[binutils-gdb] gdb: replace architecture_changed with new_architecture observer

[Andrew Burgess]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=4b2f71e6c67060e2aa0d35652de80fdc1f810ce8

commit 4b2f71e6c67060e2aa0d35652de80fdc1f810ce8
Author: Andrew Burgess <aburgess@redhat.com>
Date:   Wed Oct 11 10:30:35 2023 +0100

    gdb: replace architecture_changed with new_architecture observer
    
    This commit replaces the architecture_changed observer with a
    new_architecture observer.
    
    Currently the only user of the architecture_changed observer is the
    Python code, which uses this observer to register the Python unwinder
    with the architecture.
    
    The problem is that the architecture_changed observer is triggered
    from inferior::set_arch(), which only sees the inferior-wide gdbarch
    value.  For targets that use thread-specific architectures, these
    never trigger the architecture_changed observer, and so never have the
    Python unwinder registered with them.
    
    When it comes to unwinding GDB makes use of the frame's gdbarch, which
    is based on the thread's regcache gdbarch, which is set in
    get_thread_regcache to the value returned from
    target_thread_architecture, which is not always the inferiors gdbarch
    value, it might be a thread-specific gdbarch which has not passed
    through inferior::set_arch().
    
    The new_architecture observer will be triggered from
    gdbarch_find_by_info, whenever a new gdbarch is created and
    initialised.  As GDB caches and reuses gdbarch values, we should
    expect to see each new architecture trigger the new_architecture
    observer just once.
    
    After this commit, targets that make use of thread-specific
    architectures should be able to make use of Python unwinders.
    
    As I don't have access to a machine that makes use of thread-specific
    architectures right now, I asked Luis to confirm that an AArch64
    target that uses SVE/SME can't use the Python unwinders in threads
    that are using a thread-specific architectures, and he confirmed that
    this is indeed the case, see this discussion:
    
      https://inbox.sourceware.org/gdb/87wmvsat8i.fsf@redhat.com
    
    Tested-By: Lancelot Six <lancelot.six@amd.com>
    Tested-By: Luis Machado <luis.machado@arm.com>
    Reviewed-By: Luis Machado <luis.machado@arm.com>
    Approved-By: Simon Marchi <simon.marchi@efficios.com>

Diff:
---
 gdb/arch-utils.c       | 2 ++
 gdb/inferior.c         | 1 -
 gdb/observable.c       | 2 +-
 gdb/observable.h       | 7 +++----
 gdb/python/py-unwind.c | 5 ++---
 5 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
index 60a50ea5b2c..c64584c5760 100644
--- a/gdb/arch-utils.c
+++ b/gdb/arch-utils.c
@@ -1479,6 +1479,8 @@ gdbarch_find_by_info (struct gdbarch_info info)
   if (gdbarch_debug)
     gdbarch_dump (new_gdbarch, gdb_stdlog);
 
+  gdb::observers::new_architecture.notify (new_gdbarch);
+
   return new_gdbarch;
 }
 
diff --git a/gdb/inferior.c b/gdb/inferior.c
index efe57cceae3..1778723863e 100644
--- a/gdb/inferior.c
+++ b/gdb/inferior.c
@@ -179,7 +179,6 @@ inferior::set_arch (gdbarch *arch)
   gdb_assert (arch != nullptr);
   gdb_assert (gdbarch_initialized_p (arch));
   m_gdbarch = arch;
-  gdb::observers::architecture_changed.notify (this, arch);
 
   process_stratum_target *proc_target = this->process_target ();
   if (proc_target != nullptr)
diff --git a/gdb/observable.c b/gdb/observable.c
index 09613b2ddda..f2e65b11604 100644
--- a/gdb/observable.c
+++ b/gdb/observable.c
@@ -52,7 +52,7 @@ DEFINE_OBSERVABLE (about_to_proceed);
 DEFINE_OBSERVABLE (breakpoint_created);
 DEFINE_OBSERVABLE (breakpoint_deleted);
 DEFINE_OBSERVABLE (breakpoint_modified);
-DEFINE_OBSERVABLE (architecture_changed);
+DEFINE_OBSERVABLE (new_architecture);
 DEFINE_OBSERVABLE (thread_ptid_changed);
 DEFINE_OBSERVABLE (inferior_added);
 DEFINE_OBSERVABLE (inferior_appeared);
diff --git a/gdb/observable.h b/gdb/observable.h
index acb05e9b535..a535eedcd38 100644
--- a/gdb/observable.h
+++ b/gdb/observable.h
@@ -153,10 +153,9 @@ extern observable<struct breakpoint */* b */> breakpoint_deleted;
    is the modified breakpoint.  */
 extern observable<struct breakpoint */* b */> breakpoint_modified;
 
-/* INF's architecture has changed.  The argument NEWARCH is a
-   pointer to the new architecture.  */
-extern observable<inferior */* inf */, struct gdbarch */* newarch */>
-  architecture_changed;
+/* GDB has instantiated a new architecture, NEWARCH is a pointer to the new
+   architecture.  */
+extern observable<struct gdbarch */* newarch */> new_architecture;
 
 /* The thread's ptid has changed.  The OLD_PTID parameter specifies
    the old value, and NEW_PTID specifies the new value.  */
diff --git a/gdb/python/py-unwind.c b/gdb/python/py-unwind.c
index f8b142dd52c..ee50c51b531 100644
--- a/gdb/python/py-unwind.c
+++ b/gdb/python/py-unwind.c
@@ -945,7 +945,7 @@ static const registry<gdbarch>::key<pyuw_gdbarch_data_type> pyuw_gdbarch_data;
    intermediary.  */
 
 static void
-pyuw_on_new_gdbarch (inferior *inf, gdbarch *newarch)
+pyuw_on_new_gdbarch (gdbarch *newarch)
 {
   struct pyuw_gdbarch_data_type *data = pyuw_gdbarch_data.get (newarch);
   if (data == nullptr)
@@ -974,8 +974,7 @@ pyuw_on_new_gdbarch (inferior *inf, gdbarch *newarch)
 static int CPYCHECKER_NEGATIVE_RESULT_SETS_EXCEPTION
 gdbpy_initialize_unwind (void)
 {
-  gdb::observers::architecture_changed.attach (pyuw_on_new_gdbarch,
-					       "py-unwind");
+  gdb::observers::new_architecture.attach (pyuw_on_new_gdbarch, "py-unwind");
 
   if (PyType_Ready (&pending_frame_object_type) < 0)
     return -1;