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build failure and warnings on aarch64 and redhat 7.x

From: Mark W. Krentel
Subject: build failure and warnings on aarch64 and redhat 7.x
Date: Mon, 22 Jun 2020 23:50:21 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

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:

   In function 'load_debug_frame':
   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,, 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

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.



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