.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/private_typeinfo.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/stdlib_new_delete.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_exception.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_personality.cpp.o src/CMakeFiles/cxxabi_shared_objects.dir/cxa_thread_atexit.cpp.o  -Wl,-rpath,"\$ORIGIN/../lib" -lpthread -lc -lgcc_s && :
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `__cxa_unexpected_handler' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: in function `std::set_unexpected(void (*)())':
cxa_default_handlers.cpp:(.text+0x18): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `__cxa_terminate_handler' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: in function `std::set_terminate(void (*)())':
cxa_default_handlers.cpp:(.text+0x234): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `_ZTISt9exception' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: in function `demangling_terminate_handler()':
cxa_default_handlers.cpp:(.text+0x2c0): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `__cxa_terminate_handler' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: in function `__cxx_global_var_init':
cxa_default_handlers.cpp:(.text.startup+0x8): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `__cxa_unexpected_handler' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_default_handlers.cpp.o: in function `__cxx_global_var_init.1':
cxa_default_handlers.cpp:(.text.startup+0x24): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `_ZTISt8bad_cast' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o: in function `__cxa_bad_cast':
cxa_aux_runtime.cpp:(.text+0x10): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `_ZNSt8bad_castD1Ev' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: cxa_aux_runtime.cpp:(.text+0x18): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `_ZTISt10bad_typeid' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o: in function `__cxa_bad_typeid':
cxa_aux_runtime.cpp:(.text+0x54): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `_ZNSt10bad_typeidD1Ev' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: cxa_aux_runtime.cpp:(.text+0x5c): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `_ZTISt20bad_array_new_length' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o: in function `__cxa_throw_bad_array_new_length':
cxa_aux_runtime.cpp:(.text+0x98): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_aux_runtime.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `_ZNSt20bad_array_new_lengthD1Ev' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: cxa_aux_runtime.cpp:(.text+0xa0): dangerous relocation: unsupported relocation
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_demangle.cpp.o: relocation R_AARCH64_ADR_PREL_PG_HI21 against symbol `stderr@@GLIBC_2.17' which may bind externally can not be used when making a shared object; recompile with -fPIC
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: src/CMakeFiles/cxxabi_shared_objects.dir/cxa_demangle.cpp.o(.text+0x17c8): unresolvable R_AARCH64_ADR_PREL_PG_HI21 relocation against symbol `stderr@@GLIBC_2.17'
/work/b.rzycki/4/bin/aarch64-sarc-linux-gnu-ld: final link failed: bad value
clang-9: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

[sbc100]

Oops. Sorry that is a different issue. Fixed in https://reviews.llvm.org/D60158

In D60049#1451682, @brzycki wrote:

Hello @sbc100, I'm seeing a build break caused by D60005 when I attempt to do the following:

1. compile llvm for aarch64 with a gnu toolchain sysroot
2. use the freshly built llvm aarch64 compiler to build libcxxabi aarch64 before building libcxx aarch64

[ldionne]

In D60049#1450593, @sbc100 wrote:

My understanding is that POSITION_INDEPENDENT_CODE should always be on when building shared libraries. What would it mean to build a non-PIC shared library?

The motivation for this change is allow me to disable PIC code in non-shared libraries. For WebAssembly we currently disable shared libraries so increasing the reach of this option wouldn't break us, but I'm not sure I see any reason to do that.

For example, Apple's libc++.dylib is not built with -fPIC. I believe this is because of the shared linker cache but I should investigate TBH. But regardless, it does make sense not to force shared libraries to be built with -fPIC if the user doesn't want that.

However, there's a twist here. We're using an object library to create the shared library so the CMake default for enabling -fPIC (on for shared libraries, off for static libraries) is not going to trigger, and the shared library will never be built with -fPIC. We need a bit of refactoring for this to be possible.

[sbc100]

• control pic for shared library too

[sbc100]

OK I made LIBCXXABI_ENABLE_PIC effect both shared and non-shared versions of the library.

[ldionne]

In D60049#1452223, @sbc100 wrote:

OK I made LIBCXXABI_ENABLE_PIC effect both shared and non-shared versions of the library.

Let's wait for https://reviews.llvm.org/D60166 to land, and then you can rebase on top of it. This is going to fix the issue I mentioned.

libcxxabi/src/CMakeLists.txt

183

This doesn't work because it's an object library. Now libc++abi.dylib will never be built with -fPIC.

