zlib/inffast.c

   1 /* inffast.c -- fast decoding
   2  * Copyright (C) 1995-2008, 2010, 2013 Mark Adler
   3  * For conditions of distribution and use, see copyright notice in zlib.h
   4  */
   5
   6 #include "zutil.h"
   7 #include "inftrees.h"
   8 #include "inflate.h"
   9 #include "inffast.h"
  10
  11 #ifndef ASMINF
  12
  13 /*
  14    Decode literal, length, and distance codes and write out the resulting
  15    literal and match bytes until either not enough input or output is
  16    available, an end-of-block is encountered, or a data error is encountered.
  17    When large enough input and output buffers are supplied to inflate(), for
  18    example, a 16K input buffer and a 64K output buffer, more than 95% of the
  19    inflate execution time is spent in this routine.
  20
  21    Entry assumptions:
  22
  23         state->mode == LEN
  24         strm->avail_in >= 6
  25         strm->avail_out >= 258
  26         start >= strm->avail_out
  27         state->bits < 8
  28
  29    On return, state->mode is one of:
  30
  31         LEN -- ran out of enough output space or enough available input
  32         TYPE -- reached end of block code, inflate() to interpret next block
  33         BAD -- error in block data
  34
  35    Notes:
  36
  37     - The maximum input bits used by a length/distance pair is 15 bits for the
  38       length code, 5 bits for the length extra, 15 bits for the distance code,
  39       and 13 bits for the distance extra.  This totals 48 bits, or six bytes.
  40       Therefore if strm->avail_in >= 6, then there is enough input to avoid
  41       checking for available input while decoding.
  42
  43     - The maximum bytes that a single length/distance pair can output is 258
  44       bytes, which is the maximum length that can be coded.  inflate_fast()
  45       requires strm->avail_out >= 258 for each loop to avoid checking for
  46       output space.
  47  */
  48 void ZLIB_INTERNAL inflate_fast(strm, start)
  49 z_streamp strm;
  50 unsigned start;         /* inflate()'s starting value for strm->avail_out */
  51 {
  52     struct inflate_state FAR *state;
  53     z_const unsigned char FAR *in;      /* local strm->next_in */
  54     z_const unsigned char FAR *last;    /* have enough input while in < last */
  55     unsigned char FAR *out;     /* local strm->next_out */
  56     unsigned char FAR *beg;     /* inflate()'s initial strm->next_out */
  57     unsigned char FAR *end;     /* while out < end, enough space available */
  58 #ifdef INFLATE_STRICT
  59     unsigned dmax;              /* maximum distance from zlib header */
  60 #endif
  61     unsigned wsize;             /* window size or zero if not using window */
  62     unsigned whave;             /* valid bytes in the window */
  63     unsigned wnext;             /* window write index */
  64     unsigned char FAR *window;  /* allocated sliding window, if wsize != 0 */
  65     unsigned long hold;         /* local strm->hold */
  66     unsigned bits;              /* local strm->bits */
  67     code const FAR *lcode;      /* local strm->lencode */
  68     code const FAR *dcode;      /* local strm->distcode */
  69     unsigned lmask;             /* mask for first level of length codes */
  70     unsigned dmask;             /* mask for first level of distance codes */
  71     code here;                  /* retrieved table entry */
  72     unsigned op;                /* code bits, operation, extra bits, or */
  73                                 /*  window position, window bytes to copy */
  74     unsigned len;               /* match length, unused bytes */
  75     unsigned dist;              /* match distance */
  76     unsigned char FAR *from;    /* where to copy match from */
  77
  78     /* copy state to local variables */
  79     state = (struct inflate_state FAR *)strm->state;
  80     in = strm->next_in;
  81     last = in + (strm->avail_in - 5);
  82     out = strm->next_out;
  83     beg = out - (start - strm->avail_out);
  84     end = out + (strm->avail_out - 257);
  85 #ifdef INFLATE_STRICT
  86     dmax = state->dmax;
  87 #endif
  88     wsize = state->wsize;
  89     whave = state->whave;
  90     wnext = state->wnext;
  91     window = state->window;
  92     hold = state->hold;
  93     bits = state->bits;
  94     lcode = state->lencode;
  95     dcode = state->distcode;
  96     lmask = (1U << state->lenbits) - 1;
  97     dmask = (1U << state->distbits) - 1;
  98
  99     /* decode literals and length/distances until end-of-block or not enough
 100        input data or output space */
 101     do {
 102         if (bits < 15) {
 103             hold += (unsigned long)(*in++) << bits;
 104             bits += 8;
 105             hold += (unsigned long)(*in++) << bits;
 106             bits += 8;
 107         }
 108         here = lcode[hold & lmask];
 109       dolen:
 110         op = (unsigned)(here.bits);
 111         hold >>= op;
 112         bits -= op;
 113         op = (unsigned)(here.op);
