H264 Decoder: move cut off logic from Construct to ModifyRefPicList().

content/common/gpu/media/h264_decoder.cc

Index: content/common/gpu/media/h264_decoder.cc
diff --git a/content/common/gpu/media/h264_decoder.cc b/content/common/gpu/media/h264_decoder.cc
index 9953ff21a593d28c9508f51715091f8b10f81203..07466e74d49d3f3384488ccdab6b476b0cdd31b5 100644
--- a/content/common/gpu/media/h264_decoder.cc
+++ b/content/common/gpu/media/h264_decoder.cc
@@ -362,9 +362,6 @@ void H264Decoder::ConstructReferencePicListsP(
   dpb_.GetLongTermRefPicsAppending(&ref_pic_list_p0_);
   std::sort(ref_pic_list_p0_.begin() + num_short_refs, ref_pic_list_p0_.end(),
             LongTermPicNumAscCompare());
-
-  // Cut off if we have more than requested in slice header.
-  ref_pic_list_p0_.resize(slice_hdr->num_ref_idx_l0_active_minus1 + 1);
 }
 
 struct POCAscCompare {
@@ -436,12 +433,6 @@ void H264Decoder::ConstructReferencePicListsB(
       std::equal(ref_pic_list_b0_.begin(), ref_pic_list_b0_.end(),
                  ref_pic_list_b1_.begin()))
     std::swap(ref_pic_list_b1_[0], ref_pic_list_b1_[1]);
-
-  // Per 8.2.4.2 it's possible for num_ref_idx_lX_active_minus1 to indicate
-  // there should be more ref pics on list than we constructed.
-  // Those superfluous ones should be treated as non-reference.
-  ref_pic_list_b0_.resize(slice_hdr->num_ref_idx_l0_active_minus1 + 1);
-  ref_pic_list_b1_.resize(slice_hdr->num_ref_idx_l1_active_minus1 + 1);
 }
 
 // See 8.2.4
@@ -492,6 +483,15 @@ bool H264Decoder::ModifyReferencePicList(media::H264SliceHeader* slice_hdr,
   int num_ref_idx_lX_active_minus1;
   media::H264ModificationOfPicNum* list_mod;
 
+  // Cut off if we have more than requested in slice header.

[Pawel Osciak, 2015/09/02 03:34:58]

Hm, this should probably say resize instead of cut off...

How about:

Resize the list to the size requested in the slice header.
Note that per 8.2.4.2 it's possible for num_ref_idx_lX_active_minus1 to
indicate there should be more ref pics on list than we constructed.
Those superfluous ones should be treated as non-reference and will be
initialized to nullptr, which must be handled by clients.

[hshi1, 2015/09/02 03:49:25]

Done.

+  // Note that per 8.2.4.2 it's possible for num_ref_idx_lX_active_minus1 to
+  // indicate there should be more ref pics on list than we constructed.
+  // Those superfluous ones should be treated as non-reference.
+  num_ref_idx_lX_active_minus1 = list == 0
+                                     ? slice_hdr->num_ref_idx_l0_active_minus1
+                                     : slice_hdr->num_ref_idx_l1_active_minus1;
+  ref_pic_listx->resize(num_ref_idx_lX_active_minus1 + 1);
+

[Pawel Osciak, 2015/09/02 03:34:58]

I'm on the fence here, so completely up to you, but what would you think about:

  bool ref_pic_list_modification_flag_lX;
  size_t num_ref_idx_lX_active_minus1;

  if (list == 0) {
    ref_pic_list_modification_flag_lX =
        slice_hdr->ref_pic_list_modification_flag_l0;
    num_ref_idx_lX_active_minus1 =
        slice_hdr->num_ref_idx_l0_active_minus1;
    list_mod = slice_hdr->ref_list_l0_modifications;
  } else {
    ref_pic_list_modification_flag_lX =
        slice_hdr->ref_pic_list_modification_flag_l1;
    num_ref_idx_lX_active_minus1 =
        slice_hdr->num_ref_idx_l1_active_minus1;
    list_mod = slice_hdr->ref_list_l1_modifications;
  }

  // Comment...
  ref_pic_listx->resize(num_ref_idx_lX_active_minus1 + 1);

  if (!ref_pic_list_modification_flag_lX)
    return true;

[hshi1, 2015/09/02 03:49:25]

Done.

   // This can process either ref_pic_list0 or ref_pic_list1, depending on
   // the list argument. Set up pointers to proper list to be processed here.
   if (list == 0) {
@@ -506,9 +506,6 @@ bool H264Decoder::ModifyReferencePicList(media::H264SliceHeader* slice_hdr,
     list_mod = slice_hdr->ref_list_l1_modifications;
   }
 
-  num_ref_idx_lX_active_minus1 = ref_pic_listx->size() - 1;
-  DCHECK_GE(num_ref_idx_lX_active_minus1, 0);
-
   // Spec 8.2.4.3:
   // Reorder pictures on the list in a way specified in the stream.
   int pic_num_lx_pred = curr_pic_->pic_num;