[sbc100]

In D60049#1452325, @ldionne wrote:

In D60049#1452223, @sbc100 wrote:

OK I made LIBCXXABI_ENABLE_PIC effect both shared and non-shared versions of the library.

Let's wait for https://reviews.llvm.org/D60166 to land, and then you can rebase on top of it. This is going to fix the issue I mentioned.

In D60049#1452325, @ldionne wrote:

In D60049#1452223, @sbc100 wrote:

OK I made LIBCXXABI_ENABLE_PIC effect both shared and non-shared versions of the library.

Let's wait for https://reviews.llvm.org/D60166 to land, and then you can rebase on top of it. This is going to fix the issue I mentioned.

IIUC this change will still be required with way since we want to default to using PIC in the static library. In which case if you don't mind I'd rather land mine first since its smaller and ready to go. In other words I don't see any reason to block this change on your refactor, unless I'm missing something.

[sbc100]

Let me re-phrase that. I feel like I broke stuff in https://reviews.llvm.org/D60005 and I want to get it fixed. Since the current state is broken I'd rather not refactor without first with fixing (via this change) or reverting https://reviews.llvm.org/D60005.

[sbc100]

libcxxabi/src/CMakeLists.txt

183

I have another minor fix to this line as a separate change: https://reviews.llvm.org/D60158. It does seem seem to work and builds the library code with fPIC.

[ldionne]

libcxxabi/src/CMakeLists.txt

183

Hmm yes, indeed my last comment doesn't make sense since you explicitly set POSITION_INDEPENDENT_CODE ON on the object library for the shared library. So this will work and my refactoring is not necessary prior to this change. We can land this and then I'll rebase my refactoring on top.

[sbc100]

• rebase

libcxxabi/CMakeLists.txt

 #===============================================================================
 include(CMakeDependentOption)
 include(HandleCompilerRT)
 
 # Define options.
 option(LIBCXXABI_ENABLE_EXCEPTIONS "Use exceptions." ON)
 option(LIBCXXABI_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
 option(LIBCXXABI_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
+option(LIBCXXABI_ENABLE_PIC "Build Position-Independent Code, even in static library" ON)
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 option(LIBCXXABI_ENABLE_STATIC_UNWINDER "Statically link the LLVM unwinder." OFF)
 option(LIBCXXABI_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBCXXABI_ENABLE_THREADS "Build with threads enabled" ON)
 option(LIBCXXABI_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 option(LIBCXXABI_HAS_EXTERNAL_THREAD_API
   "Build libc++abi with an externalized threading API.

libcxxabi/src/CMakeLists.txt

set_target_properties(${name}
                             OFF
                           CXX_STANDARD
                             11
                           CXX_STANDARD_REQUIRED
                             ON
                           COMPILE_FLAGS
                             "${LIBCXXABI_COMPILE_FLAGS}")
 
+  if(LIBCXXABI_ENABLE_PIC)
+    set_target_properties(${name} PROPERTIES POSITION_INDEPENDENT_CODE ON)
+  endif()
+
   if(ARGS_DEFINES)
     target_compile_definitions(${name} PRIVATE ${ARGS_DEFINES})
   endif()
 
   if(ARGS_FLAGS)
     target_compile_options(${name} PRIVATE ${ARGS_FLAGS})
   endif()
 endmacro()
 
 if(LIBCXXABI_HERMETIC_STATIC_LIBRARY)
   append_flags_if_supported(CXXABI_STATIC_OBJECTS_FLAGS -fvisibility=hidden)
   append_flags_if_supported(CXXABI_STATIC_OBJECTS_FLAGS -fvisibility-global-new-delete-hidden)
   cxxabi_object_library(cxxabi_static_objects
     DEFINES
       _LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS
       _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
     FLAGS ${CXXABI_STATIC_OBJECTS_FLAGS})
 else()
   cxxabi_object_library(cxxabi_static_objects)
 endif()
 
-cxxabi_object_library(cxxabi_shared_objects POSITION_INDEPENDENT_CODE ON)
+cxxabi_object_library(cxxabi_shared_objects)
 set(cxxabi_static_sources $<TARGET_OBJECTS:cxxabi_static_objects>)
 set(cxxabi_shared_sources $<TARGET_OBJECTS:cxxabi_shared_objects>)
 
 # Build the shared library.
 if (LIBCXXABI_ENABLE_SHARED)
   add_library(cxxabi_shared SHARED ${cxxabi_shared_sources})
   if(COMMAND llvm_setup_rpath)
     llvm_setup_rpath(cxxabi_shared)