slight improvement to scavenging of update frames when a collision has occurred

rts/sm/Scav.c

@@ -1575,11 +1575,13 @@ scavenge_stack(StgPtr p, StgPtr stack_end)
 	// threadPaused().  We could traverse the whole stack again
 	// before GC, but that seems like overkill.
 	//
-	// Scavenging this update frame as normal would be disastrous;
-	// the updatee would end up pointing to the value.  So we turn
-	// the indirection into an IND_PERM, so that evacuate will
-	// copy the indirection into the old generation instead of
-	// discarding it.
+        // So, if the frame points to an indirection, it will get
+        // shorted out when we evacuate.  If this happens, we have no
+        // closure to update any more.  In the past we solved this by
+        // replacing the IND with an IND_PERM, but a better solution
+        // is to replace the update frame with a frame that no longer
+        // does the update and just uses the value already computed by
+        // the other thread, so that is what we now do.
         //
         // Note [upd-black-hole]
         // One slight hiccup is that the THUNK_SELECTOR machinery can
@@ -1590,22 +1592,16 @@ scavenge_stack(StgPtr p, StgPtr stack_end)
         // the updatee is never a THUNK_SELECTOR and we're ok.
         // NB. this is a new invariant: blackholing is not optional.
     {
-        nat type;
-        const StgInfoTable *i;
-        StgClosure *updatee;
-
-        updatee = ((StgUpdateFrame *)p)->updatee;
-        i = updatee->header.info;
-        if (!IS_FORWARDING_PTR(i)) {
-            type = get_itbl(updatee)->type;
-            if (type == IND) {
-                updatee->header.info = &stg_IND_PERM_info;
-            } else if (type == IND_OLDGEN) {
-                updatee->header.info = &stg_IND_OLDGEN_PERM_info;
-            }            
+        StgClosure *v;
+        StgUpdateFrame *frame = (StgUpdateFrame *)p;
+
+        evacuate(&frame->updatee);
+        v = frame->updatee;
+        if (GET_CLOSURE_TAG(v) != 0 ||
+            (get_itbl(v)->type != BLACKHOLE &&
+             get_itbl(v)->type != CAF_BLACKHOLE)) {
+            frame->header.info = (const StgInfoTable*)&stg_gc_unpt_r1_info;
         }
-        evacuate(&((StgUpdateFrame *)p)->updatee);
-        ASSERT(GET_CLOSURE_TAG(((StgUpdateFrame *)p)->updatee) == 0);
         p += sizeofW(StgUpdateFrame);
         continue;
     }