prof: Fix heap census for large ARR_WORDS (#11627)

The heap census now handles large ARR_WORDS objects which have
been shrunk by shrinkMutableByteArray# or resizeMutableByteArray#.

Test Plan: ./validate && make test WAY=profasm

Reviewers: hvr, bgamari, austin, thomie

Reviewed By: thomie

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2005

GHC Trac Issues: #11627

1 files changed, 14 insertions, 0 deletions

diff --git a/rts/ProfHeap.c b/rts/ProfHeap.c
index a7ea3eb565..819faeb9be 100644
--- a/rts/ProfHeap.c
+++ b/rts/ProfHeap.c
@@ -937,6 +937,20 @@ heapCensusChain( Census *census, bdescr *bd )
         }
 
         p = bd->start;
+
+        // When we shrink a large ARR_WORDS, we do not adjust the free pointer
+        // of the associated block descriptor, thus introducing slop at the end
+        // of the object.  This slop remains after GC, violating the assumption
+        // of the loop below that all slop has been eliminated (#11627).
+        // Consequently, we handle large ARR_WORDS objects as a special case.
+        if (bd->flags & BF_LARGE
+            && get_itbl((StgClosure *)p)->type == ARR_WORDS) {
+            size = arr_words_sizeW((StgArrBytes *)p);
+            prim = rtsTrue;
+            heapProfObject(census, (StgClosure *)p, size, prim);
+            continue;
+        }
+
         while (p < bd->free) {
             info = get_itbl((StgClosure *)p);
             prim = rtsFalse;