zopfli: some optimizations

This include _BitScanReverse support which I sent a PR:
google/zopfli#102 and some other minor changes.

Author: JayXon

lib/zopfli/squeeze.c

@@ -282,14 +282,14 @@ static double GetBestLengths(ZopfliBlockState *s,
       }
     }
     /* Lengths. */
-    for (k = 3; k <= leng && i + k <= inend; k++) {
-      double newCost;
-
+    double kmincost = mincost + costs[j];
+    int kend = leng <= inend - i ? leng : inend - i;
+    for (k = 3; k <= kend; k++) {
       /* Calling the cost model is expensive, avoid this if we are already at
       the minimum possible cost that it can return. */
-     if (costs[j + k] - costs[j] <= mincost) continue;
+      if (costs[j + k] <= kmincost) continue;
 
-      newCost = costs[j] + costmodel(k, sublen[k], costcontext);
+      double newCost = costs[j] + costmodel(k, sublen[k], costcontext);
       assert(newCost >= 0);
       if (newCost < costs[j + k]) {
         assert(k <= ZOPFLI_MAX_MATCH);

lib/zopfli/util.c

@@ -33,15 +33,22 @@ Author: [email protected] (Jyrki Alakuijala)
 /* __builtin_clz available beginning with GCC 3.4 */
 #elif __GNUC__ * 100 + __GNUC_MINOR__ >= 304
 # define HAS_BUILTIN_CLZ
+/* _BitScanReverse available beginning with Visual Studio 2005 */
+#elif _MSC_VER >= 1400
+# include <intrin.h> 
+# define HAS_BITSCANREVERSE
 #endif
 
 int ZopfliGetDistExtraBits(int dist) {
-#ifdef HAS_BUILTIN_CLZ
   if (dist < 5) return 0;
+#ifdef HAS_BUILTIN_CLZ
   return (31 ^ __builtin_clz(dist - 1)) - 1; /* log2(dist - 1) - 1 */
+#elif defined HAS_BITSCANREVERSE
+  unsigned long l;
+  _BitScanReverse(&l, dist - 1);
+  return l - 1;
 #else
-  if (dist < 5) return 0;
-  else if (dist < 9) return 1;
+  if (dist < 9) return 1;
   else if (dist < 17) return 2;
   else if (dist < 33) return 3;
   else if (dist < 65) return 4;
@@ -65,6 +72,14 @@ int ZopfliGetDistExtraBitsValue(int dist) {
     int l = 31 ^ __builtin_clz(dist - 1); /* log2(dist - 1) */
     return (dist - (1 + (1 << l))) & ((1 << (l - 1)) - 1);
   }
+#elif defined HAS_BITSCANREVERSE
+  if (dist < 5) {
+    return 0;
+  } else {
+    unsigned long l;
+    _BitScanReverse(&l, dist - 1);
+    return (dist - (1 + (1 << l))) & ((1 << (l - 1)) - 1);
+  }
 #else
   if (dist < 5) return 0;
   else if (dist < 9) return (dist - 5) & 1;
@@ -92,6 +107,15 @@ int ZopfliGetDistSymbol(int dist) {
     int r = ((dist - 1) >> (l - 1)) & 1;
     return l * 2 + r;
   }
+#elif defined HAS_BITSCANREVERSE
+  if (dist < 5) {
+    return dist - 1;
+  } else {
+    unsigned long l;
+    _BitScanReverse(&l, dist - 1);
+    int r = ((dist - 1) >> (l - 1)) & 1;
+    return l * 2 + r;
+  }
 #else
   if (dist < 193) {
     if (dist < 13) {  /* dist 0..13. */