[libcxxabi] r228408 - unwind: use -fno-rtti -fno-exceptions -funwind-tables

[Sergey Dmitrouk]

On Thu, Feb 12, 2015 at 07:13:09AM -0800, Saleem Abdulrasool wrote:
>    I've been successful with clang as well.
>    Reproduction steps would help in identifying what is happening.

Looks like I found the culprit, it's CMake.  I'm cross compiling
libcxxabi for ARM on x86_64 host, compiler targets ARM only (so ARM is
also the default target).  cmake/config-ix.cmake contains (also note "_c_"
instead of "_cxx_", but it doesn't affect results):

    check_c_compiler_flag(-funwind-tables LIBCXXABI_HAS_FUNWIND_TABLES)

This test fails on linking stage:

    CMakeFiles/cmTryCompileExec3829915952.dir/src.cxx.o:(.ARM.exidx+0x0):
    undefined reference to `__aeabi_unwind_cpp_pr0'

because successful linking requires symbol defined in libunwind (and that's
what is being built).  Now here:

    append_if(LIBUNWIND_COMPILE_FLAGS LIBCXXABI_HAS_NO_EXCEPTIONS_FLAG -fno-exceptions)
    append_if(LIBUNWIND_COMPILE_FLAGS LIBCXXABI_HAS_NO_RTTI_FLAG -fno-rtti)
    append_if(LIBUNWIND_COMPILE_FLAGS LIBCXXABI_HAS_FUNWIND_TABLES -funwind-tables)

`-fno-exceptions` is added, but not `-funwind-tables`.  As a result
libunwind sees "can't unwind" marker in exidx records and gives up.

I found this bug report, which seems to be related:
    http://www.cmake.org/Bug/view.php?id=15264

Not adding `-fno-exceptions` flag if `-funwind-tables` isn't available seems
to be reasonable by itself, but will also work around this issue with CMake.

-- 
Sergey