Re: [PATCH 7/7] gdb: add special handling for frame level 0 in frame_info_ptr

[Simon Marchi <simon.marchi@polymtl.ca>]

On 11/10/22 11:28, Bruno Larsen wrote:
> On 08/11/2022 17:19, Simon Marchi wrote:
>> On 11/8/22 05:40, Bruno Larsen wrote:
>>> On 07/11/2022 16:53, Simon Marchi via Gdb-patches wrote:
>>>> From: Simon Marchi <simon.marchi@efficios.com>
>>>>
>>>> I noticed this problem while preparing the initial submission for the
>>>> ROCm GDB port.  One particularity of this patch set is that it does not
>>>> support unwinding frames, that requires support of some DWARF extensions
>>>> that will come later.  It was still possible to run to a breakpoint and
>>>> print frame #0, though.
>>>>
>>>> When rebasing on top of the frame_info_ptr work, GDB started tripping on
>>>> a prepare_reinflate call, making it not possible anymore to event print
>>>> the frame when stopping on a breakpoint.  One thing to know about frame
>>>> 0 is that its id is lazily computed when something requests it through
>>>> get_frame_id.  See:
>>>>
>>>>     https://gitlab.com/gnutools/binutils-gdb/-/blob/23912acd402f5af9caf91b257e5209ec4c58a09c/gdb/frame.c#L2070-2080
>>>>
>>>> So, up to that prepare_reinflate call, frame 0's id was not computed,
>>>> and prepare_reinflate, calling get_frame_id, forces it to be computed.
>>>> Computing the frame id generally requires unwinding the previous frame,
>>>> which with my ROCm GDB patch fails.  An exception is thrown and the
>>>> printing of the frame is simply abandonned.
>>>>
>>>> Regardless of this ROCm GDB problem (which is admittedly temporary, it
>>>> will be possible to unwind with subsequent patches), we want to avoid
>>>> prepare_reinflate to force the computing of the frame id, for the same
>>>> reasons we lazily compute it in the first place.
>>>>
>>>> In addition, frame 0's id is subject to change across a frame cache
>>>> reset.  This is why save_selected_frame and restore_selected_frame have
>>>> special handling for frame 0:
>>>>
>>>>     https://gitlab.com/gnutools/binutils-gdb/-/blob/23912acd402f5af9caf91b257e5209ec4c58a09c/gdb/frame.c#L1841-1863
>>>>
>>>> For this last reason, we also need to handle frame 0 specially in
>>>> prepare_reinflate / reinflate.  Because the frame id of frame 0 can
>>>> change across a frame cache reset, we must not rely on the frame id from
>>>> that frame to reinflate it.  We should instead just re-fetch the current
>>>> frame at that point.
>>>>
>>>> This patch adds a frame_info_ptr::m_cached_level field, set in
>>>> frame_info_ptr::prepare_reinflate, so we can tell if a frame is frame 0.
>>>> There are cases where a frame_info_ptr object wraps a sentinel frame,
>>>> for which frame_relative_level returns -1, so I have chosen the value -2
>>>> to represent "invalid frame level", for when the frame_info_ptr object
>>>> is empty.
>>>>
>>>> In frame_info_ptr::prepare_reinflate, only cache the frame id if the
>>>> frame level is not 0.  It's fine to cache the frame id for the sentinel
>>>> frame, it will be properly handled by frame_find_by_id later.
>>>>
>>>> In frame_info_ptr::reinflate, if the frame level is 0, call
>>>> get_current_frame to get the target's current frame.  Otherwise, use
>>>> frame_find_by_id just as before.
>>>>
>>>> This patch should not have user-visible changes with upstream GDB.  But
>>>> it will avoid forcing the computation of frame 0's when calling
>>>> prepare_reinflate.  And, well, it fixes the upcoming ROCm GDB patch
>>>> series.
>>>>
>>>> Change-Id: I176ed7ee9317ddbb190acee8366e087e08e4d266
>>> This all makes sense. I have a small style preference below, but even if you dislike it, the code is still fine.
>>>
>>> Reviewed-By: Bruno Larsen <blarsen@redhat.com>
>>>
>>>> ---
>>>>    gdb/frame-info.c | 24 ++++++++++++++++++++----
>>>>    gdb/frame-info.h | 20 ++++++++++++++++++--
>>>>    2 files changed, 38 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/gdb/frame-info.c b/gdb/frame-info.c
>>>> index 584222dc490f..e3ee9f8174e1 100644
>>>> --- a/gdb/frame-info.c
>>>> +++ b/gdb/frame-info.c
>>>> @@ -31,7 +31,11 @@ intrusive_list<frame_info_ptr> frame_info_ptr::frame_list;
>>>>    void
>>>>    frame_info_ptr::prepare_reinflate ()
>>>>    {
>>>> -  m_cached_id = get_frame_id (*this);
>>>> +  m_cached_level = frame_relative_level (*this);
>>>> +  gdb_assert (m_cached_level >= -1);
>>> Since you have declared invalid_level = -2 for this class, I feel like it would be more better to have the assert be
>>>
>>> gdb_assert (m_cached_level > invalid_level);
>> This form assumes that invalid_level is -2, defeating the purpose to
>> have the abstraction in the first place.  If we changed invalid_level to
>> be INT_MAX, for intsance, the assertion wouldn't be right anymore.
>>
>>> This way there is no need to wonder why -1 is a valid level, and makes it easier to grep for the comment in the file, should someone want to know.
>> In my vision of things, the sentinel frame having level -1 is well
>> known, because it's the frame just below the current frame, which is
>> known to have level 0.  So while it looks like a magic random value,
>> it's not really.  The numerical value has a meaning.  We wouldn't want
>> to change the sentinel frame's level value to be any other arbitrary
>> numerical value.
> Ok, I see your point. It does make sense when you put it like that.
>>
>> Here, I can just drop the assert.  It's basically just checking that
>> frame_relative_level didn't return something that doesn't make sense.
>> But there's no reason for frame_relative_level to return something that
>> doesn't make sense in the first place.  Other callers of
>> frame_relative_level don't do this assert, they just trust that
>> frame_relative_level returns something that makes sense, nothing really
>> different here.
>>
>>>> +
>>>> +  if (m_cached_level != 0)
>>>> +    m_cached_id = get_frame_id (*this);
>>>>    }
>>>>      /* See frame-info-ptr.h.  */
>>>> @@ -39,9 +43,21 @@ frame_info_ptr::prepare_reinflate ()
>>>>    void
>>>>    frame_info_ptr::reinflate ()
>>>>    {
>>>> -  gdb_assert (frame_id_p (m_cached_id));
>>>> +  gdb_assert (m_cached_level >= -1);
>>> Likewise
>> Here, I could add a comment like:
>>
>>    /* Ensure we have a valid frame level, indicating prepare_reinflate
>>       was called.  */
> 
> Yeah, this comment fixes any possible confusion. You convinced me :-)
> 
> Reviewed-By: Bruno Larsen <blarsen@redhat.com>

Thanks to you and Tom for your review on all patches, I will push
shortly.

Simon