rts: Expand and add more notes regarding slop

1 files changed, 15 insertions, 2 deletions

diff --git a/rts/ProfHeap.c b/rts/ProfHeap.c
index 6c26de1699..26f4de468d 100644
--- a/rts/ProfHeap.c
+++ b/rts/ProfHeap.c
@@ -1275,8 +1275,21 @@ heapCensusChain( Census *census, bdescr *bd )
             heapProfObject(census,(StgClosure*)p,size,prim);
 
             p += size;
-            /* skip over slop */
-            while (p < bd->free && !*p) p++; // skip slop
+
+            /* skip over slop, see Note [slop on the heap] */
+            while (p < bd->free && !*p) p++;
+            /* Note [skipping slop in the heap profiler]
+             *
+             * We make sure to zero slop that can remain after a major GC so
+             * here we can assume any slop words we see until the block's free
+             * pointer are zero. Since info pointers are always nonzero we can
+             * use this to scan for the next valid heap closure.
+             *
+             * Note that not all types of slop are relevant here, only the ones
+             * that can reman after major GC. So essentially just large objects
+             * and pinned objects. All other closures will have been packed nice
+             * and thight into fresh blocks.
+             */
         }
     }
 }