Fix non-4:2:0 chroma MV calculation for SPLITMV

The previous code was somewhat vestigial for 16x16 MI units, but was
incorrect when called with chroma blocks larger than 4x4 because the
block index caused a reference to a non-existent BMI. This patch uses
the same MV for all chroma subblocks in SPLITMV mode, which is
suboptimal for non-4:2:0 subsamplings, but as SPLITMV may be removed
in the near future, will use this as a stop gap.

Change-Id: I3211cee5ccf1cfb426e5eef5353b0ce5bb92b4cd

vp9/common/vp9_reconinter.c

@@ -257,22 +257,19 @@ static INLINE int round_mv_comp_q4(int value) {
   return (value < 0 ? value - 2 : value + 2) / 4;
 }
 
-#define IDX1 2
-#define IDX2 3
-
-static int mi_mv_pred_row_q4(MACROBLOCKD *mb, int off, int idx) {
-  const int temp = mb->mode_info_context->bmi[off + 0].as_mv[idx].as_mv.row +
-                   mb->mode_info_context->bmi[off + 1].as_mv[idx].as_mv.row +
-                   mb->mode_info_context->bmi[off + IDX1].as_mv[idx].as_mv.row +
-                   mb->mode_info_context->bmi[off + IDX2].as_mv[idx].as_mv.row;
+static int mi_mv_pred_row_q4(MACROBLOCKD *mb, int idx) {
+  const int temp = mb->mode_info_context->bmi[0].as_mv[idx].as_mv.row +
+                   mb->mode_info_context->bmi[1].as_mv[idx].as_mv.row +
+                   mb->mode_info_context->bmi[2].as_mv[idx].as_mv.row +
+                   mb->mode_info_context->bmi[3].as_mv[idx].as_mv.row;
   return round_mv_comp_q4(temp);
 }
 
-static int mi_mv_pred_col_q4(MACROBLOCKD *mb, int off, int idx) {
-  const int temp = mb->mode_info_context->bmi[off + 0].as_mv[idx].as_mv.col +
-                   mb->mode_info_context->bmi[off + 1].as_mv[idx].as_mv.col +
-                   mb->mode_info_context->bmi[off + IDX1].as_mv[idx].as_mv.col +
-                   mb->mode_info_context->bmi[off + IDX2].as_mv[idx].as_mv.col;
+static int mi_mv_pred_col_q4(MACROBLOCKD *mb, int idx) {
+  const int temp = mb->mode_info_context->bmi[0].as_mv[idx].as_mv.col +
+                   mb->mode_info_context->bmi[1].as_mv[idx].as_mv.col +
+                   mb->mode_info_context->bmi[2].as_mv[idx].as_mv.col +
+                   mb->mode_info_context->bmi[3].as_mv[idx].as_mv.col;
   return round_mv_comp_q4(temp);
 }
 
@@ -351,9 +348,12 @@ static void build_inter_predictors(int plane, int block,
       if (plane == 0) {
         mv = &xd->mode_info_context->bmi[block].as_mv[which_mv].as_mv;
       } else {
-        const int y_block = (block & 2) * 4 + (block & 1) * 2;
-        split_chroma_mv.row = mi_mv_pred_row_q4(xd, y_block, which_mv);
-        split_chroma_mv.col = mi_mv_pred_col_q4(xd, y_block, which_mv);
+        // TODO(jkoleszar): All chroma MVs in SPLITMV mode are taken as the
+        // same MV (the average of the 4 luma MVs) but we could do something
+        // smarter for non-4:2:0. Just punt for now, pending the changes to get
+        // rid of SPLITMV mode entirely.
+        split_chroma_mv.row = mi_mv_pred_row_q4(xd, which_mv);
+        split_chroma_mv.col = mi_mv_pred_col_q4(xd, which_mv);
         mv = &split_chroma_mv;
       }
     } else {