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

[Sergey Dmitrouk]

There is no need to revert it completely, not adding -fno-exceptions if
-funwind-tables is not available should be enough.  This is a workaround
for CMake issue, but is also a correct thing to do by itself.
Specifying one option without another literally breaks libunwind, these
options dependent on each other and should either be both applied or
none of them should be applied.

-- 
Sergey

On Fri, Feb 13, 2015 at 10:48:14AM -0800, Anton Korobeynikov wrote:
> Since the stuff is heavily broken at this moment, shall we revert the
> detection stuff for a moment?
>
> On Fri, Feb 13, 2015 at 7:36 PM, Saleem Abdulrasool
> <compnerd at compnerd.org> wrote:
> > On Thu, Feb 12, 2015 at 1:13 PM, Sergey Dmitrouk
> > <sdmitrouk at accesssoftek.com> wrote:
> >>
> >> How about the attached patch?  Seems to work for me (it's trivial, but
> >> syntax is weird, so I wasn't sure).
> >
> >
> > It would prevent building unwind as a DSO without libc++abi linked in, so,
> > this isn't the way to handle this.  I think you need to tweak the detection
> > of the -funwind-tables option.
> >
> >>
> >>
> >> --
> >> Sergey
> >>
> >> On Thu, Feb 12, 2015 at 07:36:14AM -0800, Sergey Dmitrouk wrote:
> >> > 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
> >
> > --
> > Saleem Abdulrasool
> > compnerd (at) compnerd (dot) org
> >
> > ____________
>
> --
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University