From: Bruno Larsen <blarsen@redhat.com>
To: Carl Love <cel@us.ibm.com>, gdb-patches@sourceware.org
Cc: Rogerio Alves <rogealve@br.ibm.com>
Subject: Re: [PATCH] Updated, fix reverse stepping multiple contiguous PC ranges
Date: Wed, 9 Mar 2022 09:23:15 -0300 [thread overview]
Message-ID: <3b2ab293-92d9-aa9b-bff2-3b6eaff27d59@redhat.com> (raw)
In-Reply-To: <0fbaf3783401f8aa8e76a4d3928efff46485ab8d.camel@us.ibm.com>
On 3/8/22 19:01, Carl Love wrote:
> 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 have recently worked on gdb.base/until-trailing-insns.exp, that uses the dwarf assembler quite a bit to create a line table. I am not sure how one would go about making contiguous ranges, but maybe you can find something in gdb/testsuite/lib/dwarf.exp to help you.
>
> 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);
>>
>>
>
--
Cheers!
Bruno Larsen
next prev parent reply other threads:[~2022-03-09 12:23 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-23 16:39 Carl Love
2022-02-28 18:02 ` Carl Love
2022-03-08 20:21 ` Bruno Larsen
2022-03-08 22:01 ` Carl Love
2022-03-09 12:23 ` Bruno Larsen [this message]
2022-03-09 20:52 ` Carl Love
2022-03-10 13:49 ` Bruno Larsen
2022-03-09 14:53 ` Luis Machado
2022-03-10 11:13 ` Luis Machado
2022-03-10 13:19 ` Bruno Larsen
2022-03-10 13:33 ` Luis Machado
2022-03-22 15:28 ` Carl Love
2022-03-22 17:05 ` [PATCH V2] " Carl Love
2022-03-22 17:10 ` Luis Machado
2022-03-23 12:20 ` Bruno Larsen
2022-03-23 15:43 ` [PATCH V3] " Carl Love
2022-03-31 13:52 ` [PATCH, v2] Fix reverse stepping multiple contiguous PC ranges over the line table Luis Machado
2022-04-04 16:55 ` will schmidt
2022-04-05 8:36 ` Luis Machado
2022-04-05 15:15 ` will schmidt
2022-04-05 15:24 ` Luis Machado
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