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[213.175.37.12]) by smtp.gmail.com with ESMTPSA id ew13-20020a05622a514d00b0039cc944ebdasm11499112qtb.54.2022.11.10.08.28.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 10 Nov 2022 08:28:50 -0800 (PST) Message-ID: <306755c6-eeb3-63a1-4d9a-a4678d13b8a4@redhat.com> Date: Thu, 10 Nov 2022 17:28:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.1 Subject: Re: [PATCH 7/7] gdb: add special handling for frame level 0 in frame_info_ptr To: Simon Marchi , gdb-patches@sourceware.org Cc: Simon Marchi References: <20221107155310.2590069-1-simon.marchi@polymtl.ca> <20221107155310.2590069-7-simon.marchi@polymtl.ca> <24a4edb5-4081-7b34-a5ca-291d51decea4@redhat.com> <4ea9edd4-b268-5e8c-b697-b5dd09e2e25a@polymtl.ca> From: Bruno Larsen In-Reply-To: <4ea9edd4-b268-5e8c-b697-b5dd09e2e25a@polymtl.ca> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-11.7 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: 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 >>> >>> 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 >> >>> --- >>>   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_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 -- Cheers, Bruno > > Simon >