[Bug target/18748] New: first instruction in each procedure isn't unwindable

["davidm at hpl dot hp dot com" <gcc-bugzilla@gcc.gnu.org>]

I'm reporting this against x86_64-suse-linux since that's where I noticed the
problem first, but I believe the issue affects all platforms which use DWARF
unwind-info.

The problem is that when looking up unwind info when the instruction-pointer
(IP) points to X, we're actually looking up (X-1) in the unwind tables.  The
motivation for this fudging is that X may never be reached (because of
non-return functions).  Unfortunately, for the first instruction in a procedure,
looking up (X-1) may cause us to find the unwind-info for the preceeding
function which, in general, will give wrong results.

I see only two possible solutions:

 (1) Change the compiler(s) so it emits a dummy (nop) instruction in front
     of each procedure and include that (never-to-be-executed) dummy-instruction
     as part of the unwind-info.

 (2) Stop the IP-fudging. This occasionally requires emitting a dummy (nop)
     instruction after invoking a no-return functions.

This problem can easily be reproduced by single-stepping through a program and
attempting to unwind after each instruction.  The libunwind [1] package contains
a test-case for this which can be invoked like this:

 tests/test-ptrace -c -n -t tests/test-ptrace-misc

In theory, this test should succeed with no error.  Due to the above bug, it
will fail to unwind whenever execution reaches the first instruction in a procedure.

   --david

[1] http://www.hpl.hp.com/research/linux/libunwind/

-- 
           Summary: first instruction in each procedure isn't unwindable
           Product: gcc
           Version: 3.3.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: davidm at hpl dot hp dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: x86_64-suse-linux
  GCC host triplet: x86_64-suse-linux
GCC target triplet: x86_64-suse-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18748