A small GC optimisation

Store the *number* of the destination generation in the Bdescr struct,
so that in evacuate() we don't have to deref gen to get it.
This is another improvement ported over from my GC branch.

8 files changed, 105 insertions, 81 deletions

diff --git a/includes/rts/storage/Block.h b/includes/rts/storage/Block.h
index f39b99c0dc..c73c9af90a 100644
--- a/includes/rts/storage/Block.h
+++ b/includes/rts/storage/Block.h
@@ -46,26 +46,45 @@
  * on a 32-bit machine.
  */
 
+// Note: fields marked with [READ ONLY] must not be modified by the
+// client of the block allocator API.  All other fields can be
+// freely modified.
+
 #ifndef CMINUSMINUS
 typedef struct bdescr_ {
-    StgPtr start;              /* start addr of memory */
-    StgPtr free;               /* first free byte of memory */
-    struct bdescr_ *link;      /* used for chaining blocks together */
+
+    StgPtr start;              // [READ ONLY] start addr of memory
+
+    StgPtr free;               // first free byte of memory.
+                               // NB. during use this value should lie
+                               // between start and start + blocks *
+                               // BLOCK_SIZE.  Values outside this
+                               // range are reserved for use by the
+                               // block allocator.  In particular, the
+                               // value (StgPtr)(-1) is used to
+                               // indicate that a block is unallocated.
+
+    struct bdescr_ *link;      // used for chaining blocks together
+
     union {
-        struct bdescr_ *back;  /* used (occasionally) for doubly-linked lists*/
-        StgWord *bitmap;
-        StgPtr  scan;           /* scan pointer for copying GC */
+        struct bdescr_ *back;  // used (occasionally) for doubly-linked lists
+        StgWord *bitmap;       // bitmap for marking GC
+        StgPtr  scan;          // scan pointer for copying GC
     } u;
 
-    struct generation_ *gen;   /* generation */
-    struct generation_ *dest;  /* destination gen */
+    struct generation_ *gen;   // generation
+
+    StgWord16 gen_no;          // gen->no, cached
+    StgWord16 dest_no;         // number of destination generation
+    StgWord16 _pad1;
+
+    StgWord16 flags;           // block flags, see below
 
-    StgWord32 blocks;		/* no. of blocks (if grp head, 0 otherwise) */
+    StgWord32 blocks;          // [READ ONLY] no. of blocks in a group
+                               // (if group head, 0 otherwise)
 
-    StgWord16 gen_no;
-    StgWord16 flags;            /* block flags, see below */
 #if SIZEOF_VOID_P == 8
-    StgWord32 _padding[2];
+    StgWord32 _padding[3];
 #else
     StgWord32 _padding[0];
 #endif
diff --git a/includes/rts/storage/GC.h b/includes/rts/storage/GC.h
index c412c96617..7cee670303 100644
--- a/includes/rts/storage/GC.h
+++ b/includes/rts/storage/GC.h
@@ -194,9 +194,9 @@ extern rtsBool keepCAFs;
 
 INLINE_HEADER void initBdescr(bdescr *bd, generation *gen, generation *dest)
 {
-    bd->gen    = gen;
-    bd->gen_no = gen->no;
-    bd->dest   = dest;
+    bd->gen     = gen;
+    bd->gen_no  = gen->no;
+    bd->dest_no = dest->no;
 }
 
 #endif /* RTS_STORAGE_GC_H */
diff --git a/rts/Arena.c b/rts/Arena.c
index 6d7a65b0f3..653eb69706 100644
--- a/rts/Arena.c
+++ b/rts/Arena.c
@@ -86,7 +86,7 @@ arenaAlloc( Arena *arena, size_t size )
 
 	bd->gen_no  = 0;
 	bd->gen     = NULL;
-	bd->dest    = NULL;
+        bd->dest_no = 0;
 	bd->flags   = 0;
 	bd->free    = bd->start;
 	bd->link    = arena->current;
diff --git a/rts/sm/Evac.c b/rts/sm/Evac.c
index 9a5f27887e..18ace21b9e 100644
--- a/rts/sm/Evac.c
+++ b/rts/sm/Evac.c
@@ -51,7 +51,7 @@ STATIC_INLINE void evacuate_large(StgPtr p);
    -------------------------------------------------------------------------- */
 
