--- misc/zlib-1.2.3/deflate.c
+++ misc/build/zlib-1.2.3/deflate.c
@@ -288,6 +288,8 @@
     s->prev   = (Posf *)  ZALLOC(strm, s->w_size, sizeof(Pos));
     s->head   = (Posf *)  ZALLOC(strm, s->hash_size, sizeof(Pos));
 
+    s->high_water = 0;      /* nothing written to s->window yet */
+
     s->lit_bufsize = 1 << (memLevel + 6); /* 16K elements by default */
 
     overlay = (ushf *) ZALLOC(strm, s->lit_bufsize, sizeof(ush)+2);
@@ -1355,6 +1355,40 @@
          */
 
     } while (s->lookahead < MIN_LOOKAHEAD && s->strm->avail_in != 0);
+
+    /* If the WIN_INIT bytes after the end of the current data have never been
+     * written, then zero those bytes in order to avoid memory check reports of
+     * the use of uninitialized (or uninitialised as Julian writes) bytes by
+     * the longest match routines.  Update the high water mark for the next
+     * time through here.  WIN_INIT is set to MAX_MATCH since the longest match
+     * routines allow scanning to strstart + MAX_MATCH, ignoring lookahead.
+     */
+    if (s->high_water < s->window_size) {
+        ulg curr = s->strstart + (ulg)(s->lookahead);
+        ulg init;
+
+        if (s->high_water < curr) {
+            /* Previous high water mark below current data -- zero WIN_INIT
+             * bytes or up to end of window, whichever is less.
+             */
+            init = s->window_size - curr;
+            if (init > WIN_INIT)
+                init = WIN_INIT;
+            zmemzero(s->window + curr, (unsigned)init);
+            s->high_water = curr + init;
+        }
+        else if (s->high_water < (ulg)curr + WIN_INIT) {
+            /* High water mark at or above current data, but below current data
+             * plus WIN_INIT -- zero out to current data plus WIN_INIT, or up
+             * to end of window, whichever is less.
+             */
+            init = (ulg)curr + WIN_INIT - s->high_water;
+            if (init > s->window_size - s->high_water)
+                init = s->window_size - s->high_water;
+            zmemzero(s->window + s->high_water, (unsigned)init);
+            s->high_water += init;
+        }
+    }
 }
 
 /* ===========================================================================
--- misc/zlib-1.2.3/deflate.h
+++ misc/build/zlib-1.2.3/deflate.h
@@ -260,6 +260,12 @@
      * are always zero.
      */
 
+    ulg high_water;
+    /* High water mark offset in window for initialized bytes -- bytes above
+     * this are set to zero in order to avoid memory check warnings when
+     * longest match routines access bytes past the input.  This is then
+     * updated to the new high water mark.
+     */
 } FAR deflate_state;
 
 /* Output a byte on the stream.
@@ -278,6 +284,10 @@
  * distances are limited to MAX_DIST instead of WSIZE.
  */
 
+#define WIN_INIT MAX_MATCH
+/* Number of bytes after end of data in window to initialize in order to avoid
+   memory checker errors from longest match routines */
+
         /* in trees.c */
 void _tr_init         OF((deflate_state *s));
 int  _tr_tally        OF((deflate_state *s, unsigned dist, unsigned lc));