Add LIT_MEM define to use more memory for a small deflate speedup.

A bug fix in zlib 1.2.12 resulted in a slight slowdown (1-2%) of
deflate. This commit provides the option to #define LIT_MEM, which
uses more memory to reverse most of that slowdown. The memory for
the pending buffer and symbol buffers is increased by 25%, which
increases the total memory usage with the default parameters by
about 6%.

1 files changed, 16 insertions, 2 deletions

diff --git a/trees.c b/trees.c
index 8dbdc40..5ca23e9 100644
--- a/trees.c
+++ b/trees.c
@@ -899,14 +899,19 @@ local void compress_block(deflate_state *s, const ct_data *ltree,
                           const ct_data *dtree) {
     unsigned dist;      /* distance of matched string */
     int lc;             /* match length or unmatched char (if dist == 0) */
-    unsigned sx = 0;    /* running index in sym_buf */
+    unsigned sx = 0;    /* running index in symbol buffers */
     unsigned code;      /* the code to send */
     int extra;          /* number of extra bits to send */
 
     if (s->sym_next != 0) do {
+#ifdef LIT_MEM
+        dist = s->d_buf[sx];
+        lc = s->l_buf[sx++];
+#else
         dist = s->sym_buf[sx++] & 0xff;
         dist += (unsigned)(s->sym_buf[sx++] & 0xff) << 8;
         lc = s->sym_buf[sx++];
+#endif
         if (dist == 0) {
             send_code(s, lc, ltree); /* send a literal byte */
             Tracecv(isgraph(lc), (stderr," '%c' ", lc));
@@ -931,8 +936,12 @@ local void compress_block(deflate_state *s, const ct_data *ltree,
             }
         } /* literal or match pair ? */
 
-        /* Check that the overlay between pending_buf and sym_buf is ok: */
+        /* Check for no overlay of pending_buf on needed symbols */
+#ifdef LIT_MEM
+        Assert(s->pending < (s->lit_bufsize << 1) + sx, "pendingBuf overflow");
+#else
         Assert(s->pending < s->lit_bufsize + sx, "pendingBuf overflow");
+#endif
 
     } while (sx < s->sym_next);
 
@@ -1082,9 +1091,14 @@ void ZLIB_INTERNAL _tr_flush_block(deflate_state *s, charf *buf,
  * the current block must be flushed.
  */
 int ZLIB_INTERNAL _tr_tally(deflate_state *s, unsigned dist, unsigned lc) {
+#ifdef LIT_MEM
+    s->d_buf[s->sym_next] = (ush)dist;
+    s->l_buf[s->sym_next++] = (uch)lc;
+#else
     s->sym_buf[s->sym_next++] = (uch)dist;
     s->sym_buf[s->sym_next++] = (uch)(dist >> 8);
     s->sym_buf[s->sym_next++] = (uch)lc;
+#endif
     if (dist == 0) {
         /* lc is the unmatched char */
         s->dyn_ltree[lc].Freq++;