Make maximum obmc process unit 64x64

For 128x128 level blocks, process the overlapped prediction in
the unit of 64x64. This allows hardware design to reuse the 64x64
processing unit in 128x128 level block coding.

Change-Id: I3967b8e3c1c697f96a50e59a0957fc69b67e6f8e

av1/common/reconinter.c

diff --git a/av1/common/reconinter.c b/av1/common/reconinter.c
index 7a21e07..ba25631 100644
--- a/av1/common/reconinter.c
+++ b/av1/common/reconinter.c
@@ -2154,7 +2154,9 @@
     const BLOCK_SIZE a_bsize = AOMMAX(BLOCK_8X8, above_mbmi->sb_type);
     MB_MODE_INFO backup_mbmi;
 
-    mi_step = AOMMIN(xd->n8_w, mi_size_wide[a_bsize]);
+    const int above_step =
+        AOMMIN(mi_size_wide[a_bsize], mi_size_wide[BLOCK_64X64]);
+    mi_step = AOMMIN(xd->n8_w, above_step);
 
     if (!is_neighbor_overlappable(above_mbmi)) continue;
 
@@ -2201,6 +2203,7 @@
       bw = (mi_step * MI_SIZE) >> pd->subsampling_x;
       bh = AOMMAX((num_4x4_blocks_high_lookup[bsize] * 2) >> pd->subsampling_y,
                   4);
+      bh = AOMMIN(bh, block_size_high[BLOCK_64X64] >> (pd->subsampling_y + 1));
 
       if (skip_u4x4_pred_in_obmc(bsize, pd, 0)) continue;
       build_inter_predictors(cm, xd, j, mi_col_offset, mi_row_offset, 0, bw, bh,
@@ -2250,8 +2253,9 @@
 
     const BLOCK_SIZE l_bsize = AOMMAX(left_mbmi->sb_type, BLOCK_8X8);
     MB_MODE_INFO backup_mbmi;
-
-    mi_step = AOMMIN(xd->n8_h, mi_size_high[l_bsize]);
+    const int left_step =
+        AOMMIN(mi_size_high[l_bsize], mi_size_high[BLOCK_64X64]);
+    mi_step = AOMMIN(xd->n8_h, left_step);
 
     if (!is_neighbor_overlappable(left_mbmi)) continue;
 
@@ -2297,6 +2301,7 @@
       const struct macroblockd_plane *pd = &xd->plane[j];
       bw = AOMMAX((num_4x4_blocks_wide_lookup[bsize] * 2) >> pd->subsampling_x,
                   4);
+      bw = AOMMIN(bw, block_size_wide[BLOCK_64X64] >> (pd->subsampling_x + 1));
       bh = (mi_step << MI_SIZE_LOG2) >> pd->subsampling_y;
 
       if (skip_u4x4_pred_in_obmc(bsize, pd, 1)) continue;