[PATCH 3/8] displaced_step_finish: Don't fetch the regcache of exited threads

[Pedro Alves <pedro@palves.net>]

displaced_step_finish can be called with event_status.kind ==
TARGET_WAITKIND_THREAD_EXITED, and in that case it is not possible to
get at the already-exited thread's registers.

This patch moves the get_thread_regcache calls to branches that
actually need it, where we know the thread is still alive.

It also adds an assertion to get_thread_regcache, to help catching
these broken cases sooner.

Change-Id: I63b5eacb3e02a538fc5087c270d8025adfda88c3
---
 gdb/infrun.c   | 19 ++++++++++++-------
 gdb/regcache.c |  2 ++
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 6dc0a2bb9a5..76693a30611 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -2021,8 +2021,6 @@ displaced_step_finish (thread_info *event_thread,
 		       const target_waitstatus &event_status)
 {
   /* Check whether the parent is displaced stepping.  */
-  struct regcache *regcache = get_thread_regcache (event_thread);
-  struct gdbarch *gdbarch = regcache->arch ();
   inferior *parent_inf = event_thread->inf;
 
   /* If this was a fork/vfork/clone, this event indicates that the
@@ -2040,10 +2038,15 @@ displaced_step_finish (thread_info *event_thread,
      gdbarch_displaced_step_restore_all_in_ptid.  This is not enforced
      during gdbarch validation to support architectures which support
      displaced stepping but not forks.  */
-  if (event_status.kind () == TARGET_WAITKIND_FORKED
-      && gdbarch_supports_displaced_stepping (gdbarch))
-    gdbarch_displaced_step_restore_all_in_ptid
-      (gdbarch, parent_inf, event_status.child_ptid ());
+  if (event_status.kind () == TARGET_WAITKIND_FORKED)
+    {
+      struct regcache *parent_regcache = get_thread_regcache (event_thread);
+      struct gdbarch *gdbarch = parent_regcache->arch ();
+
+      if (gdbarch_supports_displaced_stepping (gdbarch))
+	gdbarch_displaced_step_restore_all_in_ptid
+	  (gdbarch, parent_inf, event_status.child_ptid ());
+    }
 
   displaced_step_thread_state *displaced = &event_thread->displaced_step_state;
 
@@ -2082,11 +2085,13 @@ displaced_step_finish (thread_info *event_thread,
 	 child hasn't been added to the inferior list yet at this
 	 point.  */
 
+      struct regcache *parent_regcache = get_thread_regcache (event_thread);
+      struct gdbarch *gdbarch = parent_regcache->arch ();
       struct regcache *child_regcache
 	= get_thread_arch_regcache (parent_inf, event_status.child_ptid (),
 				    gdbarch);
       /* Read PC value of parent.  */
-      CORE_ADDR parent_pc = regcache_read_pc (regcache);
+      CORE_ADDR parent_pc = regcache_read_pc (parent_regcache);
 
       displaced_debug_printf ("write child pc from %s to %s",
 			      paddress (gdbarch,
diff --git a/gdb/regcache.c b/gdb/regcache.c
index e46a0b58f50..f9bf1ecbe12 100644
--- a/gdb/regcache.c
+++ b/gdb/regcache.c
@@ -411,6 +411,8 @@ get_thread_regcache (process_stratum_target *target, ptid_t ptid)
 struct regcache *
 get_thread_regcache (thread_info *thread)
 {
+  gdb_assert (thread->state != THREAD_EXITED);
+
   return get_thread_regcache (thread->inf->process_target (),
 			      thread->ptid);
 }
-- 
2.43.0