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      1 /*
      2  * Copyright (c) 2016, Alliance for Open Media. All rights reserved
      3  *
      4  * This source code is subject to the terms of the BSD 2 Clause License and
      5  * the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
      6  * was not distributed with this source code in the LICENSE file, you can
      7  * obtain it at www.aomedia.org/license/software. If the Alliance for Open
      8  * Media Patent License 1.0 was not distributed with this source code in the
      9  * PATENTS file, you can obtain it at www.aomedia.org/license/patent.
     10  */
     11 
     12 #include "av1/common/common.h"
     13 #include "av1/common/pred_common.h"
     14 #include "av1/common/reconinter.h"
     15 #include "av1/common/reconintra.h"
     16 #include "av1/common/seg_common.h"
     17 
     18 // Returns a context number for the given MB prediction signal
     19 static InterpFilter get_ref_filter_type(const MB_MODE_INFO *ref_mbmi,
     20                                         const MACROBLOCKD *xd, int dir,
     21                                         MV_REFERENCE_FRAME ref_frame) {
     22   (void)xd;
     23 
     24   return ((ref_mbmi->ref_frame[0] == ref_frame ||
     25            ref_mbmi->ref_frame[1] == ref_frame)
     26               ? av1_extract_interp_filter(ref_mbmi->interp_filters, dir & 0x01)
     27               : SWITCHABLE_FILTERS);
     28 }
     29 
     30 int av1_get_pred_context_switchable_interp(const MACROBLOCKD *xd, int dir) {
     31   const MB_MODE_INFO *const mbmi = xd->mi[0];
     32   const int ctx_offset =
     33       (mbmi->ref_frame[1] > INTRA_FRAME) * INTER_FILTER_COMP_OFFSET;
     34   assert(dir == 0 || dir == 1);
     35   const MV_REFERENCE_FRAME ref_frame = mbmi->ref_frame[0];
     36   // Note:
     37   // The mode info data structure has a one element border above and to the
     38   // left of the entries corresponding to real macroblocks.
     39   // The prediction flags in these dummy entries are initialized to 0.
     40   int filter_type_ctx = ctx_offset + (dir & 0x01) * INTER_FILTER_DIR_OFFSET;
     41   int left_type = SWITCHABLE_FILTERS;
     42   int above_type = SWITCHABLE_FILTERS;
     43 
     44   if (xd->left_available)
     45     left_type = get_ref_filter_type(xd->mi[-1], xd, dir, ref_frame);
     46 
     47   if (xd->up_available)
     48     above_type =
     49         get_ref_filter_type(xd->mi[-xd->mi_stride], xd, dir, ref_frame);
     50 
     51   if (left_type == above_type) {
     52     filter_type_ctx += left_type;
     53   } else if (left_type == SWITCHABLE_FILTERS) {
     54     assert(above_type != SWITCHABLE_FILTERS);
     55     filter_type_ctx += above_type;
     56   } else if (above_type == SWITCHABLE_FILTERS) {
     57     assert(left_type != SWITCHABLE_FILTERS);
     58     filter_type_ctx += left_type;
     59   } else {
     60     filter_type_ctx += SWITCHABLE_FILTERS;
     61   }
     62 
     63   return filter_type_ctx;
     64 }
     65 
     66 static void palette_add_to_cache(uint16_t *cache, int *n, uint16_t val) {
     67   // Do not add an already existing value
     68   if (*n > 0 && val == cache[*n - 1]) return;
     69 
     70   cache[(*n)++] = val;
     71 }
     72 
     73 int av1_get_palette_cache(const MACROBLOCKD *const xd, int plane,
     74                           uint16_t *cache) {
     75   const int row = -xd->mb_to_top_edge >> 3;
     76   // Do not refer to above SB row when on SB boundary.
     77   const MB_MODE_INFO *const above_mi =
     78       (row % (1 << MIN_SB_SIZE_LOG2)) ? xd->above_mbmi : NULL;
     79   const MB_MODE_INFO *const left_mi = xd->left_mbmi;
     80   int above_n = 0, left_n = 0;
     81   if (above_mi) above_n = above_mi->palette_mode_info.palette_size[plane != 0];
     82   if (left_mi) left_n = left_mi->palette_mode_info.palette_size[plane != 0];
     83   if (above_n == 0 && left_n == 0) return 0;
     84   int above_idx = plane * PALETTE_MAX_SIZE;
     85   int left_idx = plane * PALETTE_MAX_SIZE;
     86   int n = 0;
     87   const uint16_t *above_colors =
     88       above_mi ? above_mi->palette_mode_info.palette_colors : NULL;
     89   const uint16_t *left_colors =
     90       left_mi ? left_mi->palette_mode_info.palette_colors : NULL;
     91   // Merge the sorted lists of base colors from above and left to get
     92   // combined sorted color cache.
