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

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

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
---
 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);
+
+  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);
+
+  if (m_ptr != nullptr)
+    {
+      /* The frame_info wasn't invalidated, no need to reinflate.  */
+      return;
+    }
+
+  if (m_cached_level == 0)
+    m_ptr = get_current_frame ().get ();
+  else
+    {
+      gdb_assert (frame_id_p (m_cached_id));
+      m_ptr = frame_find_by_id (m_cached_id).get ();
+    }
 
-  if (m_ptr == nullptr)
-    m_ptr = frame_find_by_id (m_cached_id).get ();
   gdb_assert (m_ptr != nullptr);
 }
diff --git a/gdb/frame-info.h b/gdb/frame-info.h
index 1d2d4bdc7e68..3369b114326f 100644
--- a/gdb/frame-info.h
+++ b/gdb/frame-info.h
@@ -56,16 +56,21 @@ class frame_info_ptr : public intrusive_list_node<frame_info_ptr>
   }
 
   frame_info_ptr (const frame_info_ptr &other)
-    : m_ptr (other.m_ptr), m_cached_id (other.m_cached_id)
+    : m_ptr (other.m_ptr),
+      m_cached_id (other.m_cached_id),
+      m_cached_level (other.m_cached_level)
   {
     frame_list.push_back (*this);
   }
 
   frame_info_ptr (frame_info_ptr &&other)
-    : m_ptr (other.m_ptr), m_cached_id (other.m_cached_id)
+    : m_ptr (other.m_ptr),
+      m_cached_id (other.m_cached_id),
+      m_cached_level (other.m_cached_level)
   {
     other.m_ptr = nullptr;
     other.m_cached_id = null_frame_id;
+    other.m_cached_level = invalid_level;
     frame_list.push_back (*this);
   }
 
@@ -78,6 +83,7 @@ class frame_info_ptr : public intrusive_list_node<frame_info_ptr>
   {
     m_ptr = other.m_ptr;
     m_cached_id = other.m_cached_id;
+    m_cached_level = other.m_cached_level;
     return *this;
   }
 
@@ -85,6 +91,7 @@ class frame_info_ptr : public intrusive_list_node<frame_info_ptr>
   {
     m_ptr = nullptr;
     m_cached_id = null_frame_id;
+    m_cached_level = invalid_level;
     return *this;
   }
 
@@ -92,8 +99,10 @@ class frame_info_ptr : public intrusive_list_node<frame_info_ptr>
   {
     m_ptr = other.m_ptr;
     m_cached_id = other.m_cached_id;
+    m_cached_level = other.m_cached_level;
     other.m_ptr = nullptr;
     other.m_cached_id = null_frame_id;
+    other.m_cached_level = invalid_level;
     return *this;
   }
 
@@ -136,6 +145,10 @@ class frame_info_ptr : public intrusive_list_node<frame_info_ptr>
   void reinflate ();
 
 private:
+  /* We sometimes need to construct frame_info_ptr objects around the
+     sentinel_frame, which has level -1.  Therefore, make the invalid frame
+     level value -2.  */
+  static constexpr int invalid_level = -2;
 
   /* The underlying pointer.  */
   frame_info *m_ptr = nullptr;
@@ -143,6 +156,9 @@ class frame_info_ptr : public intrusive_list_node<frame_info_ptr>
   /* The frame_id of the underlying pointer.  */
   frame_id m_cached_id = null_frame_id;
 
+  /* The frame level of the underlying pointer.  */
+  int m_cached_level = invalid_level;
+
   /* All frame_info_ptr objects are kept on an intrusive list.
      This keeps their construction and destruction costs
      reasonably small.  */
-- 
2.38.1