exp/1155: s/390 Linux: GDB can't reselect the right frame after an inferior function call

[jimb at redhat dot com]

>Number:         1155
>Category:       exp
>Synopsis:       s/390 Linux: GDB can't reselect the right frame after an inferior function call
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Thu Mar 27 17:08:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     jimb at redhat dot com
>Release:        unknown-1.0
>Organization:
>Environment:
s390-ibm-linux, GDB 2003-03-27-cvs, gcc (GCC) 3.2.1 20021207 (Red Hat Linux 8.0 3.2.1-2)
>Description:
If you ask GDB to make an inferior function call when the youngest frame is a frameless function, and the selected frame is the second-to-youngest frame, then GDB will not properly re-select the second-to-youngest frame when the inferior call returns.

On the S/390, if a function doesn't use alloca, then the compiler just uses the stack pointer as the frame pointer.  This means that GDB's frame_info structures use the address of the low end of the frame as the frame base, not the high end.  (The S/390 stack grows downwards.)  So, if the youngest function call hasn't allocated any stack space, then its frame base address is equal to that of its caller.  This means that frame_find_by_id is unable to distinguish between the two.

There are comments in various versions of the code that suggest that Andrew (who has been doing lots of frame hacking recently) knows about this; as of 2003-11-29, frame_find_by_id said:

      /* FIXME: cagney/2002-04-21: This isn't sufficient.  It should
	 use id.pc / this.pc to check that the two frames belong to
	 the same function.  Otherwise we'll do things like match
	 dummy frames or mis-match frameless functions.  However,
	 until someone notices, stick with the existing behavour.  */

Okay --- somebody has noticed!  :)

As of 2003-3-27, that comment was gone, but frame_id_eq says:

  /* Add a test to check that the frame ID's are for the same function
     here.  */
  return 1;
>How-To-Repeat:
Run the test script gdb.c++/overload.exp.  If the code generated for marker1 in overload.cc does not allocate any stack space, then after the first call to foo_instance1.overloadargs, all subsequent calls will fail, since foo_instance1 is local to main, but marker1's frame becomes selected after the first call returns.

GNU gdb 2003-03-27-cvs
Copyright 2003 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "s390-ibm-linux"...
(gdb) break marker1
Breakpoint 1 at 0x40052e: file /home/cygnus/jimb/src/gdb/testsuite/gdb.c++/overload.cc, line 54.
(gdb) run
Starting program: /home/cygnus/jimb/s390/gdb/build/gdb/testsuite/gdb.c++/overload 

Breakpoint 1, marker1() () at /home/cygnus/jimb/src/gdb/testsuite/gdb.c++/overload.cc:54
54      {}
(gdb) up
#1  0x00400612 in main () at /home/cygnus/jimb/src/gdb/testsuite/gdb.c++/overload.cc:83
83          marker1();
(gdb) print foo_instance1.overloadargs (1)
$1 = 1
(gdb) frame
#0  marker1() () at /home/cygnus/jimb/src/gdb/testsuite/gdb.c++/overload.cc:54
54      {}
(gdb) print foo_instance1.overloadargs (1)
No symbol "foo_instance1" in current context.
(gdb) 
>Fix:
It's easy enough for the test suite to recognize and report this failure, but then finish the call to marker1 and continue the overloading tests.  I've attached a patch that does that.
>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: application/octet-stream; name="gdb-5.3post-tests-overload-mar2003.patch"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="gdb-5.3post-tests-overload-mar2003.patch"

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