[layout] Comment
diff --git a/src/hb-ot-layout-gsubgpos.hh b/src/hb-ot-layout-gsubgpos.hh
index 8d0f400..bf74672 100644
--- a/src/hb-ot-layout-gsubgpos.hh
+++ b/src/hb-ot-layout-gsubgpos.hh
@@ -4021,6 +4021,11 @@
HB_VAR_ARRAY * sizeof (hb_accelerate_subtables_context_t::hb_applicable_t) +
count * sizeof (hb_accelerate_subtables_context_t::hb_applicable_t);
+ /* The following is a calloc because when we are collecting subtables,
+ * some of them might be invalid and hence not collect; as a result,
+ * we might not fill in all the count entries of the subtables array.
+ * Zeroing it allows the set digest to gatekeep it without having to
+ * initialize it further. */
auto *thiz = (hb_ot_layout_lookup_accelerator_t *) hb_calloc (1, size);
if (unlikely (!thiz))
return nullptr;
