[Bug target/71951] libgcc_s built with -fomit-frame-pointer on aarch64 is broken

["wdijkstr at arm dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71951

--- Comment #11 from Wilco <wdijkstr at arm dot com> ---
(In reply to Icenowy Zheng from comment #10)
> In my environment (glibc 2.25, and both the building scripts of glibc and
> gcc have -fomit-frame-pointer automatically enabled), this bug is not fully
> resolved yet.
> 
> With GCC upgraded to 6.4.0, GDB debugger started to work correctly. (With
> GCC 6.3.0 GDB cannot even work and segfault at unwind code in libgcc).
> 
> However, if I still build GCC with -fomit-frame-pointer in CFLAGS, the
> backtrace() function of glibc cannot work, and segfault at line 240 of
> libgcc/unwind-dw2.c .
> 
> By reading the source code, I think the unwind code is still trying to get
> CFA from the register x29, and when debugging I found that the x29 register
> in the unwind context is 0 (because of -fomit-frame-pointer), so line 240 is
> dereferencing a NULL pointer, so it segfaulted.
> 
> Maybe the behavior that accessing x29 register to get CFA is not correct?

Well if everything built with -fomit-frame-pointer then it is definitely wrong
to read x29. Can you give more info similar to comment #3?