 STATIC_INLINE StgPtr
-alloc_for_copy (nat size, generation *gen)
+alloc_for_copy (nat size, nat gen_no)
 {
     StgPtr to;
     gen_workspace *ws;
@@ -61,17 +61,16 @@ alloc_for_copy (nat size, generation *gen)
      * evacuate to an older generation, adjust it here (see comment
      * by evacuate()).
      */
-    if (gen < gct->evac_gen) {
+    if (gen_no < gct->evac_gen_no) {
 	if (gct->eager_promotion) {
-	    gen = gct->evac_gen;
+            gen_no = gct->evac_gen_no;
 	} else {
 	    gct->failed_to_evac = rtsTrue;
 	}
     }
     
-    ws = &gct->gens[gen->no];
-    // this compiles to a single mem access to gen->abs_no only
-    
+    ws = &gct->gens[gen_no];  // zero memory references here
+
     /* chain a new block onto the to-space for the destination gen if
      * necessary.
      */
@@ -91,12 +90,12 @@ alloc_for_copy (nat size, generation *gen)
 
 STATIC_INLINE GNUC_ATTR_HOT void
 copy_tag(StgClosure **p, const StgInfoTable *info, 
-         StgClosure *src, nat size, generation *gen, StgWord tag)
+         StgClosure *src, nat size, nat gen_no, StgWord tag)
 {
     StgPtr to, from;
     nat i;
 
-    to = alloc_for_copy(size,gen);
+    to = alloc_for_copy(size,gen_no);
     
     from = (StgPtr)src;
     to[0] = (W_)info;
@@ -133,12 +132,12 @@ copy_tag(StgClosure **p, const StgInfoTable *info,
 #if defined(PARALLEL_GC)
 STATIC_INLINE void
 copy_tag_nolock(StgClosure **p, const StgInfoTable *info, 
-         StgClosure *src, nat size, generation *gen, StgWord tag)
+         StgClosure *src, nat size, nat gen_no, StgWord tag)
 {
     StgPtr to, from;
     nat i;
 
-    to = alloc_for_copy(size,gen);
+    to = alloc_for_copy(size,gen_no);
 
     from = (StgPtr)src;
     to[0] = (W_)info;
@@ -170,7 +169,7 @@ copy_tag_nolock(StgClosure **p, const StgInfoTable *info,
  */
 static rtsBool
 copyPart(StgClosure **p, StgClosure *src, nat size_to_reserve, 
-         nat size_to_copy, generation *gen)
+         nat size_to_copy, nat gen_no)
 {
     StgPtr to, from;
     nat i;
@@ -194,7 +193,7 @@ spin:
     info = (W_)src->header.info;
 #endif
 
-    to = alloc_for_copy(size_to_reserve, gen);
+    to = alloc_for_copy(size_to_reserve, gen_no);
 
     from = (StgPtr)src;
     to[0] = info;
@@ -222,9 +221,9 @@ spin:
 /* Copy wrappers that don't tag the closure after copying */
 STATIC_INLINE GNUC_ATTR_HOT void
 copy(StgClosure **p, const StgInfoTable *info, 
-     StgClosure *src, nat size, generation *gen)
+     StgClosure *src, nat size, nat gen_no)
 {
-    copy_tag(p,info,src,size,gen,0);
+    copy_tag(p,info,src,size,gen_no,0);
 }
 
 /* -----------------------------------------------------------------------------
@@ -243,10 +242,12 @@ evacuate_large(StgPtr p)
 {
   bdescr *bd;
   generation *gen, *new_gen;
+  nat gen_no, new_gen_no;
   gen_workspace *ws;
     
   bd = Bdescr(p);
   gen = bd->gen;
+  gen_no = bd->gen_no;
   ACQUIRE_SPIN_LOCK(&gen->sync_large_objects);
 
