Re: Who's insane: gdb or I?

[Bernd Jendrissek <berndj@prism.co.za>]

On Wed, Jul 03, 2002 at 03:15:47PM +0200, Bernd Jendrissek wrote:
> 1083            OVERtime+=(etv.tv_sec - stv.tv_sec) * 1000;
> (gdb) s
> 1084            OVERtime+=(etv.tv_usec - stv.tv_usec) / 1000;
> (gdb) s
> 1083            OVERtime+=(etv.tv_sec - stv.tv_sec) * 1000;
> (gdb) s
> 1084            OVERtime+=(etv.tv_usec - stv.tv_usec) / 1000;
> (gdb) s
> 1083            OVERtime+=(etv.tv_sec - stv.tv_sec) * 1000;
> (gdb) s
> 1084            OVERtime+=(etv.tv_usec - stv.tv_usec) / 1000;
> (gdb) list
> 1079            return;
> 1080        }
> 1081        if (PERMaccessconf->nnrpdoverstats) {
> 1082            gettimeofday(&etv, NULL);
> 1083            OVERtime+=(etv.tv_sec - stv.tv_sec) * 1000;
> 1084            OVERtime+=(etv.tv_usec - stv.tv_usec) / 1000;
> 1085        }

I then wondered why gdb hops between lines 1083 and 1084 a few times.

>     if (PERMaccessconf->nnrpdoverstats) {
>  804cb25:       a1 e8 9b 06 08          mov    0x8069be8,%eax
>  804cb2a:       83 78 74 00             cmpl   $0x0,0x74(%eax)
>  804cb2e:       74 48                   je     804cb78 <CMDxover+0x15c>
>         gettimeofday(&etv, NULL);
>  804cb30:       83 c4 f8                add    $0xfffffff8,%esp
>  804cb33:       6a 00                   push   $0x0
>  804cb35:       8d 5d d0                lea    0xffffffd0(%ebp),%ebx
>  804cb38:       53                      push   %ebx
>  804cb39:       e8 b6 d9 ff ff          call   804a4f4 <_init+0x780>
>         OVERtime+=(etv.tv_sec - stv.tv_sec) * 1000;
>  804cb3e:       8b 45 d8                mov    0xffffffd8(%ebp),%eax
>  804cb41:       8b 4d d0                mov    0xffffffd0(%ebp),%ecx
>  804cb44:       29 c1                   sub    %eax,%ecx
>         OVERtime+=(etv.tv_usec - stv.tv_usec) / 1000;
>  804cb46:       8b 45 dc                mov    0xffffffdc(%ebp),%eax
>  804cb49:       8b 73 04                mov    0x4(%ebx),%esi
>  804cb4c:       69 c9 e8 03 00 00       imul   $0x3e8,%ecx,%ecx
>  804cb52:       29 c6                   sub    %eax,%esi
>  804cb54:       bb d3 4d 62 10          mov    $0x10624dd3,%ebx
>  804cb59:       89 f0                   mov    %esi,%eax
>  804cb5b:       f7 eb                   imul   %ebx,%eax
>  804cb5d:       03 0d b0 9b 06 08       add    0x8069bb0,%ecx
>  804cb63:       c1 fa 06                sar    $0x6,%edx
>  804cb66:       89 f0                   mov    %esi,%eax
>  804cb68:       c1 f8 1f                sar    $0x1f,%eax
>  804cb6b:       29 c2                   sub    %eax,%edx
>  804cb6d:       01 d1                   add    %edx,%ecx
>  804cb6f:       89 0d b0 9b 06 08       mov    %ecx,0x8069bb0
>     }
>  804cb75:       83 c4 10                add    $0x10,%esp

I am happy with the explanation that I should get used to this when running
optimized code under gdb.  With a look at the assembly (disassembled from
the actual binary) I can convince myself that the alternation between lines
1083 and 1084 is due to fancy instruction scheduling and strength reduction
on gcc's part.  (gcc 2.95.2 BTW, and I know it's ancient.  But it works.)

Thanks to all who looked at this.  Unless anyone else thinks there's
something wrong here, I'm not too bothered about it.  At -O2 I think I
deserve to be confused by gdb! :)

Carlo, do you still want me to look at -gstabs vs. -gdwarf?

bernd

-- 
berndj@users.sourceforge.net is probably better to bookmark than any
employer-specific email address I may have appearing in the headers.