RE: [PATCH] Updated, fix reverse stepping multiple contiguous PC ranges

[Carl Love <cel@us.ibm.com>]

On Tue, 2022-03-08 at 17:21 -0300, Bruno Larsen wrote:

<snip>
> 
> 
> Thanks for looking at this. Since I don't test on aarch64 often, I am
> not sure if I see regressions or racy testcases, but it does fix the
> issue you mentioned, and there doesn't seem to be regressions on
> x86_64 hardware. I have a few nits, but the main feedback is: could
> you add a testcase for this, using the dwarf assembler and manually
> creating contiguous PC ranges, so we can confirm that this is not
> regressed in the future on any hardware?
> 
> Also, I can't approve a patch, but with the testcase this patch is
> mostly ok by me
> 

I have not writen anything in dwarf assembler in the past.  Off the top
of my head, don't know how to create such a test case.  It does seem
that there are the two testcases  gdb.reverse/solib-precsave.exp and
gdb.reverse/solib-reverse.exp which the patch fixes.  I guess the dwarf
assembly test would be similar to the C level code.  

Do you have an example of how to write dwarf assembly or a pointer to a
tutorial on writting dwarf?

I will work on the two comments you had on the patch.  Thanks for your
time reviewing the patch.

                    Carl 


> > -----------------------------------------------------------
> > From: Luis Machado <luis.machado@linaro.org>
> > Date: Mon, 21 Feb 2022 23:11:23 +0000
> > Subject: [PATCH] Fix reverse stepping multiple contiguous PC ranges
> > 
> > When running GDB's testsuite on aarch64-linux/Ubuntu 20.04, I
> > noticed some
> > failures in gdb.reverse/solib-precsave.exp and gdb.reverse/solib-
> > reverse.exp.
> > 
> > The failure happens around the following code:
> > 
> > 38  b[1] = shr2(17);            /* middle part two */
> > 40  b[0] = 6;   b[1] = 9;       /* generic statement, end part two
> > */
> > 42  shr1 ("message 1\n");       /* shr1 one */
> > 
> > Normal execution:
> > 
> > - step from line 1 will land on line 2.
> > - step from line 2 will land on line 3.
> > 
> > Reverse execution:
> > 
> > - step from line 3 will land on line 2.
> > - step from line 2 will land on line 2.
> > - step from line 2 will land on line 1.
> > 
> > The problem here is that line 40 contains two contiguous but
> > distinct
> > PC ranges, like so:
> > 
> > Line 40 - [0x7ec ~ 0x7f4]
> > Line 40 - [0x7f4 ~ 0x7fc]
> > 
> > When stepping forward from line 2, we skip both of these ranges and
> > land on
> > line 42. When stepping backward from line 3, we stop at the start
> > PC of the
> > second (or first, going backwards) range of line 40.
> > 
> > This happens because we have this check in
> > infrun.c:process_event_stop_test:
> > 
> >        /* When stepping backward, stop at beginning of line range
> >           (unless it's the function entry point, in which case
> >           keep going back to the call point).  */
> >        CORE_ADDR stop_pc = ecs->event_thread->suspend.stop_pc;
> >        if (stop_pc == ecs->event_thread->control.step_range_start
> >            && stop_pc != ecs->stop_func_start
> >            && execution_direction == EXEC_REVERSE)
> >          end_stepping_range (ecs);
> >        else
> >          keep_going (ecs);
> > 
> > Since we've reached ecs->event_thread->control.step_range_start, we
> > stop
> > stepping backwards.
> > 
> > The right thing to do is to look for adjacent PC ranges for the
> > same line,
> > until we notice a line change. Then we take that as the start PC of
> > the
> > range.
> > 
> > Another solution I thought about is to merge the contiguous ranges
> > when
> > we are reading the line tables. Though I'm not sure if we really
> > want to process
> > that data as opposed to keeping it as the compiler created, and
> > then working
> > around that.
> > 
> > In any case, the following patch addresses this problem.
> > 
> > I'm not particularly happy with how we go back in the ranges (using
> > "pc - 1").
> > Feedback would be welcome.
> > 
> > Validated on aarch64-linux/Ubuntu 20.04/18.04.
> > 
> > Ubuntu 18.04 doesn't actually run into these failures because the
> > compiler
> > doesn't generate distinct PC ranges for the same line.
> > 
> > gdb/ChangeLog:
> > 
> > YYYY-MM-DD  Luis Machado
> > 
> >          * infrun.c (process_event_stop_test): Handle backward
> > stepping
> >          across multiple ranges for the same line.
> >          * symtab.c (find_line_range_start): New function.
> >          * symtab.h (find_line_range_start): New prototype.
> > 
> > 
> > Co-authored-by: Carl Love <cel@us.ibm.com>
> > ---
> >   gdb/infrun.c | 24 +++++++++++++++++++++++-
> >   gdb/symtab.c | 35 +++++++++++++++++++++++++++++++++++
