[layout] Limit how far we skip when looking back
See comments.
diff --git a/src/hb-ot-layout-gsubgpos.hh b/src/hb-ot-layout-gsubgpos.hh
index bf74672..8ca6918 100644
--- a/src/hb-ot-layout-gsubgpos.hh
+++ b/src/hb-ot-layout-gsubgpos.hh
@@ -578,6 +578,13 @@
unsigned stop = num_items - 1;
if (c->buffer->flags & HB_BUFFER_FLAG_PRODUCE_UNSAFE_TO_CONCAT)
stop = 1 - 1;
+
+ /* When looking back, limit how far we search; this function is mostly
+ * used for looking back for base glyphs when attaching marks. If we
+ * don't limit, we can get O(n^2) behavior where n is the number of
+ * consecutive marks. */
+ stop = (unsigned) hb_max ((int) stop, (int) idx - HB_MAX_CONTEXT_LENGTH);
+
while (idx > stop)
{
idx--;