   // already evacuated? 
@@ -254,7 +255,7 @@ 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 (gen < gct->evac_gen) {
+    if (gen_no < gct->evac_gen_no) {
 	gct->failed_to_evac = rtsTrue;
 	TICK_GC_FAILED_PROMOTION();
     }
@@ -274,16 +275,18 @@ evacuate_large(StgPtr p)
   
   /* link it on to the evacuated large object list of the destination gen
    */
-  new_gen = bd->dest;
-  if (new_gen < gct->evac_gen) {
+  new_gen_no = bd->dest_no;
+
+  if (new_gen_no < gct->evac_gen_no) {
       if (gct->eager_promotion) {
-	  new_gen = gct->evac_gen;
+          new_gen_no = gct->evac_gen_no;
       } else {
 	  gct->failed_to_evac = rtsTrue;
       }
   }
 
-  ws = &gct->gens[new_gen->no];
+  ws = &gct->gens[new_gen_no];
+  new_gen = &generations[new_gen_no];
 
   bd->flags |= BF_EVACUATED;
   initBdescr(bd, new_gen, new_gen->to);
@@ -352,7 +355,7 @@ REGPARM1 GNUC_ATTR_HOT void
 evacuate(StgClosure **p)
 {
   bdescr *bd = NULL;
-  generation *gen;
+  nat gen_no;
   StgClosure *q;
   const StgInfoTable *info;
   StgWord tag;
@@ -475,7 +478,7 @@ loop:
           // We aren't copying this object, so we have to check
           // whether it is already in the target generation.  (this is
           // the write barrier).
-	  if (bd->gen < gct->evac_gen) {
+          if (bd->gen_no < gct->evac_gen_no) {
 	      gct->failed_to_evac = rtsTrue;
 	      TICK_GC_FAILED_PROMOTION();
 	  }
@@ -499,7 +502,7 @@ loop:
       return;
   }
       
-  gen = bd->dest;
+  gen_no = bd->dest_no;
 
   info = q->header.info;
   if (IS_FORWARDING_PTR(info))
@@ -522,8 +525,8 @@ loop:
      */
       StgClosure *e = (StgClosure*)UN_FORWARDING_PTR(info);
       *p = TAG_CLOSURE(tag,e);
-      if (gen < gct->evac_gen) {  // optimisation 
-	  if (Bdescr((P_)e)->gen < gct->evac_gen) {
+      if (gen_no < gct->evac_gen_no) {  // optimisation
+          if (Bdescr((P_)e)->gen_no < gct->evac_gen_no) {
 	      gct->failed_to_evac = rtsTrue;
 	      TICK_GC_FAILED_PROMOTION();
 	  }
@@ -540,7 +543,7 @@ loop:
   case MUT_VAR_DIRTY:
   case MVAR_CLEAN:
   case MVAR_DIRTY:
-      copy(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen);
+      copy(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen_no);
       return;
 
   // For ints and chars of low value, save space by replacing references to
@@ -552,7 +555,7 @@ loop:
   case CONSTR_0_1:
   {   
 #if defined(__PIC__) && defined(mingw32_HOST_OS) 
-      copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen,tag);
+      copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen_no,tag);
 #else
       StgWord w = (StgWord)q->payload[0];
       if (info == Czh_con_info &&
@@ -569,7 +572,7 @@ loop:
 			     );
       }
       else {
-          copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen,tag);
+          copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen_no,tag);
       }
 #endif
       return;
@@ -578,12 +581,12 @@ loop:
   case FUN_0_1:
   case FUN_1_0:
   case CONSTR_1_0:
-      copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen,tag);
+      copy_tag_nolock(p,info,q,sizeofW(StgHeader)+1,gen_no,tag);
       return;
 