     93   while (above_n > 0 && left_n > 0) {
     94     uint16_t v_above = above_colors[above_idx];
     95     uint16_t v_left = left_colors[left_idx];
     96     if (v_left < v_above) {
     97       palette_add_to_cache(cache, &n, v_left);
     98       ++left_idx, --left_n;
     99     } else {
    100       palette_add_to_cache(cache, &n, v_above);
    101       ++above_idx, --above_n;
    102       if (v_left == v_above) ++left_idx, --left_n;
    103     }
    104   }
    105   while (above_n-- > 0) {
    106     uint16_t val = above_colors[above_idx++];
    107     palette_add_to_cache(cache, &n, val);
    108   }
    109   while (left_n-- > 0) {
    110     uint16_t val = left_colors[left_idx++];
    111     palette_add_to_cache(cache, &n, val);
    112   }
    113   assert(n <= 2 * PALETTE_MAX_SIZE);
    114   return n;
    115 }
    116 
    117 // The mode info data structure has a one element border above and to the
    118 // left of the entries corresponding to real macroblocks.
    119 // The prediction flags in these dummy entries are initialized to 0.
    120 // 0 - inter/inter, inter/--, --/inter, --/--
    121 // 1 - intra/inter, inter/intra
    122 // 2 - intra/--, --/intra
    123 // 3 - intra/intra
    124 int av1_get_intra_inter_context(const MACROBLOCKD *xd) {
    125   const MB_MODE_INFO *const above_mbmi = xd->above_mbmi;
    126   const MB_MODE_INFO *const left_mbmi = xd->left_mbmi;
    127   const int has_above = xd->up_available;
    128   const int has_left = xd->left_available;
    129 
    130   if (has_above && has_left) {  // both edges available
    131     const int above_intra = !is_inter_block(above_mbmi);
    132     const int left_intra = !is_inter_block(left_mbmi);
    133     return left_intra && above_intra ? 3 : left_intra || above_intra;
    134   } else if (has_above || has_left) {  // one edge available
    135     return 2 * !is_inter_block(has_above ? above_mbmi : left_mbmi);
    136   } else {
    137     return 0;
    138   }
    139 }
    140 
    141 #define CHECK_BACKWARD_REFS(ref_frame) \
    142   (((ref_frame) >= BWDREF_FRAME) && ((ref_frame) <= ALTREF_FRAME))
    143 #define IS_BACKWARD_REF_FRAME(ref_frame) CHECK_BACKWARD_REFS(ref_frame)
    144 
    145 int av1_get_reference_mode_context(const MACROBLOCKD *xd) {
    146   int ctx;
    147   const MB_MODE_INFO *const above_mbmi = xd->above_mbmi;
    148   const MB_MODE_INFO *const left_mbmi = xd->left_mbmi;
    149   const int has_above = xd->up_available;
    150   const int has_left = xd->left_available;
    151 
    152   // Note:
    153   // The mode info data structure has a one element border above and to the
    154   // left of the entries corresponding to real macroblocks.
    155   // The prediction flags in these dummy entries are initialized to 0.
    156   if (has_above && has_left) {  // both edges available
    157     if (!has_second_ref(above_mbmi) && !has_second_ref(left_mbmi))
    158       // neither edge uses comp pred (0/1)
    159       ctx = IS_BACKWARD_REF_FRAME(above_mbmi->ref_frame[0]) ^
    160             IS_BACKWARD_REF_FRAME(left_mbmi->ref_frame[0]);
    161     else if (!has_second_ref(above_mbmi))
    162       // one of two edges uses comp pred (2/3)
    163       ctx = 2 + (IS_BACKWARD_REF_FRAME(above_mbmi->ref_frame[0]) ||
    164                  !is_inter_block(above_mbmi));
    165     else if (!has_second_ref(left_mbmi))
    166       // one of two edges uses comp pred (2/3)
    167       ctx = 2 + (IS_BACKWARD_REF_FRAME(left_mbmi->ref_frame[0]) ||
    168                  !is_inter_block(left_mbmi));
    169     else  // both edges use comp pred (4)
    170       ctx = 4;
    171   } else if (has_above || has_left) {  // one edge available
    172     const MB_MODE_INFO *edge_mbmi = has_above ? above_mbmi : left_mbmi;
    173 
    174     if (!has_second_ref(edge_mbmi))
    175       // edge does not use comp pred (0/1)
    176       ctx = IS_BACKWARD_REF_FRAME(edge_mbmi->ref_frame[0]);
    177     else
    178       // edge uses comp pred (3)
    179       ctx = 3;
    180   } else {  // no edges available (1)
    181     ctx = 1;
    182   }
    183   assert(ctx >= 0 && ctx < COMP_INTER_CONTEXTS);
    184   return ctx;
    185 }
    186 
    187 int av1_get_comp_reference_type_context(const MACROBLOCKD *xd) {
    188   int pred_context;
    189   const MB_MODE_INFO *const above_mbmi = xd->above_mbmi;
    190   const MB_MODE_INFO *const left_mbmi = xd->left_mbmi;
    191   const int above_in_image = xd->up_available;
    192   const int left_in_image = xd->left_available;
    193 
    194   if (above_in_image && left_in_image) {  // both edges available
    195     const int above_intra = !is_inter_block(above_mbmi);
    196     const int left_intra = !is_inter_block(left_mbmi);
    197 
    198     if (above_intra && left_intra) {  // intra/intra
    199       pred_context = 2;
    200     } else if (above_intra || left_intra) {  // intra/inter
    201       const MB_MODE_INFO *inter_mbmi = above_intra ? left_mbmi : above_mbmi;
    202 
    203       if (!has_second_ref(inter_mbmi))  // single pred
    204         pred_context = 2;
    205       else  // comp pred
    206         pred_context = 1 + 2 * has_uni_comp_refs(inter_mbmi);
    207     } else {  // inter/inter
    208       const int a_sg = !has_second_ref(above_mbmi);
    209       const int l_sg = !has_second_ref(left_mbmi);
    210       const MV_REFERENCE_FRAME frfa = above_mbmi->ref_frame[0];
    211       const MV_REFERENCE_FRAME frfl = left_mbmi->ref_frame[0];
    212 
    213       if (a_sg && l_sg) {  // single/single
    214         pred_context = 1 + 2 * (!(IS_BACKWARD_REF_FRAME(frfa) ^
    215                                   IS_BACKWARD_REF_FRAME(frfl)));
    216       } else if (l_sg || a_sg) {  // single/comp
    217         const int uni_rfc =
    218             a_sg ? has_uni_comp_refs(left_mbmi) : has_uni_comp_refs(above_mbmi);
    219 
    220         if (!uni_rfc)  // comp bidir
    221           pred_context = 1;
    222         else  // comp unidir
    223           pred_context = 3 + (!(IS_BACKWARD_REF_FRAME(frfa) ^
    224                                 IS_BACKWARD_REF_FRAME(frfl)));
    225       } else {  // comp/comp
    226         const int a_uni_rfc = has_uni_comp_refs(above_mbmi);
    227         const int l_uni_rfc = has_uni_comp_refs(left_mbmi);
    228 
    229         if (!a_uni_rfc && !l_uni_rfc)  // bidir/bidir
    230           pred_context = 0;
    231         else if (!a_uni_rfc || !l_uni_rfc)  // unidir/bidir
    232           pred_context = 2;
    233         else  // unidir/unidir
    234           pred_context =
    235               3 + (!((frfa == BWDREF_FRAME) ^ (frfl == BWDREF_FRAME)));
    236       }
    237     }
    238   } else if (above_in_image || left_in_image) {  // one edge available
    239     const MB_MODE_INFO *edge_mbmi = above_in_image ? above_mbmi : left_mbmi;
    240 
    241     if (!is_inter_block(edge_mbmi)) {  // intra
    242       pred_context = 2;
    243     } else {                           // inter
    244       if (!has_second_ref(edge_mbmi))  // single pred
    245         pred_context = 2;
    246       else  // comp pred
    247         pred_context = 4 * has_uni_comp_refs(edge_mbmi);
    248     }
    249   } else {  // no edges available
    250     pred_context = 2;
    251   }
    252 
    253   assert(pred_context >= 0 && pred_context < COMP_REF_TYPE_CONTEXTS);
    254   return pred_context;
    255 }
    256 
    257 // Returns a context number for the given MB prediction signal
    258 //
    259 // Signal the uni-directional compound reference frame pair as either
    260 // (BWDREF, ALTREF), or (LAST, LAST2) / (LAST, LAST3) / (LAST, GOLDEN),
    261 // conditioning on the pair is known as uni-directional.
