cbs_mpeg2: Improve performance of writing slices

Instead of using a combination of bitreader and -writer for copying data,
one can byte-align the (obsolete and removed) bitreader to improve performance.
One can even use memcpy in the normal case.
This improved the time needed for writing the slicedata from 33618 to
2370 decicycles when tested on a video originating from a DVD (4194394
runs).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@googlemail.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>

libavcodec/cbs_mpeg2.c

@@ -264,8 +264,6 @@ static int cbs_mpeg2_write_slice(CodedBitstreamContext *ctx,
                                  PutBitContext *pbc)
 {
     MPEG2RawSlice *slice = unit->content;
-    GetBitContext gbc;
-    size_t bits_left;
     int err;
 
     err = cbs_mpeg2_write_slice_header(ctx, pbc, &slice->header);
@@ -273,21 +271,38 @@ static int cbs_mpeg2_write_slice(CodedBitstreamContext *ctx,
         return err;
 
     if (slice->data) {
+        size_t rest = slice->data_size - (slice->data_bit_start + 7) / 8;
+        uint8_t *pos = slice->data + slice->data_bit_start / 8;
+
+        av_assert0(slice->data_bit_start >= 0 &&
+                   8 * slice->data_size > slice->data_bit_start);
+
         if (slice->data_size * 8 + 8 > put_bits_left(pbc))
             return AVERROR(ENOSPC);
 
-        init_get_bits(&gbc, slice->data, slice->data_size * 8);
-        skip_bits_long(&gbc, slice->data_bit_start);
-
-        while (get_bits_left(&gbc) > 15)
-            put_bits(pbc, 16, get_bits(&gbc, 16));
+        // First copy the remaining bits of the first byte
+        if (slice->data_bit_start % 8)
+            put_bits(pbc, 8 - slice->data_bit_start % 8,
+                     *pos++ & MAX_UINT_BITS(8 - slice->data_bit_start % 8));
+
+        if (put_bits_count(pbc) % 8 == 0) {
+            // If the writer is aligned at this point,
+            // memcpy can be used to improve performance.
+            // This is the normal case.
+            flush_put_bits(pbc);
+            memcpy(put_bits_ptr(pbc), pos, rest);
+            skip_put_bytes(pbc, rest);
+        } else {
+            // If not, we have to copy manually:
+            for (; rest > 3; rest -= 4, pos += 4)
+                put_bits32(pbc, AV_RB32(pos));
 
-        bits_left = get_bits_left(&gbc);
-        put_bits(pbc, bits_left, get_bits(&gbc, bits_left));
+            for (; rest; rest--, pos++)
+                put_bits(pbc, 8, *pos);
 
-        // Align with zeroes.
-        while (put_bits_count(pbc) % 8 != 0)
-            put_bits(pbc, 1, 0);
+            // Align with zeros
+            put_bits(pbc, 8 - put_bits_count(pbc) % 8, 0);
+        }
     }
 
     return 0;