avcodec/mpegvideo: Use offset instead of pointer for vbv_delay

An offset has the advantage of not needing to be updated
when the buffer is reallocated. Furthermore, the way the pointer
is currently updated is undefined behaviour in case the pointer
is not already set (i.e. when not encoding MPEG-1/2), because
it calculates the nonsense NULL - s->pb.buf.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

3 files changed, 9 insertions, 10 deletions

diff --git a/libavcodec/mpeg12enc.c b/libavcodec/mpeg12enc.c
index ecb90d1a41..9c0be69ded 100644
--- a/libavcodec/mpeg12enc.c
+++ b/libavcodec/mpeg12enc.c
@@ -465,7 +465,7 @@ void ff_mpeg1_encode_picture_header(MpegEncContext *s, int picture_number)
              (s->picture_number - mpeg12->gop_picture_number) & 0x3ff);
     put_bits(&s->pb, 3, s->pict_type);
 
-    s->vbv_delay_ptr = s->pb.buf + put_bytes_count(&s->pb, 0);
+    s->vbv_delay_pos = put_bytes_count(&s->pb, 0);
     put_bits(&s->pb, 16, 0xFFFF);               /* vbv_delay */
 
     // RAL: Forward f_code also needed for B-frames
diff --git a/libavcodec/mpegvideo.h b/libavcodec/mpegvideo.h
index 86954348f0..84d79d2e59 100644
--- a/libavcodec/mpegvideo.h
+++ b/libavcodec/mpegvideo.h
@@ -432,7 +432,7 @@ typedef struct MpegEncContext {
 
     /* MPEG-1 specific */
     int last_mv_dir;         ///< last mv_dir, used for B-frame encoding
-    uint8_t *vbv_delay_ptr;  ///< pointer to vbv_delay in the bitstream
+    int vbv_delay_pos;       ///< offset of vbv_delay in the bitstream
 
     /* MPEG-2-specific - I wished not to have to support this mess. */
     int progressive_sequence;
diff --git a/libavcodec/mpegvideo_enc.c b/libavcodec/mpegvideo_enc.c
index ead78e13ad..33f2937217 100644
--- a/libavcodec/mpegvideo_enc.c
+++ b/libavcodec/mpegvideo_enc.c
@@ -1840,8 +1840,9 @@ vbv_retry:
             double inbits  = avctx->rc_max_rate *
                              av_q2d(avctx->time_base);
             int    minbits = s->frame_bits - 8 *
-                             (s->vbv_delay_ptr - s->pb.buf - 1);
+                             (s->vbv_delay_pos - 1);
             double bits    = s->rc_context.buffer_index + minbits - inbits;
+            uint8_t *const vbv_delay_ptr = s->pb.buf + s->vbv_delay_pos;
 
             if (bits < 0)
                 av_log(avctx, AV_LOG_ERROR,
@@ -1857,11 +1858,11 @@ vbv_retry:
 
             av_assert0(vbv_delay < 0xFFFF);
 
-            s->vbv_delay_ptr[0] &= 0xF8;
-            s->vbv_delay_ptr[0] |= vbv_delay >> 13;
-            s->vbv_delay_ptr[1]  = vbv_delay >> 5;
-            s->vbv_delay_ptr[2] &= 0x07;
-            s->vbv_delay_ptr[2] |= vbv_delay << 3;
+            vbv_delay_ptr[0] &= 0xF8;
+            vbv_delay_ptr[0] |= vbv_delay >> 13;
+            vbv_delay_ptr[1]  = vbv_delay >> 5;
+            vbv_delay_ptr[2] &= 0x07;
+            vbv_delay_ptr[2] |= vbv_delay << 3;
 
             props = av_cpb_properties_alloc(&props_size);
             if (!props)
@@ -2721,7 +2722,6 @@ int ff_mpv_reallocate_putbitbuffer(MpegEncContext *s, size_t threshold, size_t s
         && s->slice_context_count == 1
         && s->pb.buf == s->avctx->internal->byte_buffer) {
         int lastgob_pos = s->ptr_lastgob - s->pb.buf;
-        int vbv_pos     = s->vbv_delay_ptr - s->pb.buf;
 
         uint8_t *new_buffer = NULL;
         int new_buffer_size = 0;
@@ -2744,7 +2744,6 @@ int ff_mpv_reallocate_putbitbuffer(MpegEncContext *s, size_t threshold, size_t s
         s->avctx->internal->byte_buffer_size = new_buffer_size;
         rebase_put_bits(&s->pb, new_buffer, new_buffer_size);
         s->ptr_lastgob   = s->pb.buf + lastgob_pos;
-        s->vbv_delay_ptr = s->pb.buf + vbv_pos;
     }
     if (put_bytes_left(&s->pb, 0) < threshold)
         return AVERROR(EINVAL);