[PING][PATCH] Fix reverse stepping multiple contiguous PC ranges

[Luis Machado <luis.machado@linaro.org>]

Ping!

On 3/19/21 3:17 PM, Luis Machado wrote:
> Polite ping.
> 
> On 3/12/21 12:36 PM, Luis Machado wrote:
>> Polite ping.
>>
>> On 3/4/21 11:30 AM, Luis Machado wrote:
>>> On 2/26/21 3:58 PM, Luis Machado wrote:
>>>> On 2/11/21 8:38 AM, Luis Machado wrote:
>>>>> On 2/1/21 4:19 PM, Luis Machado wrote:
>>>>>> 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  <luis.machado@linaro.org>
>>>>>>
>>>>>>     * 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.
>>>>>> ---
>>>>>>   gdb/infrun.c | 22 +++++++++++++++++++++-
>>>>>>   gdb/symtab.c | 35 +++++++++++++++++++++++++++++++++++
>>>>>>   gdb/symtab.h | 16 ++++++++++++++++
>>>>>>   3 files changed, 72 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/gdb/infrun.c b/gdb/infrun.c
>>>>>> index e070eff33d..5bb268529d 100644
>>>>>> --- a/gdb/infrun.c
>>>>>> +++ b/gdb/infrun.c
>>>>>> @@ -6534,11 +6534,31 @@ process_event_stop_test (struct 
>>>>>> execution_control_state *ecs)
>>>>>>        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);
>>>>>> +
>>>>>> +      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->suspend.stop_pc;
>>>>>> -      if (stop_pc == ecs->event_thread->control.step_range_start
>>>>>> +      if (stop_pc == range_start
>>>>>>         && stop_pc != ecs->stop_func_start
>>>>>>         && execution_direction == EXEC_REVERSE)
>>>>>>       end_stepping_range (ecs);
>>>>>> diff --git a/gdb/symtab.c b/gdb/symtab.c
>>>>>> index 7ffb52a943..e8f5301951 100644
>>>>>> --- a/gdb/symtab.c
>>>>>> +++ b/gdb/symtab.c
>>>>>> @@ -3382,6 +3382,41 @@ 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;
>>>>>> +    }
>>>>>> +
>>>>>> +  return prev_pc;
>>>>>> +}
>>>>>> +
>>>>>>   /* See symtab.h.  */
>>>>>>   struct symtab *
>>>>>> diff --git a/gdb/symtab.h b/gdb/symtab.h
>>>>>> index f060e0ebc1..659cb46292 100644
>>>>>> --- a/gdb/symtab.h
>>>>>> +++ b/gdb/symtab.h
>>>>>> @@ -1916,6 +1916,22 @@ 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);
>>>>>>