Re: [PATCH 4/8] Force to insert software single step breakpoint

[Pedro Alves <palves@redhat.com>]

On 03/04/2016 10:44 AM, Yao Qi wrote:
> GDB doesn't insert software single step breakpoint if the instruction
> branches to itself, so that the program can't stop after command "si".
>
> (gdb) b 32
> Breakpoint 2 at 0x8680: file git/gdb/testsuite/gdb.base/branch-to-self.c, line 32.
> (gdb) c
> Continuing.
>
> Breakpoint 2, main () at gdb/git/gdb/testsuite/gdb.base/branch-to-self.c:32
> 32	  asm (".Lhere: " BRANCH_INSN " .Lhere"); /* loop-line */
> (gdb) si
> infrun: clear_proceed_status_thread (Thread 3991.3991)
> infrun: proceed (addr=0xffffffff, signal=GDB_SIGNAL_DEFAULT)
> infrun: step-over queue now empty
> infrun: resuming [Thread 3991.3991] for step-over
> infrun: skipping breakpoint: stepping past insn at: 0x8680
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Sending packet: $Z0,8678,4#f3...Packet received: OK
> infrun: skipping breakpoint: stepping past insn at: 0x8680
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Sending packet: $Z0,b6fe86c8,4#82...Packet received: OK
> infrun: resume (step=1, signal=GDB_SIGNAL_0), trap_expected=1, current thread [Thread 3991.3991] at 0x868
>
> breakpoint.c:should_be_inserted thinks the breakpoint shouldn't be
> inserted, which is wrong.  This patch restrict the condition that only
> return false if breakpoint is NOT single step breakpoint.

Hmm, I think we might need to do something else.

If you put a breakpoint there, then the instruction under
the breakpoint won't execute at all.

If it's a conditional branch, and the condition is false,
we will fail to ever advance past the instruction.

Similarly if the branch instruction happens to have important
side effects (flags? counters?).

Thanks,
Pedro Alves