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

[Bruno Larsen <blarsen@redhat.com>]

On 3/10/22 08:13, Luis Machado wrote:
> Hi!
> 
> On 3/8/22 20:21, Bruno Larsen via Gdb-patches wrote:
>> On 2/23/22 13:39, Carl Love via Gdb-patches wrote:
>>>
>>> GCC maintainers:
>>>
>>> The following patch was posted by Luis Machado on 2/1/2021.  There was
>>> a little discussion on the patch but it was never fully reviewed and
>>> approved.  The email for Luis <luis.machado@linaro.org> no longer
>>> works.
>>>
>>> As of 2/21/2022, the patch does not compile.  I made a small fix to get
>>> it to compile.  I commented out the original line in gdb/infrun.c and
>>> added a new line with the fix and the comment //carll fix to indicate
>>> what I changed.  Clearly, the comment needs to be removed if the patch
>>> is accepted but I wanted to show what I changed.
>>>
>>> That said, I tested the patch on Powerpc and it fixed the 5 test
>>> failures in gdb.reverse/solib-precsave.exp and 5 test failures in
>>> gdb.reverse/solib-reverse.exp.  I tested on Intel 64-bit.  The two
>>> tests solib-precsave.exp and solib-reverse.exp both initially passed on
>>> Intel.  No additional regression failures were seen with the patch.
>>>
>>> Please let me know if you have comments on the patch or if it is
>>> acceptable as is.  Thank you.
>>>
>>>                       Carl Love
>>
>> Hello Carl!
>>
>> 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
>>
>>> -----------------------------------------------------------
>>> 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.
>>
> 
> Do you mean moving the range_start check downwards below the ecs->stop_func_start check?

I meant that they can be in the same line:
	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 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.
>>
> 
> I went through this again, and I don't think prev-sal.line needs to be checked. At this point we know current_sal.line is sane, so anything that differs from the current line, we bail out and return the address we currently have.
> 
> Does that make sense?
> 
It does.  My main point was asking if finding a 0 was cause for leaving with an error, instead of leaving with the answer that could be wrong. But the base answer is already wrong, so this would probably be a less wrong answer, so this is fine

-- 
Cheers!
Bruno Larsen