Instrcutions that must not be stepped.

Sat Jun 10 00:33:00 GMT 2006

On Fri, 2006-06-09 at 10:12 -0400, Daniel Jacobowitz wrote:
. . .
> There is no way to do this for most targets.  Paul's probably talking
> about native PowerPC GDB, not any kind of remote stub.  Even at that
> point, I think the performance impact here would be unacceptable; the
> repeated single step path is one of the few time-sensitive bits of
GDB.

Daniel is correct: my specific need is for native PowerPC, but the need
for dealing with atomic sequences is more universal.  I would like to
come up with a solution for my need that would not hinder a more
universal solution at some later point.

_________
BAD NEWS:
~~~~~~~~~

I tried to measure the impact of reading the instruction about to be
stepped and stumbled across a *horrible* GDB bug.  When the number of
instructions stepped in the "Single stepping until exit from function"
mode is large enough (haven't pinned it down yet), the system starts
thrashing until X freezes up and then, after a time, the system crashes.

This is with an un-modified GDB, both cvs-head and the 6.5 tarball.
Here is the setup:

I have a stripped shared library called libtest.so in a test directory.
Here is it's source, "libtest.c":
        char *
        slow_hello ()
        {
          char message[] = "Hello, World";
          static char out_buf[sizeof(message)];
          int i;
          char *cp;

          for (cp=message; *cp; ++cp)
              for (i=0; i<10000; ++i)
                out_buf[cp - message] = *cp;

          return out_buf;
        }

The main program, "testprog.c":
        #include <stdio.h>

        void slow_help ();

        int
        main ()
        {
          printf ("%s\n", slow_hello ());
          return 0;
        }

Here is how I built the executable:
        cc -fPIC -c libtest.c
        cc -shared libtest.o -o libtest.so
        cc -c testprog.c
        cc -L. -ltest testprog.o -o testprog

I used the following gdb script ("gdb-script"):
        start
        b slow_hello
        c
        s
        c
        q

Here is what it looks like when I run GDB:
        [pgilliam@dufur]$ LD_LIBRARY_PATH=. gdb -x gdb-script testprog
        GNU gdb 6.5.50.20060608-cvs
        Copyright (C) 2006 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 "powerpc64-unknown-linux-gnu"...Using
        host libthread_db library "/lib/libthread_db.so.1".

        Breakpoint 1 at 0x100015a4: file testprog.c, line 8.
        main () at testprog.c:8
        8         printf ("%s\n", slow_hello ());
        Breakpoint 2 at 0xffaf70c

        Breakpoint 2, 0x0ffaf70c in slow_hello () from ./libtest.so
        Single stepping until exit from function slow_hello,
        which has no line number information.

        Program received signal SIGINT, Interrupt.
        0x0ffaf77c in slow_hello () from ./libtest.so
        Hello, World

        Program exited normally.
        [pgilliam@dufur]$ cat libtest.c

If I didn't interrupt it with a <cntl-c> within a minute or so of it's
start, the system would start thrashing and, sometime overnight, it
crashed.

Again, both cvs-head and 6.5 exhibited this behavior, but
(HERE'S THE KICKER) the GDB distributed with the system did not.

Here is with the distributed GDB:
        [pgilliam@dufur]$ time LD_LIBRARY_PATH=. gdb -x gdb-script
        testprog
        GNU gdb Red Hat Linux (6.3.0.0-1.21rh)
        Copyright 2004 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 "ppc-redhat-linux-gnu"...Using host
        libthread_db library "/lib/libthread_db.so.1".

        Breakpoint 1 at 0x100015a4: file testprog.c, line 8.
        main () at testprog.c:8
        8         printf ("%s\n", slow_hello ());
        Breakpoint 2 at 0xffaf70c

        Breakpoint 2, 0x0ffaf70c in slow_hello () from ./libtest.so
        Single stepping until exit from function slow_hello,
        which has no line number information.
        Hello, World
        main () at testprog.c:9
        9         return 0;

        Program exited normally.

        real    0m55.448s
        user    0m12.933s
        sys     0m30.534s
        [pgilliam@dufur]$

I tried it with vanilla 6.4 with no problem.

I also tried it with cvs-head on an x86 system with no problem, but with
something strange:

here is with the distributed gdb on x86 (slight mod to gdb-script
needed):
        pgilliam> time LD_LIBRARY_PATH=. gdb -x gdb-script ./testprog
        GNU gdb 6.1
        Copyright 2004 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 "i586-suse-linux"...Using host
        libthread_db library "/lib/tls/libthread_db.so.1".

        Breakpoint 1 at 0x80484e2

        Breakpoint 1, 0x080484e2 in main ()
        Breakpoint 2 at 0x400186ae

        Breakpoint 2, 0x400186ae in slow_hello () from ./libtest.so
        Single stepping until exit from function slow_hello,
        which has no line number information.
        0x080484f7 in main ()
        Hello, World

        Program exited normally.

        real    0m37.366s
        user    0m14.006s
        sys     0m25.242s
        pgilliam> 

And here it is with cvs-head on x86:
        pgilliam> time LD_LIBRARY_PATH=. ../gdb -x gdb-script ./testprog
        GNU gdb 6.5.50.20060609-cvs
        Copyright (C) 2006 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 "i686-pc-linux-gnu"...Using host
        libthread_db library "/lib/tls/libthread_db.so.1".

        Breakpoint 1 at 0x80484e2

        Breakpoint 1, 0x080484e2 in main ()
        Breakpoint 2 at 0x400186ae

        Breakpoint 2, 0x400186ae in slow_hello () from ./libtest.so
        Single stepping until exit from function slow_hello,
        which has no line number information.
        0x080484f7 in main ()
        Hello, World

        Program exited normally.

        real    2m4.106s
        user    0m42.448s
        sys     1m25.191s
        pgilliam> 

Wow! Why did it take so much longer?  

-=# Paul #=- 