> >   gdb/symtab.h | 16 ++++++++++++++++
> >   3 files changed, 74 insertions(+), 1 deletion(-)
> > 
> > diff --git a/gdb/infrun.c b/gdb/infrun.c
> > index 376a541faf6..997042d3e45 100644
> > --- a/gdb/infrun.c
> > +++ b/gdb/infrun.c
> > @@ -6782,11 +6782,33 @@ if (ecs->event_thread-
> > >control.proceed_to_finish
> >   	 have software watchpoints).  */
> >         ecs->event_thread->control.may_range_step = 1;
> >   
> > +      /* When we are stepping inside a particular line range, in
> > reverse,
> > +	 and we are sitting at the first address of that range, we need
> > to
> > +	 check if this address also shows up in another line range as
> > the
> > +	 end address.
> > +
> > +	 If so, we need to check what line such a step range points to.
> > +	 If it points to the same line as the current step range, that
> > +	 means we need to keep going in order to reach the first
> > address
> > +	 of the line range.  We repeat this until we eventually get to
> > the
> > +	 first address of a particular line we're stepping through.  */
> > +      CORE_ADDR range_start = ecs->event_thread-
> > >control.step_range_start;
> > +      if (execution_direction == EXEC_REVERSE)
> > +	{
> > +	  gdb::optional<CORE_ADDR> real_range_start
> > +	    //	 = find_line_range_start (ecs->event_thread-
> > >suspend.stop_pc);
> > +	    = find_line_range_start (ecs->event_thread-
> > >stop_pc());  //carll fi> +
> > +
> > +	  if (real_range_start.has_value ())
> > +	    range_start = *real_range_start;
> > +	}
> > +
> >         /* When stepping backward, stop at beginning of line range
> >   	 (unless it's the function entry point, in which case
> >   	 keep going back to the call point).  */
> >         CORE_ADDR stop_pc = ecs->event_thread->stop_pc ();
> > -      if (stop_pc == ecs->event_thread->control.step_range_start
> > +      if (stop_pc == range_start
> >   	  && stop_pc != ecs->stop_func_start
> 
> I think this could be moved to the line above.
> 
> >   	  && execution_direction == EXEC_REVERSE)
> >   	end_stepping_range (ecs);
> > diff --git a/gdb/symtab.c b/gdb/symtab.c
> > index 1a39372aad0..c40739919d1 100644
> > --- a/gdb/symtab.c
> > +++ b/gdb/symtab.c
> > @@ -3425,6 +3425,41 @@ find_pc_line (CORE_ADDR pc, int notcurrent)
> >     return sal;
> >   }
> >   
> > +/* See symtah.h.  */
> > +
> > +gdb::optional<CORE_ADDR>
> > +find_line_range_start (CORE_ADDR pc)
> > +{
> > +  struct symtab_and_line current_sal = find_pc_line (pc, 0);
> > +
> > +  if (current_sal.line == 0)
> > +    return {};
> > +
> > +  struct symtab_and_line prev_sal = find_pc_line (current_sal.pc -
> > 1, 0);
> > +
> > +  /* If the previous entry is for a different line, that means we
> > are already
> > +     at the entry with the start PC for this line.  */
> > +  if (prev_sal.line != current_sal.line)
> > +    return current_sal.pc;
> > +
> > +  /* Otherwise, keep looking for entries for the same line but
> > with
> > +     smaller PC's.  */
> > +  bool done = false;
> > +  CORE_ADDR prev_pc;
> > +  while (!done)
> > +    {
> > +      prev_pc = prev_sal.pc;
> > +
> > +      prev_sal = find_pc_line (prev_pc - 1, 0);
> > +
> > +      /* Did we notice a line change?  If so, we are done with the
> > search.  */
> > +      if (prev_sal.line != current_sal.line)
> > +	done = true;
> 
> Shouldn't prev_sal.line also be checked here and return an empty
> optional? I am not sure when that happens, so please enlighten me if
> there is no need to check.
> 
> > +    }
> > +
> > +  return prev_pc;
> > +}
> > +
> >   /* See symtab.h.  */
> >   
> >   struct symtab *
> > diff --git a/gdb/symtab.h b/gdb/symtab.h
> > index d12eee6e9d8..4d893a8a3b8 100644
> > --- a/gdb/symtab.h
> > +++ b/gdb/symtab.h
> > @@ -2149,6 +2149,22 @@ extern struct symtab_and_line find_pc_line
> > (CORE_ADDR, int);
> >   extern struct symtab_and_line find_pc_sect_line (CORE_ADDR,
> >   						 struct obj_section *,
> > int);
> >   
> > +/* Given PC, and assuming it is part of a range of addresses that
> > is part of a
> > +   line, go back through the linetable and find the starting PC of
> > that
> > +   line.
> > +
> > +   For example, suppose we have 3 PC ranges for line X:
> > +
> > +   Line X - [0x0 - 0x8]
> > +   Line X - [0x8 - 0x10]
> > +   Line X - [0x10 - 0x18]
> > +
> > +   If we call the function with PC == 0x14, we want to return 0x0,
> > as that is
> > +   the starting PC of line X, and the ranges are contiguous.
> > +*/
> > +
> > +extern gdb::optional<CORE_ADDR> find_line_range_start (CORE_ADDR
> > pc);
> > +
> >   /* Wrapper around find_pc_line to just return the symtab.  */
> >   
> >   extern struct symtab *find_pc_line_symtab (CORE_ADDR);
> 
>