GC refactoring: change evac_gen to evac_step

By establishing an ordering on step pointers, we can simplify the test
  (stp->gen_no < evac_gen)
to 
  (stp < evac_step)
which is common in evacuate().

6 files changed, 95 insertions, 90 deletions

diff --git a/rts/sm/Evac.c b/rts/sm/Evac.c
index 518b383a54..14b80a77ae 100644
--- a/rts/sm/Evac.c
+++ b/rts/sm/Evac.c
@@ -39,9 +39,9 @@ alloc_for_copy (nat size, step *stp)
      * evacuate to an older generation, adjust it here (see comment
      * by evacuate()).
      */
-    if (stp->gen_no < gct->evac_gen) {
+    if (stp < gct->evac_step) {
 	if (gct->eager_promotion) {
-	    stp = &generations[gct->evac_gen].steps[0];
+	    stp = gct->evac_step;
 	} else {
 	    gct->failed_to_evac = rtsTrue;
 	}
@@ -75,9 +75,9 @@ alloc_for_copy_noscav (nat size, step *stp)
      * evacuate to an older generation, adjust it here (see comment
      * by evacuate()).
      */
-    if (stp->gen_no < gct->evac_gen) {
+    if (stp < gct->evac_step) {
 	if (gct->eager_promotion) {
-	    stp = &generations[gct->evac_gen].steps[0];
+	    stp = gct->evac_step;
 	} else {
 	    gct->failed_to_evac = rtsTrue;
 	}
@@ -113,7 +113,7 @@ copy_tag(StgClosure **p, StgClosure *src, nat size, step *stp,StgWord tag)
     } while (info == (W_)&stg_WHITEHOLE_info);
     if (info == (W_)&stg_EVACUATED_info) {
 	src->header.info = (const StgInfoTable *)info;
-	return evacuate(src); // does the failed_to_evac stuff
+	return evacuate(p); // does the failed_to_evac stuff
     }
 #else
     info = (W_)src->header.info;
@@ -169,13 +169,13 @@ copy_noscav_tag(StgClosure **p, StgClosure *src, nat size, step *stp, StgWord ta
     } while (info == (W_)&stg_WHITEHOLE_info);
     if (info == (W_)&stg_EVACUATED_info) {
 	src->header.info = (const StgInfoTable *)info;
-	return evacuate(src); // does the failed_to_evac stuff
+	return evacuate(p); // does the failed_to_evac stuff
     }
 #else
     info = (W_)src->header.info;
     src->header.info = &stg_EVACUATED_info;
 #endif
-    
+
     to = alloc_for_copy_noscav(size,stp);
     tagged_to = (StgPtr)TAG_CLOSURE(tag,(StgClosure*)to);
     *p = (StgClosure *)tagged_to;
@@ -220,7 +220,7 @@ copyPart(StgClosure **p, StgClosure *src, nat size_to_reserve, nat size_to_copy,
     } while (info == (W_)&stg_WHITEHOLE_info);
     if (info == (W_)&stg_EVACUATED_info) {
 	src->header.info = (const StgInfoTable *)info;
-	return evacuate(src); // does the failed_to_evac stuff
+	return evacuate(p); // does the failed_to_evac stuff
     }
 #else
     info = (W_)src->header.info;
@@ -296,7 +296,7 @@ evacuate_large(StgPtr p)
     /* Don't forget to set the gct->failed_to_evac flag if we didn't get
      * the desired destination (see comments in evacuate()).
      */
-    if (bd->gen_no < gct->evac_gen) {
+    if (bd->step < gct->evac_step) {
       gct->failed_to_evac = rtsTrue;
       TICK_GC_FAILED_PROMOTION();
     }
@@ -320,9 +320,9 @@ evacuate_large(StgPtr p)
   /* link it on to the evacuated large object list of the destination step
    */
   stp = bd->step->to;
-  if (stp->gen_no < gct->evac_gen) {
+  if (stp < gct->evac_step) {
       if (gct->eager_promotion) {
-	  stp = &generations[gct->evac_gen].steps[0];
+	  stp = gct->evac_step;
       } else {
 	  gct->failed_to_evac = rtsTrue;
       }
@@ -342,22 +342,22 @@ evacuate_large(StgPtr p)
    This is called (eventually) for every live object in the system.
 
    The caller to evacuate specifies a desired generation in the
-   gct->evac_gen thread-lock variable.  The following conditions apply to
+   gct->evac_step thread-local variable.  The following conditions apply to
    evacuating an object which resides in generation M when we're
    collecting up to generation N
 
-   if  M >= gct->evac_gen 
+   if  M >= gct->evac_step 
            if  M > N     do nothing
 	   else          evac to step->to
 
-   if  M < gct->evac_gen      evac to gct->evac_gen, step 0
+   if  M < gct->evac_step      evac to gct->evac_step, step 0
 
    if the object is already evacuated, then we check which generation
    it now resides in.
 
-   if  M >= gct->evac_gen     do nothing
-   if  M <  gct->evac_gen     set gct->failed_to_evac flag to indicate that we
-                         didn't manage to evacuate this object into gct->evac_gen.
+   if  M >= gct->evac_step     do nothing
+   if  M <  gct->evac_step     set gct->failed_to_evac flag to indicate that we
+                         didn't manage to evacuate this object into gct->evac_step.
 
 
    OPTIMISATION NOTES:
@@ -472,10 +472,10 @@ loop:
   if (bd->gen_no > N) {
       /* Can't evacuate this object, because it's in a generation
        * older than the ones we're collecting.  Let's hope that it's
-       * in gct->evac_gen or older, or we will have to arrange to track
+       * in gct->evac_step or older, or we will have to arrange to track
        * this pointer using the mutable list.
        */
-      if (bd->gen_no < gct->evac_gen) {
+      if (bd->step < gct->evac_step) {
 	  // nope 
 	  gct->failed_to_evac = rtsTrue;
 	  TICK_GC_FAILED_PROMOTION();
@@ -491,7 +491,7 @@ loop:
        * object twice, for example).
        */
       if (bd->flags & BF_EVACUATED) {
-	  if (bd->gen_no < gct->evac_gen) {
+	  if (bd->step < gct->evac_step) {
 	      gct->failed_to_evac = rtsTrue;
 	      TICK_GC_FAILED_PROMOTION();
 	  }
@@ -664,7 +664,7 @@ loop:
 
   case EVACUATED:
     /* Already evacuated, just return the forwarding address.
-     * HOWEVER: if the requested destination generation (gct->evac_gen) is
+     * HOWEVER: if the requested destination generation (gct->evac_step) is
      * older than the actual generation (because the object was
      * already evacuated to a younger generation) then we have to
      * set the gct->failed_to_evac flag to indicate that we couldn't 
@@ -682,8 +682,8 @@ loop:
   {
       StgClosure *e = ((StgEvacuated*)q)->evacuee;
       *p = e;
-      if (gct->evac_gen > 0 && stp->gen_no < gct->evac_gen) {  // optimisation 
-	  if (HEAP_ALLOCED(e) && Bdescr((P_)e)->gen_no < gct->evac_gen) {
+      if (stp < gct->evac_step) {  // optimisation 
+	  if (HEAP_ALLOCED(e) && Bdescr((P_)e)->step < gct->evac_step) {
 	      gct->failed_to_evac = rtsTrue;
 	      TICK_GC_FAILED_PROMOTION();
 	  }
diff --git a/rts/sm/GC.c b/rts/sm/GC.c
index d064f1d656..4ca0c9da60 100644
--- a/rts/sm/GC.c
+++ b/rts/sm/GC.c
@@ -335,11 +335,11 @@ GarbageCollect ( rtsBool force_major_gc )
   }
 
   // follow roots from the CAF list (used by GHCi)
-  gct->evac_gen = 0;
+  gct->evac_step = 0;
   markCAFs(mark_root);
 
   // follow all the roots that the application knows about.
-  gct->evac_gen = 0;
+  gct->evac_step = 0;
   GetRoots(mark_root);
 
   // Mark the weak pointer list, and prepare to detect dead weak pointers.
@@ -1287,7 +1287,7 @@ init_uncollected_gen (nat g, nat threads)
 static void
 init_gc_thread (gc_thread *t)
 {
-    t->evac_gen = 0;
+    t->evac_step = 0;
     t->failed_to_evac = rtsFalse;
     t->eager_promotion = rtsTrue;
     t->thunk_selector_depth = 0;
diff --git a/rts/sm/GC.h b/rts/sm/GC.h
index 013a901329..d45efb96e4 100644
--- a/rts/sm/GC.h
+++ b/rts/sm/GC.h
@@ -128,7 +128,7 @@ typedef struct gc_thread_ {
     // --------------------
     // evacuate flags
 
-    nat evac_gen;                  // Youngest generation that objects
+    step *evac_step;               // Youngest generation that objects
                                    // should be evacuated to in
                                    // evacuate().  (Logically an
                                    // argument to evacuate, but it's
diff --git a/rts/sm/MarkWeak.c b/rts/sm/MarkWeak.c
index 62ac8e0397..eca5c54b13 100644
--- a/rts/sm/MarkWeak.c
+++ b/rts/sm/MarkWeak.c
@@ -109,7 +109,7 @@ traverseWeakPtrList(void)
       /* doesn't matter where we evacuate values/finalizers to, since
        * these pointers are treated as roots (iff the keys are alive).
        */
-      gct->evac_gen = 0;
+      gct->evac_step = 0;
       
       last_w = &old_weak_ptr_list;
       for (w = old_weak_ptr_list; w != NULL; w = next_w) {
diff --git a/rts/sm/Scav.c b/rts/sm/Scav.c
index 080c75014e..71c2be7550 100644
--- a/rts/sm/Scav.c
+++ b/rts/sm/Scav.c
@@ -254,11 +254,11 @@ scavenge_AP (StgAP *ap)
 /* -----------------------------------------------------------------------------
    Scavenge a block from the given scan pointer up to bd->free.
 
-   evac_gen is set by the caller to be either zero (for a step in a
+   evac_step is set by the caller to be either zero (for a step in a
    generation < N) or G where G is the generation of the step being
    scavenged.  
 
-   We sometimes temporarily change evac_gen back to zero if we're
+   We sometimes temporarily change evac_step back to zero if we're
    scavenging a mutable object where eager promotion isn't such a good
    idea.  
    -------------------------------------------------------------------------- */
@@ -268,15 +268,15 @@ scavenge_block (bdescr *bd, StgPtr scan)
 {
   StgPtr p, q;
   StgInfoTable *info;
-  nat saved_evac_gen;
+  step *saved_evac_step;
 
   p = scan;
   
   debugTrace(DEBUG_gc, "scavenging block %p (gen %d, step %d) @ %p",
 	     bd->start, bd->gen_no, bd->step->no, scan);
 
-  gct->evac_gen = bd->gen_no;
-  saved_evac_gen = gct->evac_gen;
+  gct->evac_step = bd->step;
+  saved_evac_step = gct->evac_step;
   gct->failed_to_evac = rtsFalse;
 
   // we might be evacuating into the very object that we're
@@ -573,11 +573,11 @@ scavenge_block (bdescr *bd, StgPtr scan)
     case TVAR_WATCH_QUEUE:
       {
 	StgTVarWatchQueue *wq = ((StgTVarWatchQueue *) p);
-	gct->evac_gen = 0;
+	gct->evac_step = 0;
 	evacuate((StgClosure **)&wq->closure);
 	evacuate((StgClosure **)&wq->next_queue_entry);
 	evacuate((StgClosure **)&wq->prev_queue_entry);
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
 	p += sizeofW(StgTVarWatchQueue);
 	break;
@@ -586,10 +586,10 @@ scavenge_block (bdescr *bd, StgPtr scan)
     case TVAR:
       {
 	StgTVar *tvar = ((StgTVar *) p);
-	gct->evac_gen = 0;
+	gct->evac_step = 0;
 	evacuate((StgClosure **)&tvar->current_value);
 	evacuate((StgClosure **)&tvar->first_watch_queue_entry);
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
 	p += sizeofW(StgTVar);
 	break;
@@ -598,11 +598,11 @@ scavenge_block (bdescr *bd, StgPtr scan)
     case TREC_HEADER:
       {
         StgTRecHeader *trec = ((StgTRecHeader *) p);
-        gct->evac_gen = 0;
+        gct->evac_step = 0;
 	evacuate((StgClosure **)&trec->enclosing_trec);
 	evacuate((StgClosure **)&trec->current_chunk);
 	evacuate((StgClosure **)&trec->invariants_to_check);
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
 	p += sizeofW(StgTRecHeader);
         break;
@@ -613,14 +613,14 @@ scavenge_block (bdescr *bd, StgPtr scan)
 	StgWord i;
 	StgTRecChunk *tc = ((StgTRecChunk *) p);
 	TRecEntry *e = &(tc -> entries[0]);
-	gct->evac_gen = 0;
+	gct->evac_step = 0;
 	evacuate((StgClosure **)&tc->prev_chunk);
 	for (i = 0; i < tc -> next_entry_idx; i ++, e++ ) {
 	  evacuate((StgClosure **)&e->tvar);
 	  evacuate((StgClosure **)&e->expected_value);
 	  evacuate((StgClosure **)&e->new_value);
 	}
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
 	p += sizeofW(StgTRecChunk);
 	break;
@@ -629,10 +629,10 @@ scavenge_block (bdescr *bd, StgPtr scan)
     case ATOMIC_INVARIANT:
       {
         StgAtomicInvariant *invariant = ((StgAtomicInvariant *) p);
-        gct->evac_gen = 0;
+        gct->evac_step = 0;
 	evacuate(&invariant->code);
 	evacuate((StgClosure **)&invariant->last_execution);
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
 	p += sizeofW(StgAtomicInvariant);
         break;
@@ -641,11 +641,11 @@ scavenge_block (bdescr *bd, StgPtr scan)
     case INVARIANT_CHECK_QUEUE:
       {
         StgInvariantCheckQueue *queue = ((StgInvariantCheckQueue *) p);
-        gct->evac_gen = 0;
+        gct->evac_step = 0;
 	evacuate((StgClosure **)&queue->invariant);
 	evacuate((StgClosure **)&queue->my_execution);
 	evacuate((StgClosure **)&queue->next_queue_entry);
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
 	p += sizeofW(StgInvariantCheckQueue);
         break;
@@ -687,10 +687,10 @@ scavenge_mark_stack(void)
 {
     StgPtr p, q;
     StgInfoTable *info;
-    nat saved_evac_gen;
+    step *saved_evac_step;
 
-    gct->evac_gen = oldest_gen->no;
-    saved_evac_gen = gct->evac_gen;
+    gct->evac_step = &oldest_gen->steps[0];
+    saved_evac_step = gct->evac_step;
 