 114         if (op == 0) {                          /* literal */
 115             Tracevv((stderr, here.val >= 0x20 && here.val < 0x7f ?
 116                     "inflate:         literal '%c'\n" :
 117                     "inflate:         literal 0x%02x\n", here.val));
 118             *out++ = (unsigned char)(here.val);
 119         }
 120         else if (op & 16) {                     /* length base */
 121             len = (unsigned)(here.val);
 122             op &= 15;                           /* number of extra bits */
 123             if (op) {
 124                 if (bits < op) {
 125                     hold += (unsigned long)(*in++) << bits;
 126                     bits += 8;
 127                 }
 128                 len += (unsigned)hold & ((1U << op) - 1);
 129                 hold >>= op;
 130                 bits -= op;
 131             }
 132             Tracevv((stderr, "inflate:         length %u\n", len));
 133             if (bits < 15) {
 134                 hold += (unsigned long)(*in++) << bits;
 135                 bits += 8;
 136                 hold += (unsigned long)(*in++) << bits;
 137                 bits += 8;
 138             }
 139             here = dcode[hold & dmask];
 140           dodist:
 141             op = (unsigned)(here.bits);
 142             hold >>= op;
 143             bits -= op;
 144             op = (unsigned)(here.op);
 145             if (op & 16) {                      /* distance base */
 146                 dist = (unsigned)(here.val);
 147                 op &= 15;                       /* number of extra bits */
 148                 if (bits < op) {
 149                     hold += (unsigned long)(*in++) << bits;
 150                     bits += 8;
 151                     if (bits < op) {
 152                         hold += (unsigned long)(*in++) << bits;
 153                         bits += 8;
 154                     }
 155                 }
 156                 dist += (unsigned)hold & ((1U << op) - 1);
 157 #ifdef INFLATE_STRICT
 158                 if (dist > dmax) {
 159                     strm->msg = (char *)"invalid distance too far back";
 160                     state->mode = BAD;
 161                     break;
 162                 }
 163 #endif
 164                 hold >>= op;
 165                 bits -= op;
 166                 Tracevv((stderr, "inflate:         distance %u\n", dist));
 167                 op = (unsigned)(out - beg);     /* max distance in output */
 168                 if (dist > op) {                /* see if copy from window */
 169                     op = dist - op;             /* distance back in window */
 170                     if (op > whave) {
 171                         if (state->sane) {
 172                             strm->msg =
 173                                 (char *)"invalid distance too far back";
 174                             state->mode = BAD;
 175                             break;
 176                         }
 177 #ifdef INFLATE_ALLOW_INVALID_DISTANCE_TOOFAR_ARRR
 178                         if (len <= op - whave) {
 179                             do {
 180                                 *out++ = 0;
 181                             } while (--len);
 182                             continue;
 183                         }
 184                         len -= op - whave;
 185                         do {
 186                             *out++ = 0;
 187                         } while (--op > whave);
 188                         if (op == 0) {
 189                             from = out - dist;
 190                             do {
 191                                 *out++ = *from++;
 192                             } while (--len);
 193                             continue;
 194                         }
 195 #endif
 196                     }
 197                     from = window;
 198                     if (wnext == 0) {           /* very common case */
 199                         from += wsize - op;
 200                         if (op < len) {         /* some from window */
 201                             len -= op;
 202                             do {
 203                                 *out++ = *from++;
 204                             } while (--op);
 205                             from = out - dist;  /* rest from output */
 206                         }
 207                     }
 208                     else if (wnext < op) {      /* wrap around window */
 209                         from += wsize + wnext - op;
 210                         op -= wnext;
 211                         if (op < len) {         /* some from end of window */
 212                             len -= op;
 213                             do {
