Re: software single step

[Daniel Jacobowitz <drow at mvista dot com>]

On Mon, Nov 11, 2002 at 05:12:32PM +0000, Stuart Hughes wrote:
> Daniel Jacobowitz wrote:
> > 
> > On Sun, Nov 03, 2002 at 08:54:12AM -0500, Stuart Hughes wrote:
> > > Hi,
> > >
> > > I'm trying to use gdb/gdbserver to debug a strong-arm target.  I'm
> > > having problems with single stepping.  If I try to step into the loop
> > > shown below, it simply runs on printing all 10 loop iterations.
> > >
> > >     for ( i = 0; i < 10 ; i++ ) {
> > >         printf("loop count = %d\n", i);
> > >     }
> > >
> [snip]
> > >
> > > Has anyone else seen this problem ? Does anyone know of a work around,
> > > or a fix ?
> > 
> > gdbserver doesn't implement software single step; the connected GDB
> > client handles it.  You may want to 'set debug target 1' and 'set debug
> > remote 1', and watch for differences.
> 
> 
> Hi Daniel,
> 
> I've investigted this further.  I tried the things you suggested, but
> I'm finding it hard to correlate the output from the native sesion and
> the remote session.  The native session outputs a a huge slew of output
> before it first manages to control the inferior.
> 
> Looking at the ouput from the remote debugging session, it looks okay,
> it seems to insert a breakpoint (I'm guessing, it writes 10300be5 to the
> point to break at), and then continues.  When the inferior wakes up, it
> sends back a 'T' packet.  I thought this should stop gdb, but it
> doesn't.  The session carries on, without any user intervention until
> the end of the program.
> 
> The code of the loop looks like this when disassembled:
> 
>     for ( i = 0; i < 10 ; i++ ) {
>     84b0:       e3a03000        mov     r3, #0  ; 0x0
>     84b4:       e50b3010        str     r3, [fp, -#16]
>     84b8:       e51b3010        ldr     r3, [fp, -#16]
>     84bc:       e3530009        cmp     r3, #9  ; 0x9
>     84c0:       da000000        ble     84c8 <main+0x28>
>     84c4:       ea000006        b       84e4 <main+0x44>
>         printf("loop count = %d\n", i);
>     84c8:       e59f0020        ldr     r0, [pc, #32]   ; 84f0
> <main+0x50>
>     84cc:       e51b1010        ldr     r1, [fp, -#16]
>     84d0:       ebffff9d        bl      834c <_init+0x64>
>     84d4:       e51b3010        ldr     r3, [fp, -#16]
>     84d8:       e2833001        add     r3, r3, #1      ; 0x1
>     84dc:       e50b3010        str     r3, [fp, -#16]
>     84e0:       eafffff4        b       84b8 <main+0x18>
>     }
> 
> I've attached the output captured from 'set debug remote' FYI.  
> 
> 
> Do you have any suggestions where I could look next ?? 

Well, first let me explain the relevant bits of your transcript.  What
it's trying to do is to step one instruction at a time until the source
line changes.

> Sending packet: $m4000ca4c,4#ec...Ack
> Packet received: 98040000
> Sending packet: $M4000ca4c,4:01009fef#31...Ack
> Packet received: OK
> Sending packet: $m84b0,4#cb...Ack
> Packet received: 0030a0e3
> Sending packet: $M84b0,4:01009fef#10...Ack
> Packet received: OK

Set the shared library breakpoint and main breakpoint.

> Sending packet: $Hc0#db...Ack
> Packet received: OK
> Sending packet: $c#63...Ack
> Packet received: T050b:e4fcffbf;0d:d4fcffbf;0f:b0840000;

Continue; stopped at main.

> Sending packet: $M4000ca4c,4:98040000#9b...Ack
> Packet received: OK
> Sending packet: $M84b0,4:0030a0e3#d1...Ack
> Packet received: OK

Remove breakpoints.

> Breakpoint 1, main () at hello.c:7
> 7	    for ( i = 0; i < 10 ; i++ ) {
> (gdb) step
> Sending packet: $m84b0,4#cb...Ack
> Packet received: 0030a0e3
> Sending packet: $m84b4,4#cf...Ack
> Packet received: 10300be5
> Sending packet: $M84b4,4:01009fef#14...Ack
> Packet received: OK
> Sending packet: $Hc0#db...Ack
> Packet received: OK
> Sending packet: $c#63...Ack
> Packet received: T050b:e4fcffbf;0d:d4fcffbf;0f:b4840000;

Insert only the breakpoint at $pc+4, and continue; then we stop at
$pc+4.  0x84b4.

> Sending packet: $M84b4,4:10300be5#d9...Ack
> Packet received: OK

Remove the temporary step breakpoint.

> Sending packet: $m4000ca4c,4#ec...Ack
> Packet received: 98040000
> Sending packet: $M4000ca4c,4:01009fef#31...Ack
> Packet received: OK
> Sending packet: $m84b0,4#cb...Ack
> Packet received: 0030a0e3
> Sending packet: $M84b0,4:01009fef#10...Ack
> Packet received: OK

Reinsert breakpoints.

> Sending packet: $m84b4,4#cf...Ack
> Packet received: 10300be5
> Sending packet: $g#67...Ack
> Packet received: 0100000014fdffbf1cfdffbf0000000024e7014014fdffbfe882000040d9004001000000a084000080071240e4fcffbfe8fcffbfd4fcffbff0d20340b48400000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Fetch the instruction and the register values...