    262 //
    263 // 3 contexts: Voting is used to compare the count of forward references with
    264 //             that of backward references from the spatial neighbors.
    265 int av1_get_pred_context_uni_comp_ref_p(const MACROBLOCKD *xd) {
    266   const uint8_t *const ref_counts = &xd->neighbors_ref_counts[0];
    267 
    268   // Count of forward references (L, L2, L3, or G)
    269   const int frf_count = ref_counts[LAST_FRAME] + ref_counts[LAST2_FRAME] +
    270                         ref_counts[LAST3_FRAME] + ref_counts[GOLDEN_FRAME];
    271   // Count of backward references (B or A)
    272   const int brf_count = ref_counts[BWDREF_FRAME] + ref_counts[ALTREF2_FRAME] +
    273                         ref_counts[ALTREF_FRAME];
    274 
    275   const int pred_context =
    276       (frf_count == brf_count) ? 1 : ((frf_count < brf_count) ? 0 : 2);
    277 
    278   assert(pred_context >= 0 && pred_context < UNI_COMP_REF_CONTEXTS);
    279   return pred_context;
    280 }
    281 
    282 // Returns a context number for the given MB prediction signal
    283 //
    284 // Signal the uni-directional compound reference frame pair as
    285 // either (LAST, LAST2), or (LAST, LAST3) / (LAST, GOLDEN),
    286 // conditioning on the pair is known as one of the above three.
    287 //
    288 // 3 contexts: Voting is used to compare the count of LAST2_FRAME with the
    289 //             total count of LAST3/GOLDEN from the spatial neighbors.
    290 int av1_get_pred_context_uni_comp_ref_p1(const MACROBLOCKD *xd) {
    291   const uint8_t *const ref_counts = &xd->neighbors_ref_counts[0];
    292 
    293   // Count of LAST2
    294   const int last2_count = ref_counts[LAST2_FRAME];
    295   // Count of LAST3 or GOLDEN
    296   const int last3_or_gld_count =
    297       ref_counts[LAST3_FRAME] + ref_counts[GOLDEN_FRAME];
    298 
    299   const int pred_context = (last2_count == last3_or_gld_count)
    300                                ? 1
    301                                : ((last2_count < last3_or_gld_count) ? 0 : 2);
    302 
    303   assert(pred_context >= 0 && pred_context < UNI_COMP_REF_CONTEXTS);
    304   return pred_context;
    305 }
    306 
    307 // Returns a context number for the given MB prediction signal
    308 //
    309 // Signal the uni-directional compound reference frame pair as
    310 // either (LAST, LAST3) or (LAST, GOLDEN),
    311 // conditioning on the pair is known as one of the above two.
    312 //
    313 // 3 contexts: Voting is used to compare the count of LAST3_FRAME with the
    314 //             total count of GOLDEN_FRAME from the spatial neighbors.
    315 int av1_get_pred_context_uni_comp_ref_p2(const MACROBLOCKD *xd) {
    316   const uint8_t *const ref_counts = &xd->neighbors_ref_counts[0];
    317 
    318   // Count of LAST3
    319   const int last3_count = ref_counts[LAST3_FRAME];
    320   // Count of GOLDEN
    321   const int gld_count = ref_counts[GOLDEN_FRAME];
    322 
    323   const int pred_context =
    324       (last3_count == gld_count) ? 1 : ((last3_count < gld_count) ? 0 : 2);
    325 
    326   assert(pred_context >= 0 && pred_context < UNI_COMP_REF_CONTEXTS);
    327   return pred_context;
    328 }
    329 
    330 // == Common context functions for both comp and single ref ==
    331 //
    332 // Obtain contexts to signal a reference frame to be either LAST/LAST2 or
    333 // LAST3/GOLDEN.
    334 static int get_pred_context_ll2_or_l3gld(const MACROBLOCKD *xd) {
    335   const uint8_t *const ref_counts = &xd->neighbors_ref_counts[0];
    336 
    337   // Count of LAST + LAST2
    338   const int last_last2_count = ref_counts[LAST_FRAME] + ref_counts[LAST2_FRAME];
    339   // Count of LAST3 + GOLDEN
    340   const int last3_gld_count =
    341       ref_counts[LAST3_FRAME] + ref_counts[GOLDEN_FRAME];
    342 
    343   const int pred_context = (last_last2_count == last3_gld_count)
    344                                ? 1
    345                                : ((last_last2_count < last3_gld_count) ? 0 : 2);
    346 
    347   assert(pred_context >= 0 && pred_context < REF_CONTEXTS);
    348   return pred_context;
    349 }
    350 
    351 // Obtain contexts to signal a reference frame to be either LAST or LAST2.
    352 static int get_pred_context_last_or_last2(const MACROBLOCKD *xd) {
    353   const uint8_t *const ref_counts = &xd->neighbors_ref_counts[0];
    354 
    355   // Count of LAST
    356   const int last_count = ref_counts[LAST_FRAME];
    357   // Count of LAST2
    358   const int last2_count = ref_counts[LAST2_FRAME];
    359 
    360   const int pred_context =
    361       (last_count == last2_count) ? 1 : ((last_count < last2_count) ? 0 : 2);
    362 
    363   assert(pred_context >= 0 && pred_context < REF_CONTEXTS);
    364   return pred_context;
    365 }
    366 
    367 // Obtain contexts to signal a reference frame to be either LAST3 or GOLDEN.
    368 static int get_pred_context_last3_or_gld(const MACROBLOCKD *xd) {
    369   const uint8_t *const ref_counts = &xd->neighbors_ref_counts[0];
    370 
    371   // Count of LAST3
    372   const int last3_count = ref_counts[LAST3_FRAME];
    373   // Count of GOLDEN
    374   const int gld_count = ref_counts[GOLDEN_FRAME];
    375 
    376   const int pred_context =
    377       (last3_count == gld_count) ? 1 : ((last3_count < gld_count) ? 0 : 2);
    378 
    379   assert(pred_context >= 0 && pred_context < REF_CONTEXTS);
    380   return pred_context;
    381 }
    382 
    383 // Obtain contexts to signal a reference frame be either BWDREF/ALTREF2, or
    384 // ALTREF.
    385 static int get_pred_context_brfarf2_or_arf(const MACROBLOCKD *xd) {
    386   const uint8_t *const ref_counts = &xd->neighbors_ref_counts[0];
    387 
    388   // Counts of BWDREF, ALTREF2, or ALTREF frames (B, A2, or A)
    389   const int brfarf2_count =
    390       ref_counts[BWDREF_FRAME] + ref_counts[ALTREF2_FRAME];
    391   const int arf_count = ref_counts[ALTREF_FRAME];
    392 
    393   const int pred_context =
    394       (brfarf2_count == arf_count) ? 1 : ((brfarf2_count < arf_count) ? 0 : 2);
    395 
    396   assert(pred_context >= 0 && pred_context < REF_CONTEXTS);
    397   return pred_context;
    398 }
    399 
    400 // Obtain contexts to signal a reference frame be either BWDREF or ALTREF2.
    401 static int get_pred_context_brf_or_arf2(const MACROBLOCKD *xd) {
    402   const uint8_t *const ref_counts = &xd->neighbors_ref_counts[0];
    403 
    404   // Count of BWDREF frames (B)
    405   const int brf_count = ref_counts[BWDREF_FRAME];
    406   // Count of ALTREF2 frames (A2)
    407   const int arf2_count = ref_counts[ALTREF2_FRAME];
    408 
    409   const int pred_context =
    410       (brf_count == arf2_count) ? 1 : ((brf_count < arf2_count) ? 0 : 2);
    411 
    412   assert(pred_context >= 0 && pred_context < REF_CONTEXTS);
    413   return pred_context;
    414 }
    415 
    416 // == Context functions for comp ref ==
    417 //
    418 // Returns a context number for the given MB prediction signal
    419 // Signal the first reference frame for a compound mode be either
    420 // GOLDEN/LAST3, or LAST/LAST2.
