Cherry-pick potential divide-by-zero fix

source/libvpx/vp9/common/vp9_prob.h

 /*
  *  Copyright (c) 2013 The WebM project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 15 matching lines @@
 #define MAX_PROB 255
 
 #define vp9_prob_half ((vp9_prob) 128)
 
 typedef int8_t vp9_tree_index;
 
 #define TREE_SIZE(leaf_count) (2 * (leaf_count) - 2)
 
 #define vp9_complement(x) (255 - x)
 
+#define MODE_MV_COUNT_SAT 20
+
 /* We build coding trees compactly in arrays.
    Each node of the tree is a pair of vp9_tree_indices.
    Array index often references a corresponding probability table.
    Index <= 0 means done encoding/decoding and value = -Index,
    Index > 0 means need another bit, specification at index.
    Nonnegative indices are always even;  processing begins at node 0. */
 
 typedef const vp9_tree_index vp9_tree[];
 
 static INLINE vp9_prob clip_prob(int p) {
@@ skipping 16 matching lines @@
 static INLINE vp9_prob merge_probs(vp9_prob pre_prob,
                                    const unsigned int ct[2],
                                    unsigned int count_sat,
                                    unsigned int max_update_factor) {
   const vp9_prob prob = get_binary_prob(ct[0], ct[1]);
   const unsigned int count = MIN(ct[0] + ct[1], count_sat);
   const unsigned int factor = max_update_factor * count / count_sat;
   return weighted_prob(pre_prob, prob, factor);
 }
 
+// MODE_MV_MAX_UPDATE_FACTOR (128) * count / MODE_MV_COUNT_SAT;
+static const int count_to_update_factor[MODE_MV_COUNT_SAT + 1] = {
+  0, 6, 12, 19, 25, 32, 38, 44, 51, 57, 64,
+  70, 76, 83, 89, 96, 102, 108, 115, 121, 128
+};
+
+static INLINE vp9_prob mode_mv_merge_probs(vp9_prob pre_prob,
+                                           const unsigned int ct[2]) {
+  const unsigned int den = ct[0] + ct[1];
+  if (den == 0) {
+    return pre_prob;
+  } else {
+    const unsigned int count = MIN(den, MODE_MV_COUNT_SAT);
+    const unsigned int factor = count_to_update_factor[count];
+    const vp9_prob prob =
+        clip_prob(((int64_t)(ct[0]) * 256 + (den >> 1)) / den);
+    return weighted_prob(pre_prob, prob, factor);
+  }
+}
+
 void vp9_tree_merge_probs(const vp9_tree_index *tree, const vp9_prob *pre_probs,
-                          const unsigned int *counts, unsigned int count_sat,
-                          unsigned int max_update_factor, vp9_prob *probs);
+                          const unsigned int *counts, vp9_prob *probs);
 
 
 DECLARE_ALIGNED(16, extern const uint8_t, vp9_norm[256]);
 
 #ifdef __cplusplus
 }  // extern "C"
 #endif
 
 #endif  // VP9_COMMON_VP9_PROB_H_