optional non spec compliant optimizations for mpeg1

Originally committed as revision 3430 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpeg12.c

@@ -64,6 +64,7 @@ static inline int mpeg1_decode_block_inter(MpegEncContext *s,
 static inline int mpeg1_decode_block_intra(MpegEncContext *s, 
                               DCTELEM *block, 
                               int n);
+static inline int mpeg1_fast_decode_block_inter(MpegEncContext *s, DCTELEM *block, int n);
 static inline int mpeg2_decode_block_non_intra(MpegEncContext *s, 
                                         DCTELEM *block, 
                                         int n);
@@ -1382,14 +1383,25 @@ static int mpeg_decode_mb(MpegEncContext *s,
                     }
                 }
             } else {
-                for(i=0;i<6;i++) {
-                    if (cbp & 32) {
-                        if (mpeg1_decode_block_inter(s, s->pblocks[i], i) < 0)
-                            return -1;
-                    } else {
-                        s->block_last_index[i] = -1;
+                if(s->flags2 & CODEC_FLAG2_FAST){
+                    for(i=0;i<6;i++) {
+                        if (cbp & 32) {
+                            mpeg1_fast_decode_block_inter(s, s->pblocks[i], i);
+                        } else {
+                            s->block_last_index[i] = -1;
+                        }
+                        cbp+=cbp;
+                    }
+                }else{
+                    for(i=0;i<6;i++) {
+                        if (cbp & 32) {
+                            if (mpeg1_decode_block_inter(s, s->pblocks[i], i) < 0)
+                                return -1;
+                        } else {
+                            s->block_last_index[i] = -1;
+                        }
+                        cbp+=cbp;
                     }
-                    cbp+=cbp;
                 }
             }
         }else{
@@ -1604,6 +1616,76 @@ static inline int mpeg1_decode_block_inter(MpegEncContext *s,
     return 0;
 }
 
+static inline int mpeg1_fast_decode_block_inter(MpegEncContext *s, DCTELEM *block, int n)
+{
+    int level, i, j, run;
+    RLTable *rl = &rl_mpeg1;
+    uint8_t * const scantable= s->intra_scantable.permutated;
+    const int qscale= s->qscale;
+
+    {
+        int v;
+        OPEN_READER(re, &s->gb);
+        i = -1;
+        /* special case for the first coef. no need to add a second vlc table */
+        UPDATE_CACHE(re, &s->gb);
+        v= SHOW_UBITS(re, &s->gb, 2);
+        if (v & 2) {
+            LAST_SKIP_BITS(re, &s->gb, 2);
+            level= (3*qscale)>>4;
+            level= (level-1)|1;
+            if(v&1)
+                level= -level;
+            block[0] = level;
+            i++;
+        }
+
+        /* now quantify & encode AC coefs */
+        for(;;) {
+            UPDATE_CACHE(re, &s->gb);
+            GET_RL_VLC(level, run, re, &s->gb, rl->rl_vlc[0], TEX_VLC_BITS, 2);
+            
+            if(level == 127){
+                break;
+            } else if(level != 0) {
+                i += run;
+                j = scantable[i];
+                level= ((level*2+1)*qscale)>>1;
+                level= (level-1)|1;
+                level = (level ^ SHOW_SBITS(re, &s->gb, 1)) - SHOW_SBITS(re, &s->gb, 1);
+                LAST_SKIP_BITS(re, &s->gb, 1);
+            } else {
+                /* escape */
+                run = SHOW_UBITS(re, &s->gb, 6)+1; LAST_SKIP_BITS(re, &s->gb, 6);
+                UPDATE_CACHE(re, &s->gb);
+                level = SHOW_SBITS(re, &s->gb, 8); SKIP_BITS(re, &s->gb, 8);
+                if (level == -128) {
+                    level = SHOW_UBITS(re, &s->gb, 8) - 256; LAST_SKIP_BITS(re, &s->gb, 8);
+                } else if (level == 0) {
+                    level = SHOW_UBITS(re, &s->gb, 8)      ; LAST_SKIP_BITS(re, &s->gb, 8);
+                }
+                i += run;
+                j = scantable[i];
+                if(level<0){
+                    level= -level;
+                    level= ((level*2+1)*qscale)>>1;
+                    level= (level-1)|1;
+                    level= -level;
+                }else{
+                    level= ((level*2+1)*qscale)>>1;
+                    level= (level-1)|1;
+                }
+            }
+
+            block[j] = level;
+        }
+        CLOSE_READER(re, &s->gb);
+    }
+    s->block_last_index[n] = i;
+    return 0;
+}
+
+
 static inline int mpeg2_decode_block_non_intra(MpegEncContext *s, 
                                DCTELEM *block, 
                                int n)