 linear_scan:
     while (!mark_stack_empty()) {
@@ -939,11 +939,11 @@ linear_scan:
 	case TVAR_WATCH_QUEUE:
 	  {
 	    StgTVarWatchQueue *wq = ((StgTVarWatchQueue *) p);
-	    gct->evac_gen = 0;
+	    gct->evac_step = 0;
             evacuate((StgClosure **)&wq->closure);
 	    evacuate((StgClosure **)&wq->next_queue_entry);
 	    evacuate((StgClosure **)&wq->prev_queue_entry);
-	    gct->evac_gen = saved_evac_gen;
+	    gct->evac_step = saved_evac_step;
 	    gct->failed_to_evac = rtsTrue; // mutable
 	    break;
 	  }
@@ -951,10 +951,10 @@ linear_scan:
 	case TVAR:
 	  {
 	    StgTVar *tvar = ((StgTVar *) p);
-	    gct->evac_gen = 0;
+	    gct->evac_step = 0;
 	    evacuate((StgClosure **)&tvar->current_value);
 	    evacuate((StgClosure **)&tvar->first_watch_queue_entry);
-	    gct->evac_gen = saved_evac_gen;
+	    gct->evac_step = saved_evac_step;
 	    gct->failed_to_evac = rtsTrue; // mutable
 	    break;
 	  }
@@ -964,14 +964,14 @@ linear_scan:
 	    StgWord i;
 	    StgTRecChunk *tc = ((StgTRecChunk *) p);
 	    TRecEntry *e = &(tc -> entries[0]);
-	    gct->evac_gen = 0;
+	    gct->evac_step = 0;
 	    evacuate((StgClosure **)&tc->prev_chunk);
 	    for (i = 0; i < tc -> next_entry_idx; i ++, e++ ) {
 	      evacuate((StgClosure **)&e->tvar);
 	      evacuate((StgClosure **)&e->expected_value);
 	      evacuate((StgClosure **)&e->new_value);
 	    }
-	    gct->evac_gen = saved_evac_gen;
+	    gct->evac_step = saved_evac_step;
 	    gct->failed_to_evac = rtsTrue; // mutable
 	    break;
 	  }
@@ -979,11 +979,11 @@ linear_scan:
 	case TREC_HEADER:
 	  {
 	    StgTRecHeader *trec = ((StgTRecHeader *) p);
-	    gct->evac_gen = 0;
+	    gct->evac_step = 0;
 	    evacuate((StgClosure **)&trec->enclosing_trec);
 	    evacuate((StgClosure **)&trec->current_chunk);
   	    evacuate((StgClosure **)&trec->invariants_to_check);
-	    gct->evac_gen = saved_evac_gen;
+	    gct->evac_step = saved_evac_step;
 	    gct->failed_to_evac = rtsTrue; // mutable
 	    break;
 	  }
@@ -991,10 +991,10 @@ linear_scan:
         case ATOMIC_INVARIANT:
           {
             StgAtomicInvariant *invariant = ((StgAtomicInvariant *) p);
-            gct->evac_gen = 0;
+            gct->evac_step = 0;
 	    evacuate(&invariant->code);
     	    evacuate((StgClosure **)&invariant->last_execution);
-	    gct->evac_gen = saved_evac_gen;
+	    gct->evac_step = saved_evac_step;
 	    gct->failed_to_evac = rtsTrue; // mutable
             break;
           }
@@ -1002,11 +1002,11 @@ linear_scan:
         case INVARIANT_CHECK_QUEUE:
           {
             StgInvariantCheckQueue *queue = ((StgInvariantCheckQueue *) p);
-            gct->evac_gen = 0;
+            gct->evac_step = 0;
     	    evacuate((StgClosure **)&queue->invariant);
 	    evacuate((StgClosure **)&queue->my_execution);
             evacuate((StgClosure **)&queue->next_queue_entry);
-	    gct->evac_gen = saved_evac_gen;
+	    gct->evac_step = saved_evac_step;
 	    gct->failed_to_evac = rtsTrue; // mutable
             break;
           }
@@ -1018,8 +1018,8 @@ linear_scan:
 
 	if (gct->failed_to_evac) {
 	    gct->failed_to_evac = rtsFalse;
-	    if (gct->evac_gen > 0) {
-		recordMutableGen_GC((StgClosure *)q, &generations[gct->evac_gen]);
+	    if (gct->evac_step) {
+		recordMutableGen_GC((StgClosure *)q, gct->evac_step->gen);
 	    }
 	}
 	
@@ -1079,7 +1079,7 @@ static rtsBool
 scavenge_one(StgPtr p)
 {
     const StgInfoTable *info;
-    nat saved_evac_gen = gct->evac_gen;
+    step *saved_evac_step = gct->evac_step;
     rtsBool no_luck;
     
     ASSERT(LOOKS_LIKE_CLOSURE_PTR(p));
@@ -1271,11 +1271,11 @@ scavenge_one(StgPtr p)
     case TVAR_WATCH_QUEUE:
       {
 	StgTVarWatchQueue *wq = ((StgTVarWatchQueue *) p);
-	gct->evac_gen = 0;
+	gct->evac_step = 0;
         evacuate((StgClosure **)&wq->closure);
         evacuate((StgClosure **)&wq->next_queue_entry);
         evacuate((StgClosure **)&wq->prev_queue_entry);
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
 	break;
       }
@@ -1283,10 +1283,10 @@ scavenge_one(StgPtr p)
     case TVAR:
       {
 	StgTVar *tvar = ((StgTVar *) p);
-	gct->evac_gen = 0;
+	gct->evac_step = 0;
 	evacuate((StgClosure **)&tvar->current_value);
         evacuate((StgClosure **)&tvar->first_watch_queue_entry);
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
 	break;
       }
@@ -1294,11 +1294,11 @@ scavenge_one(StgPtr p)
     case TREC_HEADER:
       {
         StgTRecHeader *trec = ((StgTRecHeader *) p);
-        gct->evac_gen = 0;
+        gct->evac_step = 0;
 	evacuate((StgClosure **)&trec->enclosing_trec);
 	evacuate((StgClosure **)&trec->current_chunk);
         evacuate((StgClosure **)&trec->invariants_to_check);
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
         break;
       }
@@ -1308,14 +1308,14 @@ scavenge_one(StgPtr p)
 	StgWord i;
 	StgTRecChunk *tc = ((StgTRecChunk *) p);
 	TRecEntry *e = &(tc -> entries[0]);
-	gct->evac_gen = 0;
+	gct->evac_step = 0;
 	evacuate((StgClosure **)&tc->prev_chunk);
 	for (i = 0; i < tc -> next_entry_idx; i ++, e++ ) {
 	  evacuate((StgClosure **)&e->tvar);
 	  evacuate((StgClosure **)&e->expected_value);
 	  evacuate((StgClosure **)&e->new_value);
 	}
-	gct->evac_gen = saved_evac_gen;
+	gct->evac_step = saved_evac_step;
 	gct->failed_to_evac = rtsTrue; // mutable
 	break;
       }
@@ -1323,10 +1323,10 @@ scavenge_one(StgPtr p)
     case ATOMIC_INVARIANT:
     {
       StgAtomicInvariant *invariant = ((StgAtomicInvariant *) p);
-      gct->evac_gen = 0;
+      gct->evac_step = 0;
       evacuate(&invariant->code);
       evacuate((StgClosure **)&invariant->last_execution);
-      gct->evac_gen = saved_evac_gen;
+      gct->evac_step = saved_evac_step;
       gct->failed_to_evac = rtsTrue; // mutable
       break;
     }
@@ -1334,11 +1334,11 @@ scavenge_one(StgPtr p)
     case INVARIANT_CHECK_QUEUE:
     {
       StgInvariantCheckQueue *queue = ((StgInvariantCheckQueue *) p);
-      gct->evac_gen = 0;
+      gct->evac_step = 0;
       evacuate((StgClosure **)&queue->invariant);
       evacuate((StgClosure **)&queue->my_execution);
       evacuate((StgClosure **)&queue->next_queue_entry);
-      gct->evac_gen = saved_evac_gen;
+      gct->evac_step = saved_evac_step;
       gct->failed_to_evac = rtsTrue; // mutable
       break;
     }
@@ -1413,7 +1413,7 @@ scavenge_mutable_list(generation *gen)
 
     bd = gen->saved_mut_list;
 