   case THUNK_1_0:
   case THUNK_0_1:
-      copy(p,info,q,sizeofW(StgThunk)+1,gen);
+      copy(p,info,q,sizeofW(StgThunk)+1,gen_no);
       return;
 
   case THUNK_1_1:
@@ -592,7 +595,7 @@ loop:
 #ifdef NO_PROMOTE_THUNKS
 #error bitrotted
 #endif
-    copy(p,info,q,sizeofW(StgThunk)+2,gen);
+    copy(p,info,q,sizeofW(StgThunk)+2,gen_no);
     return;
 
   case FUN_1_1:
@@ -600,21 +603,21 @@ loop:
   case FUN_0_2:
   case CONSTR_1_1:
   case CONSTR_2_0:
-      copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,gen,tag);
+      copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,gen_no,tag);
       return;
 
   case CONSTR_0_2:
-      copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,gen,tag);
+      copy_tag_nolock(p,info,q,sizeofW(StgHeader)+2,gen_no,tag);
       return;
 
   case THUNK:
-      copy(p,info,q,thunk_sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen);
+      copy(p,info,q,thunk_sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen_no);
       return;
 
   case FUN:
   case IND_PERM:
   case CONSTR:
-      copy_tag_nolock(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen,tag);
+      copy_tag_nolock(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen_no,tag);
       return;
 
   case BLACKHOLE:
@@ -632,7 +635,7 @@ loop:
               || i == &stg_WHITEHOLE_info 
               || i == &stg_BLOCKING_QUEUE_CLEAN_info
               || i == &stg_BLOCKING_QUEUE_DIRTY_info) {
-              copy(p,info,q,sizeofW(StgInd),gen);
+              copy(p,info,q,sizeofW(StgInd),gen_no);
               return;
           }
           ASSERT(i != &stg_IND_info);
@@ -646,11 +649,11 @@ loop:
   case WEAK:
   case PRIM:
   case MUT_PRIM:
-      copy(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen);
+      copy(p,info,q,sizeW_fromITBL(INFO_PTR_TO_STRUCT(info)),gen_no);
       return;
 
   case BCO:
-      copy(p,info,q,bco_sizeW((StgBCO *)q),gen);
+      copy(p,info,q,bco_sizeW((StgBCO *)q),gen_no);
       return;
 
   case THUNK_SELECTOR:
@@ -678,20 +681,20 @@ loop:
     barf("evacuate: stack frame at %p\n", q);
 
   case PAP:
-      copy(p,info,q,pap_sizeW((StgPAP*)q),gen);
+      copy(p,info,q,pap_sizeW((StgPAP*)q),gen_no);
       return;
 
   case AP:
-      copy(p,info,q,ap_sizeW((StgAP*)q),gen);
+      copy(p,info,q,ap_sizeW((StgAP*)q),gen_no);
       return;
 
   case AP_STACK:
-      copy(p,info,q,ap_stack_sizeW((StgAP_STACK*)q),gen);
+      copy(p,info,q,ap_stack_sizeW((StgAP_STACK*)q),gen_no);
       return;
 
   case ARR_WORDS:
       // just copy the block 
-      copy(p,info,q,arr_words_sizeW((StgArrWords *)q),gen);
+      copy(p,info,q,arr_words_sizeW((StgArrWords *)q),gen_no);
       return;
 
   case MUT_ARR_PTRS_CLEAN:
@@ -699,11 +702,11 @@ loop:
   case MUT_ARR_PTRS_FROZEN:
   case MUT_ARR_PTRS_FROZEN0:
       // just copy the block 
-      copy(p,info,q,mut_arr_ptrs_sizeW((StgMutArrPtrs *)q),gen);
+      copy(p,info,q,mut_arr_ptrs_sizeW((StgMutArrPtrs *)q),gen_no);
       return;
 
   case TSO:
-      copy(p,info,q,sizeofW(StgTSO),gen);
+      copy(p,info,q,sizeofW(StgTSO),gen_no);
       return;
 
   case STACK:
@@ -718,7 +721,7 @@ loop:
           rtsBool mine;
 
