Re: [patch/rfc] Rewrite decr-pc logic, eliminate step_sp

[Andrew Cagney <cagney at gnu dot org>]

> > > > +  if (currently_stepping (ecs))
> > > > +    {
> > > > +      if (SOFTWARE_SINGLE_STEP_P ())
> > > > +    {
> > > > +      if (singlestep_breakpoints_inserted_p
> > > > +          && prev_pc == breakpoint_pc)
> > > > +        /* If we're software-single-stepping, assume we hit one of
> > > > +           the inserted software breakpoints.  */
> > > > +        write_pc_pid (breakpoint_pc, ecs->ptid);
> > > > +    }
> >
> >
> >
> > I'm pretty sure that won't work.  prev_pc is where we were stopped
> > before we decided to single step.  breakpoint_pc is where, if we have
> > hit a breakpoint, the breakpoint would be.  They won't be equal;
> > breakpoint_pc will be the following instruction, or the target of a
> > branch if *prev_pc was a taken branch.  The old code assumes we hit a
> > breakpoint if we stopped with SIGTRAP with singlestep_breakpoints_inserted_p
> > - any reason not to keep that behavior?
>
>
> It's plain wrong.  I'm pretty sure that, when doing the thread-hop, singlestep_breakpoints_inserted_p holds, but current_stepping() doesn't.
>
> > I think Alpha OSF/1 and Alpha NetBSD are the only current
> > software-single-step and decr-pc targets, which makes this case a
> > little hard to test - at least OSF/1 had dreadful test results already,
> > I'm not sure about NetBSD.  Might want to verify that it isn't
> > catastrophic, at least.
> >
> > The rest of it looks right to me, though I had to stare at it for
> > the last twenty minutes or so.
>
>
> I gave up staring at the old code, it made no sense.
>
> Attached is a revision.

I've checked this in.  In addition to PPC (h/w single step) and i386 
(decr pc after break) I gave it a sniff test on alpha GNU/Linux (hacked 
to use s/w single step).

Andrew

> 2004-05-09  Andrew Cagney  <cagney@redhat.com>
>
> 	* infrun.c (adjust_pc_after_break): Rewrite decr logic,
> 	eliminate reference to step_sp.
> 	(struct execution_control_state, init_execution_control_state)
> 	(handle_inferior_event, keep_going): Delete update_step_sp and
> 	step_sp.
> 	* infcmd.c (step_sp): Note that variable is unused.