 214                                 *out++ = *from++;
 215                             } while (--op);
 216                             from = window;
 217                             if (wnext < len) {  /* some from start of window */
 218                                 op = wnext;
 219                                 len -= op;
 220                                 do {
 221                                     *out++ = *from++;
 222                                 } while (--op);
 223                                 from = out - dist;      /* rest from output */
 224                             }
 225                         }
 226                     }
 227                     else {                      /* contiguous in window */
 228                         from += wnext - op;
 229                         if (op < len) {         /* some from window */
 230                             len -= op;
 231                             do {
 232                                 *out++ = *from++;
 233                             } while (--op);
 234                             from = out - dist;  /* rest from output */
 235                         }
 236                     }
 237                     while (len > 2) {
 238                         *out++ = *from++;
 239                         *out++ = *from++;
 240                         *out++ = *from++;
 241                         len -= 3;
 242                     }
 243                     if (len) {
 244                         *out++ = *from++;
 245                         if (len > 1)
 246                             *out++ = *from++;
 247                     }
 248                 }
 249                 else {
 250                     from = out - dist;          /* copy direct from output */
 251                     do {                        /* minimum length is three */
 252                         *out++ = *from++;
 253                         *out++ = *from++;
 254                         *out++ = *from++;
 255                         len -= 3;
 256                     } while (len > 2);
 257                     if (len) {
 258                         *out++ = *from++;
 259                         if (len > 1)
 260                             *out++ = *from++;
 261                     }
 262                 }
 263             }
 264             else if ((op & 64) == 0) {          /* 2nd level distance code */
 265                 here = dcode[here.val + (hold & ((1U << op) - 1))];
 266                 goto dodist;
 267             }
 268             else {
 269                 strm->msg = (char *)"invalid distance code";
 270                 state->mode = BAD;
 271                 break;
 272             }
 273         }
 274         else if ((op & 64) == 0) {              /* 2nd level length code */
 275             here = lcode[here.val + (hold & ((1U << op) - 1))];
 276             goto dolen;
 277         }
 278         else if (op & 32) {                     /* end-of-block */
 279             Tracevv((stderr, "inflate:         end of block\n"));
 280             state->mode = TYPE;
 281             break;
 282         }
 283         else {
 284             strm->msg = (char *)"invalid literal/length code";
 285             state->mode = BAD;
 286             break;
 287         }
 288     } while (in < last && out < end);
 289
 290     /* return unused bytes (on entry, bits < 8, so in won't go too far back) */
 291     len = bits >> 3;
 292     in -= len;
 293     bits -= len << 3;
 294     hold &= (1U << bits) - 1;
 295
 296     /* update state and return */
 297     strm->next_in = in;
 298     strm->next_out = out;
 299     strm->avail_in = (unsigned)(in < last ? 5 + (last - in) : 5 - (in - last));
 300     strm->avail_out = (unsigned)(out < end ?
 301                                  257 + (end - out) : 257 - (out - end));
 302     state->hold = hold;
 303     state->bits = bits;
 304     return;
 305 }
 306
 307 /*
 308    inflate_fast() speedups that turned out slower (on a PowerPC G3 750CXe):
 309    - Using bit fields for code structure
 310    - Different op definition to avoid & for extra bits (do & for table bits)
 311    - Three separate decoding do-loops for direct, window, and wnext == 0
 312    - Special case for distance > 1 copies to do overlapped load and store copy
 313    - Explicit branch predictions (based on measured branch probabilities)
 314    - Deferring match copy and interspersed it with decoding subsequent codes
 315    - Swapping literal/length else
 316    - Swapping window/direct else
 317    - Larger unrolled copy loops (three is about right)
 318    - Moving len -= 3 statement into middle of loop
 319  */
 320
 321 #endif /* !ASMINF */