build failure and warnings on aarch64 and redhat 7.x

[Mark W. Krentel]

Hi, I found a data point where libunwind 1.5-stable doesn't build.
I started looking into this and I have a couple questions or suggestions.
These apply to both master and 1.5-stable.

(1) --enable-debug-frame doesn't build on RedHat 7.x because elf.h is
too old.  I first noticed this on aarch64 where debug-frame is turned
on by default, but it applies anywhere with --enable-debug-frame and
RedHat 7.x.

The problem is /usr/include/elf.h is too old for libunwind 1.5 and
doesn't define SHF_COMPRESSED.  The build fails with:

   ../../libunwind-1.5-rc1/src/dwarf/Gfind_proc_info-lsb.c:
   In function 'load_debug_frame':
   ../../libunwind-1.5-rc1/src/dwarf/Gfind_proc_info-lsb.c:130:24:
   error: 'SHF_COMPRESSED' undeclared (first use in this function)
      if (shdr->sh_flags & SHF_COMPRESSED)

This fails on RedHat 7.x, but of course, the real problem is the
version of glibc, 2.17 on RH 7.  But I haven't done the bisection to
identify the rev where this was added.

For my purposes (I've recently become the maintainer for the spack
libunwind package, github.com/spack/spack), I have an easy workaround.
I just require a recent elf headers as a prereq.

But I was thinking you might want to add a configure test (see if
SHF_COMPRESSED is defined in elf.h) and turn off debug-frame if not.

----------

(2) On aarch64, there is a spew of warnings over the asm definition of
unw_tdep_getcontext().


In file included from ../include/libunwind.h:7:0,
                 from mi/backtrace.c:29:
mi/backtrace.c: In function 'unw_backtrace':
../include/libunwind-aarch64.h:236:5: warning: right-hand operand of 
comma expression has no effect [-Wunused-value]
 #define unw_tdep_getcontext(uc) (({     \
                                 ~~~~~~~~~
   unw_tdep_context_t *unw_ctx = (uc);     \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   register uint64_t *unw_base __asm__ ("x0") = (uint64_t*) 
unw_ctx->uc_mcontext.regs;  \
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   __asm__ __volatile__ (      \
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      "stp x0, x1, [%[base], #0]\n" \
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      "stp x2, x3, [%[base], #16]\n" \
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      ...

      "str x30, [%[base], #240]\n" \
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      "mov x1, sp\n" \
      ~~~~~~~~~~~~~~~~
      "stp x1, x30, [%[base], #248]\n" \
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      : [base] "+r" (unw_base) : : "x1", "memory"); \
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   }), 0)
   ~~^~~~
../include/libunwind-common.h:127:29: note: in expansion of macro 
'unw_tdep_getcontext'
 #define unw_getcontext(uc)  unw_tdep_getcontext(uc)
                             ^~~~~~~~~~~~~~~~~~~
mi/backtrace.c:71:7: note: in expansion of macro 'unw_getcontext'
       unw_getcontext (&uc);
       ^~~~~~~~~~~~~~

AFAICT, the warning is harmless, but the message is pretty verbose and
happens something like 5-6 times.  So, anyone looking at the build
will stop and wonder.

It might be worthwhile to adjust the define to silence the warning.

Thanks,

--Mark