Handle large ARR_WORDS in heap census (fix #17572)

We can do a heap census with a non-profiling RTS. With a non-profiling
RTS we don't zero superfluous bytes of shrunk arrays hence a need to
handle the case specifically to avoid a crash.

Revert part of a586b33f8e8ad60b5c5ef3501c89e9b71794bbed

1 files changed, 16 insertions, 0 deletions

diff --git a/rts/ProfHeap.c b/rts/ProfHeap.c
index c35b4bae4e..4f82b0ba83 100644
--- a/rts/ProfHeap.c
+++ b/rts/ProfHeap.c
@@ -1011,6 +1011,22 @@ 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).
+        // The slop isn't always zeroed (e.g. in non-profiling mode, cf
+        // OVERWRITING_CLOSURE_OFS).
+        // 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 = true;
+            heapProfObject(census, (StgClosure *)p, size, prim);
+            continue;
+        }
+
+
         while (p < bd->free) {
             info = get_itbl((const StgClosure *)p);
             prim = false;