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Restrict ref mv search range.

Experiment to test speed trade off of reducing the
extent of the ref mv search.

Reducing the maximum number of tested candidates to 9 had
minimal net effect on quality in any of the tests sets.

Reduction to 7 has a small negative impact (worst was STD-HD
at about -0.2%).

This change is in response to the apparently high number of
decode cycles reported in regard to mv-ref selection.

Change-Id: I0e92e92e324337689358495a1ec9ccdeb23dc774

--- a/vp9/common/blockd.h

+++ b/vp9/common/blockd.h

@@ -44,7 +44,7 @@

 /* Segment Feature Masks */

 #define SEGMENT_DELTADATA   0

 #define SEGMENT_ABSDATA     1

-#define MAX_MV_REFS 19

+#define MAX_MV_REFS 9

 typedef struct {

   int r, c;

--- a/vp9/common/mvref_common.c

+++ b/vp9/common/mvref_common.c

@@ -290,7 +290,7 @@

   // Populate a list with candidate reference vectors from the

   // spatial neighbours.

-  for (i = 2; i < MVREF_NEIGHBOURS; ++i) {

+  for (i = 2; (i < MVREF_NEIGHBOURS) && (index < (MAX_MV_REFS - 2)); ++i) {

     if (((mv_ref_search[i][0] << 7) >= xd->mb_to_left_edge) &&

         ((mv_ref_search[i][1] << 7) >= xd->mb_to_top_edge)) {

@@ -321,6 +321,11 @@

         }

       }

     }

+  }

+

+  // Make sure we are able to add 0,0

+  if (index > (MAX_MV_REFS - 1)) {

+    index = (MAX_MV_REFS - 1);

   }

   // 0,0 is always a valid reference.