[Bug ports/2682] On ARM EABI, exit should be marked EXIDX_CANTUNWIND

["drow at sources dot redhat dot com" <sourceware-bugzilla@sourceware.org>]

------- Additional Comments From drow at sources dot redhat dot com  2006-05-22 13:56 -------
Subject: Re:  On ARM EABI, exit should be marked EXIDX_CANTUNWIND

On Mon, May 22, 2006 at 11:33:55AM -0000, jsm28 at gcc dot gnu dot org wrote:
> Yes, it's a general C++ requirement from 18.3 [lib.support.start.term] paragraph 5.
> 
> If on a given platform you can tell that there isn't unwind information for the
> stack frame for exit, then the unwinding will stop there; it's a peculiarity of
> ARM unwind information that you can't tell that and need unwind information all
> the way up the stack until explicitly marked to stop unwinding.  I don't know
> how or whether this (avoiding unwinding through exit) works on IA64 where
> everything has unwind information by default.

x86-64 also produces unwind tables by default; and my system libc has
one for exit.  Here's a modified testcase:

#include <stdlib.h>

extern "C" int puts(const char *);

struct S
{
  int i;
  S() : i(0) { }
  ~S()
  {
    puts("dtor");
    throw "s";  // no handler anywhere
  }
};

S xo;

int main(int argc, char *argv[])
{
  puts("main");
  try {
    exit (0);
  } catch (...) {
    puts ("caught");
    abort ();
  }
}

It generates:

#0  0x00002aaaf66e8770 in std::terminate () from /usr/lib/libstdc++.so.6
#1  0x00002aaaf66e886a in __cxa_throw () from /usr/lib/libstdc++.so.6
#2  0x00000000004007b8 in ~S (this=0x500d54) at exit-exc2.cc:12
#3  0x0000000000400712 in __tcf_0 () at exit-exc2.cc:16
#4  0x00002aaaf6af257d in exit () from /lib/libc.so.6
#5  0x00000000004006fb in main (argc=1, argv=0x7fffb4593918) at exit-exc2.cc:22

~S has unwind info.  __tcf_0 has unwind info.  exit has unwind info. 
Yet somehow, terminate gets called anyway.  I do not have a debuggable
libgcc handy; I have not yet managed to work out why this happens. 
There's nothing obviously special in exit to account for this.

As far as I can tell it ought to have no 'L' augmentation, and thus no
LSDA, and thus fall through... ahh!

000000b0 0000002c 000000b4 FDE cie=00000000 pc=00400878..00400890
  Augmentation data:     00 00 00 00 00 00 00 00

That is an empty augmentation entry for __tcf_0, which causes
__cxa_throw to call terminate.

You might want to check what the ARM compiler and runtime do for this
case, for compatibility, but it looks to me as if the g++-generated
stubs which call destructors ought to be marked .cantunwind if anything
should.

  // If ip is not present in the table, call terminate.  This is for
  // a destructor inside a cleanup, or a library routine the compiler
  // was not expecting to throw.
  found_type = found_terminate;
  goto do_something;

There has to be an unwind entry for these functions anyway, or we
wouldn't have a hope of reaching exit, so I hope ARM didn't try to save
space by consolidating the can't unwind marker into exit.



-- 


http://sourceware.org/bugzilla/show_bug.cgi?id=2682

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