           mine = copyPart(p,(StgClosure *)stack, stack_sizeW(stack),
-                          sizeofW(StgStack), gen);
+                          sizeofW(StgStack), gen_no);
           if (mine) {
               new_stack = (StgStack *)*p;
               move_STACK(stack, new_stack);
@@ -732,7 +735,7 @@ loop:
     }
 
   case TREC_CHUNK:
-      copy(p,info,q,sizeofW(StgTRecChunk),gen);
+      copy(p,info,q,sizeofW(StgTRecChunk),gen_no);
       return;
 
   default:
@@ -836,7 +839,7 @@ selector_chain:
             unchain_thunk_selectors(prev_thunk_selector, (StgClosure *)p);
             *q = (StgClosure *)p;
             // shortcut, behave as for:  if (evac) evacuate(q);
-            if (evac && bd->gen < gct->evac_gen) {
+            if (evac && bd->gen_no < gct->evac_gen_no) {
                 gct->failed_to_evac = rtsTrue;
                 TICK_GC_FAILED_PROMOTION();
             }
@@ -1075,7 +1078,7 @@ bale_out:
     // check whether it was updated in the meantime.
     *q = (StgClosure *)p;
     if (evac) {
-        copy(q,(const StgInfoTable *)info_ptr,(StgClosure *)p,THUNK_SELECTOR_sizeW(),bd->dest);
+        copy(q,(const StgInfoTable *)info_ptr,(StgClosure *)p,THUNK_SELECTOR_sizeW(),bd->dest_no);
     }
     unchain_thunk_selectors(prev_thunk_selector, *q);
     return;
diff --git a/rts/sm/GC.c b/rts/sm/GC.c
index 38ae1a7996..c9908359b8 100644
--- a/rts/sm/GC.c
+++ b/rts/sm/GC.c
@@ -344,11 +344,11 @@ SET_GCT(gc_threads[0]);
   }
 
   // follow roots from the CAF list (used by GHCi)
-  gct->evac_gen = 0;
+  gct->evac_gen_no = 0;
   markCAFs(mark_root, gct);
 
   // follow all the roots that the application knows about.
-  gct->evac_gen = 0;
+  gct->evac_gen_no = 0;
   markSomeCapabilities(mark_root, gct, gct->thread_index, n_gc_threads,
                        rtsTrue/*prune sparks*/);
 
@@ -1062,7 +1062,7 @@ gcWorkerThread (Capability *cap)
 #endif
     
     // Every thread evacuates some roots.
-    gct->evac_gen = 0;
+    gct->evac_gen_no = 0;
     markSomeCapabilities(mark_root, gct, gct->thread_index, n_gc_threads,
                          rtsTrue/*prune sparks*/);
     scavenge_capability_mut_lists(&capabilities[gct->thread_index]);
@@ -1399,7 +1399,7 @@ init_gc_thread (gc_thread *t)
     t->scavenged_static_objects = END_OF_STATIC_LIST;
     t->scan_bd = NULL;
     t->mut_lists = capabilities[t->thread_index].mut_lists;
-    t->evac_gen = 0;
+    t->evac_gen_no = 0;
     t->failed_to_evac = rtsFalse;
     t->eager_promotion = rtsTrue;
     t->thunk_selector_depth = 0;
diff --git a/rts/sm/GCThread.h b/rts/sm/GCThread.h
index c45eb1bdec..62dd1fb73a 100644
--- a/rts/sm/GCThread.h
+++ b/rts/sm/GCThread.h
@@ -146,7 +146,7 @@ typedef struct gc_thread_ {
     // --------------------
     // evacuate flags
 
