[CMake] Fix libc++abi __aeabi_idiv() link error.

[logan]

Summary

This commit fixes __aeabi_idiv() link error when we are building
libc++abi with compiler-rt.

When compiler-rt is enabled, the option --rtlib=compiler-rt will be
added to link flags. However, if -nodefaultlibs is specified
simultaneously, --rtlib will be ignored and compiler-rt libraries will
not be linked. Consequently, several ARM built-in functions, e.g.
__aeabi_idiv(), will not be available.

[rengolin]

This looks like a work around the fact that --rtlib is being ignored with --nodefaultlibs. I'm not sure that's a sane behaviour, if you explicitly specify --rtlib.

[EricWF]

This is not correct. We need the -nodefaultlibs on most platforms to prevent libstdc++ from being linked. I think the correct fix is to change --rtlib=<foo>

[logan]

Hi @rengolin and @EricWF,

After tracing the commit log of clang, it seems that --rtlib= is ignored intentionally when -nodefaultlibs is present. (see also: rL254535)

And, unfortunately, we don't have a command line option to ask clang to link compiler-rt built-in library along (regardless the -nodefaultlibs option.)

One solution might be adding the libclang_rt.builtins.${arch}.a detection rules[1] to CMakeLists.txt, and manually specify -lclang_rt.builtins-${arch}.a when LIBCXXABI_USE_COMPILER_RT is enabled. How do you think about this solution?

[1] There is a function named ToolChain::getCompilerRT() in ${clang}/lib/Driver/ToolChain.cpp which is responsible to detect and pick the correct compiler-rt library.

[EricWF]

In D24083#534459, @logan wrote:

One solution might be adding the libclang_rt.builtins.${arch}.a detection rules[1] to CMakeLists.txt, and manually specify -lclang_rt.builtins-${arch}.a when LIBCXXABI_USE_COMPILER_RT is enabled. How do you think about this solution?

That SGTM. There's a similar workaround in libc++ to link the sanitizer run-times on OS X (found here).

[rengolin]

In D24083#534464, @EricWF wrote:

In D24083#534459, @logan wrote:

One solution might be adding the libclang_rt.builtins.${arch}.a detection rules[1] to CMakeLists.txt, and manually specify -lclang_rt.builtins-${arch}.a when LIBCXXABI_USE_COMPILER_RT is enabled. How do you think about this solution?

That SGTM. There's a similar workaround in libc++ to link the sanitizer run-times on OS X (found here).

+1

[rengolin]

Was this abandoned? Merged?

[logan]

Sorry, I don't have time to work on this recently. Here's the plan:

1. Add another flag to specify the path to libclang_rt-builtin.a.

2. If that flag was not specified and the compiler is clang, then run clang -### to extract for -L option and -l option for libclang_rt-builtin.a.

3. If neither (1) nor (2) are available, then report an error.

I hope I can get back to this next week.

src/CMakeLists.txt

else()
     list(APPEND libraries unwind)
   endif()
 else()
   append_if(libraries LIBCXXABI_HAS_GCC_S_LIB gcc_s)
 endif()
 
 # Setup flags.
 append_if(LIBCXXABI_COMPILE_FLAGS LIBCXXABI_HAS_FPIC_FLAG -fPIC)
+if (NOT LIBCXXABI_USE_COMPILER_RT)
   append_if(LIBCXXABI_LINK_FLAGS LIBCXXABI_HAS_NODEFAULTLIBS_FLAG -nodefaultlibs)
+endif()
 
 set(LIBCXXABI_SHARED_LINK_FLAGS)
 
 if ( APPLE )
   if ( CMAKE_OSX_DEPLOYMENT_TARGET STREQUAL "10.6" )
     list(APPEND LIBCXXABI_COMPILE_FLAGS "-U__STRICT_ANSI__")
     list(APPEND LIBCXXABI_SHARED_LINK_FLAGS
       "-compatibility_version 1"