Speedup for decoding lossless WebP photographs:

use bit_pos instead num_bit to update the bit-reader
37.69 MB/s -> 39.79 MB/s, 5.6 %

Change-Id: Ica476cc3e2fc2db084d6961bea8586b050bb221d

src/dec/vp8l.c

@@ -156,15 +156,15 @@ static WEBP_INLINE int PlaneCodeToDistance(int xsize, int plane_code) {
 static WEBP_INLINE int ReadSymbol(const HuffmanTree* tree,
                                   VP8LBitReader* const br) {
   const HuffmanTreeNode* node = tree->root_;
-  int num_bits = 0;
   uint32_t bits = VP8LPrefetchBits(br);
+  int bitpos = br->bit_pos_;
   assert(node != NULL);
-  while (!HuffmanTreeNodeIsLeaf(node)) {
+  while (HuffmanTreeNodeIsNotLeaf(node)) {
     node = HuffmanTreeNextNode(node, bits & 1);
     bits >>= 1;
-    ++num_bits;
+    ++bitpos;
   }
-  VP8LDiscardBits(br, num_bits);
+  VP8LSetBitPos(br, bitpos);
   return node->symbol_;
 }
 

src/utils/bit_reader.h

@@ -316,9 +316,10 @@ static WEBP_INLINE uint32_t VP8LPrefetchBits(VP8LBitReader* const br) {
   return (uint32_t)(br->val_ >> br->bit_pos_);
 }
 
-// Discard 'num_bits' bits from the cache.
-static WEBP_INLINE void VP8LDiscardBits(VP8LBitReader* const br, int num_bits) {
-  br->bit_pos_ += num_bits;
+// For jumping over a number of bits in the bit stream when accessed with
+// VP8LPrefetchBits and VP8LFillBitWindow.
+static WEBP_INLINE void VP8LSetBitPos(VP8LBitReader* const br, int val) {
+  br->bit_pos_ = val;
 }
 
 // Advances the Read buffer by 4 bytes to make room for reading next 32 bits.

src/utils/huffman.c

@@ -128,14 +128,14 @@ static int TreeAddSymbol(HuffmanTree* const tree,
     if (NodeIsEmpty(node)) {
       if (IsFull(tree)) return 0;    // error: too many symbols.
       AssignChildren(tree, node);
-    } else if (HuffmanTreeNodeIsLeaf(node)) {
+    } else if (!HuffmanTreeNodeIsNotLeaf(node)) {
       return 0;  // leaf is already occupied.
     }
     node += node->children_ + ((code >> code_length) & 1);
   }
   if (NodeIsEmpty(node)) {
     node->children_ = 0;      // turn newly created node into a leaf.
-  } else if (!HuffmanTreeNodeIsLeaf(node)) {
+  } else if (HuffmanTreeNodeIsNotLeaf(node)) {
     return 0;   // trying to assign a symbol to already used code.
   }
   node->symbol_ = symbol;  // Add symbol in this node.

src/utils/huffman.h

@@ -35,10 +35,10 @@ struct HuffmanTree {
   int num_nodes_;           // number of currently occupied nodes
 };
 
-// Returns true if the given node is a leaf of the Huffman tree.
-static WEBP_INLINE int HuffmanTreeNodeIsLeaf(
+// Returns true if the given node is not a leaf of the Huffman tree.
+static WEBP_INLINE int HuffmanTreeNodeIsNotLeaf(
     const HuffmanTreeNode* const node) {
-  return (node->children_ == 0);
+  return node->children_;
 }
 
 // Go down one level. Most critical function. 'right_child' must be 0 or 1.