-    generation *evac_gen;          // Youngest generation that objects
+    nat evac_gen_no;               // 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 72f0ade797..f4b576ac73 100644
--- a/rts/sm/MarkWeak.c
+++ b/rts/sm/MarkWeak.c
@@ -110,7 +110,7 @@ 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_gen_no = 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 d7e16eaf2a..9ac152af53 100644
--- a/rts/sm/Scav.c
+++ b/rts/sm/Scav.c
@@ -362,11 +362,11 @@ scavenge_fun_srt(const StgInfoTable *info)
 /* -----------------------------------------------------------------------------
    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_gen_no 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_gen_no back to zero if we're
    scavenging a mutable object where eager promotion isn't such a good
    idea.  
    -------------------------------------------------------------------------- */
@@ -383,7 +383,7 @@ scavenge_block (bdescr *bd)
 	     bd->start, bd->gen_no, bd->u.scan);
 
   gct->scan_bd = bd;
-  gct->evac_gen = bd->gen;
+  gct->evac_gen_no = bd->gen_no;
   saved_eager_promotion = gct->eager_promotion;
   gct->failed_to_evac = rtsFalse;
 
@@ -754,7 +754,7 @@ scavenge_mark_stack(void)
     StgInfoTable *info;
     rtsBool saved_eager_promotion;
 
-    gct->evac_gen = oldest_gen;
+    gct->evac_gen_no = oldest_gen->no;
     saved_eager_promotion = gct->eager_promotion;
 
     while ((p = pop_mark_stack())) {
@@ -1041,8 +1041,8 @@ scavenge_mark_stack(void)
 
 	if (gct->failed_to_evac) {
 	    gct->failed_to_evac = rtsFalse;
-	    if (gct->evac_gen) {
-		recordMutableGen_GC((StgClosure *)q, gct->evac_gen->no);
+            if (gct->evac_gen_no) {
+                recordMutableGen_GC((StgClosure *)q, gct->evac_gen_no);
 	    }
 	}
     } // while (p = pop_mark_stack())
@@ -1340,8 +1340,10 @@ void
 scavenge_mutable_list(bdescr *bd, generation *gen)
 {
     StgPtr p, q;
+    nat gen_no;
 
-    gct->evac_gen = gen;
+    gen_no = gen->no;
+    gct->evac_gen_no = gen_no;
     for (; bd != NULL; bd = bd->link) {
 	for (q = bd->start; q < bd->free; q++) {
 	    p = (StgPtr)*q;
@@ -1376,7 +1378,7 @@ scavenge_mutable_list(bdescr *bd, generation *gen)
 	    //
 	    switch (get_itbl((StgClosure *)p)->type) {
 	    case MUT_ARR_PTRS_CLEAN:
-		recordMutableGen_GC((StgClosure *)p,gen->no);
+                recordMutableGen_GC((StgClosure *)p,gen_no);
 		continue;
 	    case MUT_ARR_PTRS_DIRTY:
             {
@@ -1394,7 +1396,7 @@ scavenge_mutable_list(bdescr *bd, generation *gen)
 
                 gct->eager_promotion = saved_eager_promotion;
                 gct->failed_to_evac = rtsFalse;
-		recordMutableGen_GC((StgClosure *)p,gen->no);
+                recordMutableGen_GC((StgClosure *)p,gen_no);
 		continue;
             }
             default:
@@ -1404,7 +1406,7 @@ scavenge_mutable_list(bdescr *bd, generation *gen)
 	    if (scavenge_one(p)) {
 		// didn't manage to promote everything, so put the
 		// object back on the list.
-		recordMutableGen_GC((StgClosure *)p,gen->no);
+                recordMutableGen_GC((StgClosure *)p,gen_no);
 	    }
 	}
     }
@@ -1446,7 +1448,7 @@ scavenge_static(void)
 
   /* Always evacuate straight to the oldest generation for static
    * objects */
-  gct->evac_gen = oldest_gen;
+  gct->evac_gen_no = oldest_gen->no;
 
   /* keep going until we've scavenged all the objects on the linked
      list... */
@@ -1741,7 +1743,7 @@ scavenge_large (gen_workspace *ws)
     bdescr *bd;
     StgPtr p;
 
-    gct->evac_gen = ws->gen;
+    gct->evac_gen_no = ws->gen->no;
 
     bd = ws->todo_large_objects;