Sub8x8 block chroma component inter prediction

Handle the sub8x8 chroma component at the unit of 2x2/4x2/2x4 level
and use the motion vector inherited from the luma component. This
improves the coding performance:

lowres 0.4%
midres 0.25%
hdres  0.15%

Change-Id: I34dff4218cfa3e5d55e7ed0341f36f4719389f7e

av1/common/reconinter.c

diff --git a/av1/common/reconinter.c b/av1/common/reconinter.c
index c0fc494..b07a8bd 100644
--- a/av1/common/reconinter.c
+++ b/av1/common/reconinter.c
@@ -647,6 +647,87 @@
   }
 #endif
 
+#if CONFIG_SUB8X8_MC
+  if (mi->mbmi.sb_type < BLOCK_8X8 && plane > 0) {
+    // block size in log2
+    const int b4_wl = b_width_log2_lookup[mi->mbmi.sb_type];
+    const int b4_hl = b_height_log2_lookup[mi->mbmi.sb_type];
+    const int b8_sl = b_width_log2_lookup[BLOCK_8X8];
+
+    // block size
+    const int b4_w = 1 << b4_wl;
+    const int b4_h = 1 << b4_hl;
+    const int b8_s = 1 << b8_sl;
+    int idx, idy;
+
+    const int x_base = x;
+    const int y_base = y;
+
+    // processing unit size
+    const int x_step = w >> (b8_sl - b4_wl);
+    const int y_step = h >> (b8_sl - b4_hl);
+
+    for (idy = 0; idy < b8_s; idy += b4_h) {
+      for (idx = 0; idx < b8_s; idx += b4_w) {
+        const int chr_idx = (idy * 2) + idx;
+        for (ref = 0; ref < 1 + is_compound; ++ref) {
+          const struct scale_factors *const sf = &xd->block_refs[ref]->sf;
+          struct buf_2d *const pre_buf = &pd->pre[ref];
+          struct buf_2d *const dst_buf = &pd->dst;
+          uint8_t *dst = dst_buf->buf;
+          const MV mv = mi->bmi[chr_idx].as_mv[ref].as_mv;
+          const MV mv_q4 = clamp_mv_to_umv_border_sb(
+              xd, &mv, bw, bh, pd->subsampling_x, pd->subsampling_y);
+          uint8_t *pre;
+          MV32 scaled_mv;
+          int xs, ys, subpel_x, subpel_y;
+          const int is_scaled = av1_is_scaled(sf);
+
+          x = x_base + idx * x_step;
+          y = y_base + idy * y_step;
+
+          dst += dst_buf->stride * y + x;
+
+          if (is_scaled) {
+            pre =
+                pre_buf->buf + scaled_buffer_offset(x, y, pre_buf->stride, sf);
+            scaled_mv = av1_scale_mv(&mv_q4, mi_x + x, mi_y + y, sf);
+            xs = sf->x_step_q4;
+            ys = sf->y_step_q4;
+          } else {
+            pre = pre_buf->buf + y * pre_buf->stride + x;
+            scaled_mv.row = mv_q4.row;
+            scaled_mv.col = mv_q4.col;
+            xs = ys = 16;
+          }
+
+          subpel_x = scaled_mv.col & SUBPEL_MASK;
+          subpel_y = scaled_mv.row & SUBPEL_MASK;
+          pre += (scaled_mv.row >> SUBPEL_BITS) * pre_buf->stride +
+                 (scaled_mv.col >> SUBPEL_BITS);
+
+#if CONFIG_AOM_HIGHBITDEPTH
+          if (xd->cur_buf->flags & YV12_FLAG_HIGHBITDEPTH) {
+            high_inter_predictor(pre, pre_buf->stride, dst, dst_buf->stride,
+                                 subpel_x, subpel_y, sf, x_step, y_step, ref,
+                                 &mi->mbmi.interp_filter, xs, ys, xd->bd);
+          } else {
+            inter_predictor(pre, pre_buf->stride, dst, dst_buf->stride,
+                            subpel_x, subpel_y, sf, x_step, y_step, ref,
+                            &mi->mbmi.interp_filter, xs, ys);
+          }
+#else
+          inter_predictor(pre, pre_buf->stride, dst, dst_buf->stride, subpel_x,
+                          subpel_y, sf, x_step, y_step, ref,
+                          &mi->mbmi.interp_filter, xs, ys);
+#endif
+        }
+      }
+    }
+    return;
+  }
+#endif
+
   for (ref = 0; ref < 1 + is_compound; ++ref) {
     const struct scale_factors *const sf = &xd->block_refs[ref]->sf;
     struct buf_2d *const pre_buf = &pd->pre[ref];

av1/common/reconinter.h

diff --git a/av1/common/reconinter.h b/av1/common/reconinter.h
index bfa7e95..5f62f0a 100644
--- a/av1/common/reconinter.h
+++ b/av1/common/reconinter.h
@@ -50,7 +50,7 @@
     const int16_t *kernel_y =
         av1_get_interp_filter_subpel_kernel(interp_filter_params_y, subpel_y);
 #else
-  if (interp_filter_params.taps == SUBPEL_TAPS) {
+  if (interp_filter_params.taps == SUBPEL_TAPS && w > 2 && h > 2) {
     const int16_t *kernel_x =
         av1_get_interp_filter_subpel_kernel(interp_filter_params, subpel_x);
     const int16_t *kernel_y =
@@ -109,7 +109,7 @@
     const int16_t *kernel_y =
         av1_get_interp_filter_subpel_kernel(interp_filter_params_y, subpel_y);
 #else
-  if (interp_filter_params.taps == SUBPEL_TAPS) {
+  if (interp_filter_params.taps == SUBPEL_TAPS && w > 2 && h > 2) {
     const int16_t *kernel_x =
         av1_get_interp_filter_subpel_kernel(interp_filter_params, subpel_x);
     const int16_t *kernel_y =