[project @ 2003-03-24 15:33:25 by simonmar]

A couple of changes related to bug #2 in the previous commit:

 - follow IND_STATICs, now that we can check whether
   we end up in to-space or not

 - improve the commentary

diff --git a/ghc/rts/GC.c b/ghc/rts/GC.c
index 2b30d67..a0f119f 100644 (file)
--- a/ghc/rts/GC.c
+++ b/ghc/rts/GC.c
@@ -1,5 +1,5 @@
 /* -----------------------------------------------------------------------------
- * $Id: GC.c,v 1.149 2003/03/24 14:46:53 simonmar Exp $
+ * $Id: GC.c,v 1.150 2003/03/24 15:33:25 simonmar Exp $
  *
  * (c) The GHC Team 1998-2003
  *
@@ -2002,9 +2002,22 @@ eval_thunk_selector( nat field, StgSelector * p )
 
 selector_loop:
 
+    // We don't want to end up in to-space, because this causes
+    // problems when the GC later tries to evacuate the result of
+    // eval_thunk_selector().  There are various ways this could
+    // happen:
+    //
+    // - following an IND_STATIC
+    //
+    // - when the old generation is compacted, the mark phase updates
+    //   from-space pointers to be to-space pointers, and we can't
+    //   reliably tell which we're following (eg. from an IND_STATIC).
+    // 
+    // So we use the block-descriptor test to find out if we're in
+    // to-space.
+    //
     if (Bdescr((StgPtr)selectee)->flags & BF_EVACUATED) {
-       SET_INFO(p, info_ptr);
-       return NULL;
+       goto bale_out;
     }
 
     info = get_itbl(selectee);
@@ -2021,12 +2034,15 @@ selector_loop:
          ASSERT(field <  (StgWord32)(info->layout.payload.ptrs + 
                                      info->layout.payload.nptrs));
          
+         // ToDo: shouldn't we test whether this pointer is in
+         // to-space?
          return selectee->payload[field];
 
       case IND:
       case IND_PERM:
       case IND_OLDGEN:
       case IND_OLDGEN_PERM:
+      case IND_STATIC:
          selectee = ((StgInd *)selectee)->indirectee;
          goto selector_loop;
 
@@ -2036,11 +2052,6 @@ selector_loop:
          // leaks by evaluating this selector thunk anyhow.
          break;
 
-      case IND_STATIC:
-         // We can't easily tell whether the indirectee is into 
-         // from or to-space, so just bail out here.
-         break;
-
       case THUNK_SELECTOR:
       {
          StgClosure *val;
@@ -2113,6 +2124,7 @@ selector_loop:
             (int)(info->type));
     }
 
+bale_out:
     // We didn't manage to evaluate this thunk; restore the old info pointer
     SET_INFO(p, info_ptr);
     return NULL;