> Sending packet: $m84b8,4#d3...Ack
> Packet received: 10301be5
> Sending packet: $M84b8,4:01009fef#18...Ack
> Packet received: OK

Set a step breakpoint at 0x84b8.

> Sending packet: $Hc0#db...Ack
> Packet received: OK
> Sending packet: $c#63...Ack
> Packet received: T050b:e4fcffbf;0d:d4fcffbf;0f:b8840000;

Continue, stopping at 0x84b8.

> Sending packet: $M84b8,4:10301be5#de...Ack
> Packet received: OK
> Sending packet: $m84b8,4#d3...Ack
> Packet received: 10301be5
> Sending packet: $g#67...Ack
> Packet received: 0100000014fdffbf1cfdffbf0000000024e7014014fdffbfe882000040d9004001000000a084000080071240e4fcffbfe8fcffbfd4fcffbff0d20340b88400000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> Sending packet: $m84bc,4#fe...Ack
> Packet received: 090053e3
> Sending packet: $M84bc,4:01009fef#43...Ack
> Packet received: OK
> Sending packet: $c#63...Ack
> Packet received: T050b:e4fcffbf;0d:d4fcffbf;0f:bc840000;

Same, to 0x84bc.

> Sending packet: $M84bc,4:090053e3#e1...Ack
> Packet received: OK
> Sending packet: $m84bc,4#fe...Ack
> Packet received: 090053e3
> Sending packet: $g#67...Ack
> Packet received: 0100000014fdffbf1cfdffbf0000000024e7014014fdffbfe882000040d9004001000000a084000080071240e4fcffbfe8fcffbfd4fcffbff0d20340bc8400000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> Sending packet: $m84c0,4#cc...Ack
> Packet received: 000000da
> Sending packet: $M84c0,4:01009fef#11...Ack
> Packet received: OK
> Sending packet: $c#63...Ack
> Packet received: T050b:e4fcffbf;0d:d4fcffbf;0f:c0840000;

Same, to 0x84c0.

> Sending packet: $M84c0,4:000000da#cb...Ack
> Packet received: OK
> Sending packet: $m84c0,4#cc...Ack
> Packet received: 000000da
> Sending packet: $g#67...Ack
> Packet received: 0100000014fdffbf1cfdffbf0000000024e7014014fdffbfe882000040d9004001000000a084000080071240e4fcffbfe8fcffbfd4fcffbff0d20340c08400000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> Sending packet: $m84c4,4#d0...Ack
> Packet received: 060000ea
> Sending packet: $M84c4,4:01009fef#15...Ack
> Packet received: OK
> Sending packet: $c#63...Ack
> Packet received: T050b:e4fcffbf;0d:d4fcffbf;0f:c4840000;

Same, to 0x84c4.  Now this is the strange one.  What happened?  It
seems to have mispredicted the next instruction when placing the
breakpoint; the branch was presumably taken.

You might want to debug arm_get_next_pc when the argument is 0x84c0,
and see how it's making this decision.


> Sending packet: $M84c4,4:060000ea#d6...Ack
> Packet received: OK
> Sending packet: $m84c4,4#d0...Ack
> Packet received: 060000ea
> Sending packet: $g#67...Ack
> Packet received: 0f0000000f0000000f0000000a00000024e7014014fdffbfe882000040d9004001000000a084000080071240e4fcffbff0081240d4fcffbfd4840000c48400000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> Sending packet: $m84e4,4#d2...Ack
> Packet received: 0030a0e3
> Sending packet: $M84e4,4:01009fef#17...Ack
> Packet received: OK
> Sending packet: $c#63...Ack
> Packet received: T050b:e4fcffbf;0d:d4fcffbf;0f:e4840000;

Same, to 0x84e4.

> Sending packet: $M84e4,4:0030a0e3#d8...Ack
> Packet received: OK
> Sending packet: $m84e4,4#d2...Ack
> Packet received: 0030a0e3
> Sending packet: $g#67...Ack
> Packet received: 0f0000000f0000000f0000000a00000024e7014014fdffbfe882000040d9004001000000a084000080071240e4fcffbff0081240d4fcffbfd4840000e48400000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
> Sending packet: $m84e4,4#d2...Ack
> Packet received: 0030a0e3
> Sending packet: $m84e4,4#d2...Ack
> Packet received: 0030a0e3
> Sending packet: $m84a0,4#ca...Ack
> Packet received: 0dc0a0e1
> Sending packet: $m84a4,4#ce...Ack
> Packet received: 00d82de9
> Sending packet: $m84a8,4#d2...Ack
> Packet received: 04b04ce2
> Sending packet: $m84ac,4#fd...Ack
> Packet received: 04d04de2
> Sending packet: $m84b0,4#cb...Ack
> Packet received: 01009fef
> Sending packet: $m84b4,4#cf...Ack
> Packet received: 10300be5
> Sending packet: $m84b8,4#d3...Ack
> Packet received: 10301be5
> Sending packet: $m84bc,4#fe...Ack
> Packet received: 090053e3
> Sending packet: $m84c0,4#cc...Ack
> Packet received: 000000da
> Sending packet: $M4000ca4c,4:98040000#9b...Ack
> Packet received: OK
> Sending packet: $M84b0,4:0030a0e3#d1...Ack
> Packet received: OK
> 10	    return 0;
> (gdb) quit


-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer