GDB 4.16 with m68k-stub target/bad stepping.

[aleks@spoda.gentoo.com (Brian Witt)]

I'm using GDB 4.16 as cross-debugger to a M68010 machine (AT&T 3B1
hardware without an OS).  I'm having problems continuing after a
breakpoint.  Source level stepping after a breakpoint doesn't work,
but NEXTI does.

  --> GDB 4.16 (sparc-sun-solaris2.5 --target m68k-unknown-aout), 

I started the GDB 4.13 m68k-stub.c which a previous guy had used.  I
did diffs against 4.16's m68k-stub.c, and then used `patch' to forward
merge the changes.  I'm curious if these two are compatible.  Should I
try the whole forward merge again.  Is the m68k-stub a victim of bit
rot?

Should I just hack it to work?  It runs with "mc68020" defined.  I've
already modified mem2hex() to access in 16-bit words if address and
count are even.  Is "qOffsets" important.  It seems to find variables
OK without it.  The code loads into a specific location.  There is no
dynamically placed code.

(Gosh, this message got long...)
Anyone got any ideas to help me out?


Description
-----------

The problem in my code is the push before the call to sbrk() is getting
skipped.  Garbage is used and sbrk() sez "too much" and returns -1.
The second word of "pea 0x2000" turns into "movel d0,d0".

It seems the PC sent to the host is *after* the breakpoint (normal
processing for the TRAP #15 instruction).  The breakpoint occurs as it
should.  GDB resumes execution starting at the next short (2-bytes)
after the breakpoint (after the inserted TRAP instruction).  In most
cases is needs to be backed up once the original instruction is
restored by GDB.

For instance (with "set remotedebug 1 ; set remote_debug = 1")
Address 0x35c2 is an instruction boundary ("pea 0x2000") which is
"whole_block = sbrk(0x2000);".  Address 0x7efec is on the stack.


-------------------------

68-gdb> tbreak main
Breakpoint 1 at 0x35c2: file try.cc, line 455.
68-gdb> cont
Continuing.
Sending packet: $m35c2,2#c8...Ack
Packet received: 4878
Sending packet: $M35c2,2:4e4f#15...Ack		<-- inserts TRAP.
Packet received: OK
Sending packet: $m35d4,2#cb...Ack
Packet received: 4878
Sending packet: $M35d4,2:4e4f#18...Ack
Packet received: OK
Sending packet: $Hc0#db...Ack
Packet received: 
Sending packet: $c#63...Ack
Packet received: S05
Sending packet: $g#67...Ack
Packet received: 0000000100000036061c0000062000000624000006280000062c000000000000000000d8000772fc063c00000640000006440000064800000007efec0007efec00002700000035c4000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Program received signal SIGTRAP, Trace/breakpoint trap.
Sending packet: $M35c2,2:4878#bd...Ack		<-- restores original word.
Packet received: OK
Sending packet: $M35d4,2:4878#c0...Ack
Packet received: OK
0x35c4 in main () at try.cc:455
455         whole_block = sbrk(PAGE_SIZE*2);
Current language:  auto; currently c++
68-gdb> set remotedebug 0
68-gdb>  x /6i $pc
0x35c4 <main+12>:       movel %d0,%d0
0x35c6 <main+14>:       jsr 0x4fac <sbrk>
0x35cc <main+20>:       addqw #4,%sp
0x35ce <main+22>:       movel %d0,0x63be <whole_block>
0x35d4 <main+28>:       pea 0x1000
0x35d8 <main+32>:       movel %d0,%sp@-
68-gdb> x /4i $pc -2
0x35c2 <main+10>:       pea 0x2000
0x35c6 <main+14>:       jsr 0x4fac <sbrk>
0x35cc <main+20>:       addqw #4,%sp
0x35ce <main+22>:       movel %d0,0x63be <whole_block>
68-gdb> 

-------------------------

A little examination reveals GDB believes the PC has advanced, when it
really should not have.  The second "display" shows the correct opcode.

-------------------------

68-gdb> info reg pc
pc             0x35c4   0x35c4

=========================
-------------------------

Now if I NEXTI, the first instruction is at 0x35c4, which is the
operand to the "pea" where the breakpoint was set.   Uuuggghhhh!!!!

-------------------------

68-gdb> display /i $pc
1: x/i $pc  0x35c4 <main+12>:   movel %d0,%d0
68-gdb> set remotedebug 1
68-gdb> nexti
Sending packet: $m35c2,2#c8...Ack
Packet received: 4878
Sending packet: $M35c2,2:4e4f#15...Ack
Packet received: OK
Sending packet: $s#73...Ack
Packet received: S05
Sending packet: $g#67...Ack
Packet received: 0000000100000036061c0000062000000624000006280000062c000000000000000000d8000772fc063c00000640000006440000064800000007efec0007efec0000a700000035c6000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
Sending packet: $M35c2,2:4878#bd...Ack
Packet received: OK
0x35c6  455         whole_block = sbrk(PAGE_SIZE*2);
1: x/i $pc  0x35c6 <main+14>:   Sending packet: $m35c6,1#cb...Ack
Packet received: 4e
Sending packet: $m35c6,2#cc...Ack
Packet received: 4eb9
Sending packet: $m35c8,4#d0...Ack
Packet received: 00004fac
jsr 0x4fac <sbrk>
68-gdb> 

-------------------------

After a while of NEXT comamnds I start getting SIGILL instead of
breakpoints.  The source line does not change (the PC reg does not
advance)...

-------------------------

NEXT
Sending packet: $m35c2,2#c8...Ack
Packet received: 4878
Sending packet: $M35c2,2:4e4f#15...Ack
Packet received: OK
Sending packet: $S04#b7...Ack
Packet received: 
Can't send signals to this remote system.  SIGILL not sent.
Sending packet: $s#73...Ack
Packet received: S04
Sending packet: $g#67...Ack
Packet received: 000000000000002b061c0000062000000624000006280000062c00000000000000e50000ffffffef063c00000640000006440000064800000007efec0007efd80000a714000035fc000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

Program received signal SIGILL, Illegal instruction.
Sending packet: $M35c2,2:4878#bd...Ack
Packet received: OK
0x35fc in main () at try.cc:457
457         printf( "Located block at $%p (whole at $%p)\n", block, whole_block );
1: x/i $pc  0x35fc <main+68>:   Sending packet: $m35fc,1#fb...Ack
Packet received: 00
Sending packet: $m35fc,2#fc...Ack
Packet received: 000c
Sending packet: $m35fe,2#fe...Ack
Packet received: 72ff
orib #-1,%a4
68-gdb> 


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