nonmoving: Fix handling on large object marking on 32-bit

Previously we would reset the pointer pointing to the object to be
marked to the beginning of the block when marking a large object. This
did no harm on 64-bit but on 32-bit it broke, e.g. `arr020`, since we
align pinned ByteArray allocations such that the payload is 8
byte-aligned. This means that the object might not begin at the
beginning of the block.,

1 files changed, 7 insertions, 4 deletions

diff --git a/rts/sm/NonMovingMark.c b/rts/sm/NonMovingMark.c
index 638be30c20..085d7827df 100644
--- a/rts/sm/NonMovingMark.c
+++ b/rts/sm/NonMovingMark.c
@@ -62,7 +62,13 @@ static void mark_PAP_payload (MarkQueue *queue,
  * collector and moved to nonmoving_large_objects during the next major GC.
  * When this happens the block gets its BF_NONMOVING_SWEEPING flag set to
  * indicate that it is part of the snapshot and consequently should be marked by
- * the nonmoving mark phase..
+ * the nonmoving mark phase.
+ *
+ * Note that pinned object blocks are treated as large objects containing only
+ * a single object. That is, the block has a single mark flag (BF_MARKED) and we
+ * consequently will trace the pointers of only one object per block. However,
+ * this is okay since the only type of pinned object supported by GHC is the
+ * pinned ByteArray#, which has no pointers.
  */
 
 bdescr *nonmoving_large_objects = NULL;
@@ -1281,9 +1287,6 @@ mark_closure (MarkQueue *queue, const StgClosure *p0, StgClosure **origin)
             if (bd->flags & BF_MARKED) {
                 goto done;
             }
-
-            // Mark contents
-            p = (StgClosure*)bd->start;
         } else {
             struct NonmovingSegment *seg = nonmovingGetSegment((StgPtr) p);
             nonmoving_block_idx block_idx = nonmovingGetBlockIdx((StgPtr) p);