-    gct->evac_gen = gen->no;
+    gct->evac_step = &gen->steps[0];
     for (; bd != NULL; bd = bd->link) {
 	for (q = bd->start; q < bd->free; q++) {
 	    p = (StgPtr)*q;
@@ -1497,7 +1497,7 @@ scavenge_static(void)
 
   /* Always evacuate straight to the oldest generation for static
    * objects */
-  gct->evac_gen = oldest_gen->no;
+  gct->evac_step = &oldest_gen->steps[0];
 
   /* keep going until we've scavenged all the objects on the linked
      list... */
@@ -1774,9 +1774,9 @@ scavenge_stack(StgPtr p, StgPtr stack_end)
 /*-----------------------------------------------------------------------------
   scavenge the large object list.
 
-  evac_gen set by caller; similar games played with evac_gen as with
+  evac_step set by caller; similar games played with evac_step as with
   scavenge() - see comment at the top of scavenge().  Most large
-  objects are (repeatedly) mutable, so most of the time evac_gen will
+  objects are (repeatedly) mutable, so most of the time evac_step will
   be zero.
   --------------------------------------------------------------------------- */
 
@@ -1786,7 +1786,7 @@ scavenge_large (step_workspace *ws)
     bdescr *bd;
     StgPtr p;
 
-    gct->evac_gen = ws->stp->gen_no;
+    gct->evac_step = ws->stp;
 
     bd = ws->todo_large_objects;
     
diff --git a/rts/sm/Storage.c b/rts/sm/Storage.c
index 9b86b432e1..c441826a14 100644
--- a/rts/sm/Storage.c
+++ b/rts/sm/Storage.c
@@ -103,6 +103,7 @@ initStorage( void )
 {
   nat g, s;
   generation *gen;
+  step *step_arr;
 
   if (generations != NULL) {
       // multi-init protection
@@ -146,6 +147,15 @@ initStorage( void )
 					     * sizeof(struct generation_),
 					     "initStorage: gens");
 
+  /* allocate all the steps into an array.  It is important that we do
+     it this way, because we need the invariant that two step pointers
+     can be directly compared to see which is the oldest.
+     Remember that the last generation has only one step. */
+  step_arr = stgMallocBytes(sizeof(struct step_) 
+			    * (1 + ((RtsFlags.GcFlags.generations - 1)
+				    * RtsFlags.GcFlags.steps)),
+			    "initStorage: steps");
+
   /* Initialise all generations */
   for(g = 0; g < RtsFlags.GcFlags.generations; g++) {
     gen = &generations[g];
@@ -166,21 +176,19 @@ initStorage( void )
 
     /* Oldest generation: one step */
     oldest_gen->n_steps = 1;
-    oldest_gen->steps = 
-      stgMallocBytes(1 * sizeof(struct step_), "initStorage: last step");
+    oldest_gen->steps   = step_arr + (RtsFlags.GcFlags.generations - 1)
+	                              * RtsFlags.GcFlags.steps;
 
     /* set up all except the oldest generation with 2 steps */
     for(g = 0; g < RtsFlags.GcFlags.generations-1; g++) {
       generations[g].n_steps = RtsFlags.GcFlags.steps;
-      generations[g].steps  = 
-	stgMallocBytes (RtsFlags.GcFlags.steps * sizeof(struct step_),
-			"initStorage: steps");
+      generations[g].steps   = step_arr + g * RtsFlags.GcFlags.steps;
     }
     
   } else {
     /* single generation, i.e. a two-space collector */
     g0->n_steps = 1;
-    g0->steps = stgMallocBytes (sizeof(struct step_), "initStorage: steps");
+    g0->steps   = step_arr;
   }
 
 #ifdef THREADED_RTS
@@ -264,10 +272,7 @@ exitStorage (void)
 void
 freeStorage (void)
 {
-    nat g;
-
-    for(g = 0; g < RtsFlags.GcFlags.generations; g++)
-      stgFree(generations[g].steps);
+    stgFree(g0s0); // frees all the steps
     stgFree(generations);
     freeAllMBlocks();
 #if defined(THREADED_RTS)