[PATCH] do not take mutex in _Unwind_Find_registered_FDE if there is no registered objects

[Gleb Natapov]

_Unwind_Find_FDE calls _Unwind_Find_registered_FDE and it takes lock even
when there is no registered objects. As far as I see only statically
linked applications call __register_frame_info* functions, so for
dynamically linked executables taking the lock to check unseen_objects
and seen_objects is a pessimization. Since the function is called on
each thrown exception this is a lot of unneeded locking.  This patch
checks unseen_objects and seen_objects outside of the lock and returns
earlier if both are NULL.

diff --git a/libgcc/unwind-dw2-fde.c b/libgcc/unwind-dw2-fde.c
index 5b16a1f..41de746 100644
--- a/libgcc/unwind-dw2-fde.c
+++ b/libgcc/unwind-dw2-fde.c
@@ -1001,6 +1001,13 @@ _Unwind_Find_FDE (void *pc, struct dwarf_eh_bases *bases)
   struct object *ob;
   const fde *f = NULL;
 
+  /* __atomic_write is not used to modify unseen_objects and seen_objects
+     since they are modified in locked sections only and unlock provides
+     release semantics. */
+  if (!__atomic_load_n(&unseen_objects, __ATOMIC_ACQUIRE)
+      && !__atomic_load_n(&seen_objects, __ATOMIC_ACQUIRE))
+      return NULL;
+
   init_object_mutex_once ();
   __gthread_mutex_lock (&object_mutex);
 
@@ -1020,8 +1027,8 @@ _Unwind_Find_FDE (void *pc, struct dwarf_eh_bases *bases)
   while ((ob = unseen_objects))
     {
       struct object **p;
+      struct object *next = ob->next;
 
-      unseen_objects = ob->next;
       f = search_object (ob, pc);
 
       /* Insert the object into the classified list.  */
@@ -1031,6 +1038,8 @@ _Unwind_Find_FDE (void *pc, struct dwarf_eh_bases *bases)
       ob->next = *p;
       *p = ob;
 
+      unseen_objects = next;
+
       if (f)
        goto fini;
     }
--
                        Gleb.