Re: [PATCH] i386_stab_reg_to_regnum (4 <-> 5, ebp <-> esp)

[Brian Ford <ford at vss dot fsi dot com>]

On Mon, 5 Apr 2004, Jim Blandy wrote:

> Eli, here is a program you can use to see whether DJGPP's GCC and
> the current GDB disagree on how to number %ebp and %esp.

Thank you.  Thank you.  Thank you.  Did I say thank you?

> One of the reasons people would tend not to notice if %ebp and %esp
> are misnumbered in STABS is that they almost never occur.  Dwarf 2
> emits explicit location expressions to specify the "frame base" for a
> function; these expressions usually refer to %esp or %ebp.  In STABS,
> however, stack-based variables are simply marked as "LSYMS" or
> "PSYMS", whose locations are given as offsets relative to some
> implicit base that the debugger just has to know.

I expected as much since I never saw a problem in Cygwin before I tried to
add DWARF 2 support.  I just didn't know how these functions were used
well enough to find a simple test case.

> So the only way to get those register numbers to appear at all is to
> get a variable allocated to them.  You'll never get a variable
> allocated to %esp.  And you'll never get a varable allocated to %ebp
> unless you compile with -fomit-frame-pointer.  So, if I compile the
> attached program with GCC 3.3 passing -O -fomit-frame-pointer, 'n'
> gets allocated to %ebp.

FWIW, this does show the problem clearly on Cygwin.

ford@fordpc ~
$ gcc -g -O -fomit-frame-pointer frameless.c -o frameless_stab.exe

ford@fordpc ~
$ gdb frameless_stab.exe
GNU gdb 2003-09-20-cvs (cygwin-special)
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 "i686-pc-cygwin"...
(gdb) b foo
Breakpoint 1 at 0x401050: file frameless.c, line 6.
(gdb) r 2
Starting program: /home/ford/frameless_stab.exe 2

Breakpoint 1, foo (n=0) at frameless.c:6
6       {
(gdb) n
8         register int a = n * 1;
(gdb) info address n
Symbol "n" is a variable in register esp.
(gdb) p n
$1 = 2289692
(gdb) p $esp
$2 = (void *) 0x22f01c
(gdb) p $ebp
$3 = (void *) 0x2
(gdb)

-- 
Brian Ford
Senior Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
Phone: 314-551-8460
Fax:   314-551-8444