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

[Bruno Larsen <blarsen@redhat.com>]

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>

-- 
Cheers,
Bruno

>
> Simon
>