    421 int av1_get_pred_context_comp_ref_p(const MACROBLOCKD *xd) {
    422   return get_pred_context_ll2_or_l3gld(xd);
    423 }
    424 
    425 // Returns a context number for the given MB prediction signal
    426 // Signal the first reference frame for a compound mode be LAST,
    427 // conditioning on that it is known either LAST/LAST2.
    428 int av1_get_pred_context_comp_ref_p1(const MACROBLOCKD *xd) {
    429   return get_pred_context_last_or_last2(xd);
    430 }
    431 
    432 // Returns a context number for the given MB prediction signal
    433 // Signal the first reference frame for a compound mode be GOLDEN,
    434 // conditioning on that it is known either GOLDEN or LAST3.
    435 int av1_get_pred_context_comp_ref_p2(const MACROBLOCKD *xd) {
    436   return get_pred_context_last3_or_gld(xd);
    437 }
    438 
    439 // Signal the 2nd reference frame for a compound mode be either
    440 // ALTREF, or ALTREF2/BWDREF.
    441 int av1_get_pred_context_comp_bwdref_p(const MACROBLOCKD *xd) {
    442   return get_pred_context_brfarf2_or_arf(xd);
    443 }
    444 
    445 // Signal the 2nd reference frame for a compound mode be either
    446 // ALTREF2 or BWDREF.
    447 int av1_get_pred_context_comp_bwdref_p1(const MACROBLOCKD *xd) {
    448   return get_pred_context_brf_or_arf2(xd);
    449 }
    450 
    451 // == Context functions for single ref ==
    452 //
    453 // For the bit to signal whether the single reference is a forward reference
    454 // frame or a backward reference frame.
    455 int av1_get_pred_context_single_ref_p1(const MACROBLOCKD *xd) {
    456   const uint8_t *const ref_counts = &xd->neighbors_ref_counts[0];
    457 
    458   // Count of forward reference frames
    459   const int fwd_count = ref_counts[LAST_FRAME] + ref_counts[LAST2_FRAME] +
    460                         ref_counts[LAST3_FRAME] + ref_counts[GOLDEN_FRAME];
    461   // Count of backward reference frames
    462   const int bwd_count = ref_counts[BWDREF_FRAME] + ref_counts[ALTREF2_FRAME] +
    463                         ref_counts[ALTREF_FRAME];
    464 
    465   const int pred_context =
    466       (fwd_count == bwd_count) ? 1 : ((fwd_count < bwd_count) ? 0 : 2);
    467 
    468   assert(pred_context >= 0 && pred_context < REF_CONTEXTS);
    469   return pred_context;
    470 }
    471 
    472 // For the bit to signal whether the single reference is ALTREF_FRAME or
    473 // non-ALTREF backward reference frame, knowing that it shall be either of
    474 // these 2 choices.
    475 int av1_get_pred_context_single_ref_p2(const MACROBLOCKD *xd) {
    476   return get_pred_context_brfarf2_or_arf(xd);
    477 }
    478 
    479 // For the bit to signal whether the single reference is LAST3/GOLDEN or
    480 // LAST2/LAST, knowing that it shall be either of these 2 choices.
    481 int av1_get_pred_context_single_ref_p3(const MACROBLOCKD *xd) {
    482   return get_pred_context_ll2_or_l3gld(xd);
    483 }
    484 
    485 // For the bit to signal whether the single reference is LAST2_FRAME or
    486 // LAST_FRAME, knowing that it shall be either of these 2 choices.
    487 int av1_get_pred_context_single_ref_p4(const MACROBLOCKD *xd) {
    488   return get_pred_context_last_or_last2(xd);
    489 }
    490 
    491 // For the bit to signal whether the single reference is GOLDEN_FRAME or
    492 // LAST3_FRAME, knowing that it shall be either of these 2 choices.
    493 int av1_get_pred_context_single_ref_p5(const MACROBLOCKD *xd) {
    494   return get_pred_context_last3_or_gld(xd);
    495 }
    496 
    497 // For the bit to signal whether the single reference is ALTREF2_FRAME or
    498 // BWDREF_FRAME, knowing that it shall be either of these 2 choices.
    499 int av1_get_pred_context_single_ref_p6(const MACROBLOCKD *xd) {
    500   return get_pred_context_brf_or_arf2(xd);
    501 }
    502