zlib/inffast.c

   1 /* inffast.c -- fast decoding
   2  * Copyright (C) 1995-2003 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 /* Allow machine dependent optimization for post-increment or pre-increment.
  14    Based on testing to date,
  15    Pre-increment preferred for:
  16    - PowerPC G3 (Adler)
  17    - MIPS R5000 (Randers-Pehrson)
  18    Post-increment preferred for:
  19    - none
  20    No measurable difference:
  21    - Pentium III (Anderson)
  22    - 68060 (Nikl)
  23  */
  24 #ifdef POSTINC
  25 #  define OFF 0
  26 #  define PUP(a) *(a)++
  27 #else
  28 #  define OFF 1
  29 #  define PUP(a) *++(a)
  30 #endif
  31
  32 /*
  33    Decode literal, length, and distance codes and write out the resulting
  34    literal and match bytes until either not enough input or output is
  35    available, an end-of-block is encountered, or a data error is encountered.
  36    When large enough input and output buffers are supplied to inflate(), for
  37    example, a 16K input buffer and a 64K output buffer, more than 95% of the
  38    inflate execution time is spent in this routine.
  39
  40    Entry assumptions:
  41
  42         state->mode == LEN
  43         strm->avail_in >= 6
  44         strm->avail_out >= 258
  45         start >= strm->avail_out
  46         state->bits < 8
  47
  48    On return, state->mode is one of:
  49
  50         LEN -- ran out of enough output space or enough available input
  51         TYPE -- reached end of block code, inflate() to interpret next block
  52         BAD -- error in block data
  53
  54    Notes:
  55
  56     - The maximum input bits used by a length/distance pair is 15 bits for the
  57       length code, 5 bits for the length extra, 15 bits for the distance code,
  58       and 13 bits for the distance extra.  This totals 48 bits, or six bytes.
  59       Therefore if strm->avail_in >= 6, then there is enough input to avoid
  60       checking for available input while decoding.
  61
  62     - The maximum bytes that a single length/distance pair can output is 258
  63       bytes, which is the maximum length that can be coded.  inflate_fast()
  64       requires strm->avail_out >= 258 for each loop to avoid checking for
  65       output space.
  66  */
  67 void inflate_fast(strm, start)
  68 z_streamp strm;
  69 unsigned start;         /* inflate()'s starting value for strm->avail_out */
  70 {
  71     struct inflate_state FAR *state;
  72     unsigned char FAR *in;      /* local strm->next_in */
  73     unsigned char FAR *last;    /* while in < last, enough input available */
  74     unsigned char FAR *out;     /* local strm->next_out */
  75     unsigned char FAR *beg;     /* inflate()'s initial strm->next_out */
  76     unsigned char FAR *end;     /* while out < end, enough space available */
  77     unsigned wsize;             /* window size or zero if not using window */
  78     unsigned whave;             /* valid bytes in the window */
  79     unsigned write;             /* window write index */
  80     unsigned char FAR *window;  /* allocated sliding window, if wsize != 0 */
  81     unsigned long hold;         /* local strm->hold */
  82     unsigned bits;              /* local strm->bits */
  83     code const FAR *lcode;      /* local strm->lencode */
  84     code const FAR *dcode;      /* local strm->distcode */
  85     unsigned lmask;             /* mask for first level of length codes */
  86     unsigned dmask;             /* mask for first level of distance codes */
  87     code this;                  /* retrieved table entry */
  88     unsigned op;                /* code bits, operation, extra bits, or */
  89                                 /*  window position, window bytes to copy */
  90     unsigned len;               /* match length, unused bytes */
  91     unsigned dist;              /* match distance */
  92     unsigned char FAR *from;    /* where to copy match from */
  93
  94     /* copy state to local variables */
  95     state = (struct inflate_state FAR *)strm->state;
  96     in = strm->next_in - OFF;
  97     last = in + (strm->avail_in - 5);
  98     out = strm->next_out - OFF;
  99     beg = out - (start - strm->avail_out);
 100     end = out + (strm->avail_out - 257);
 101     wsize = state->wsize;
 102     whave = state->whave;
 103     write = state->write;
 104     window = state->window;
 105     hold = state->hold;
 106     bits = state->bits;
 107     lcode = state->lencode;
 108     dcode = state->distcode;
 109     lmask = (1U << state->lenbits) - 1;
 110     dmask = (1U << state->distbits) - 1;
 111
 112     /* decode literals and length/distances until end-of-block or not enough
 113        input data or output space */
 114     do {
 115         if (bits < 15) {
 116             hold += (unsigned long)(PUP(in)) << bits;
 117             bits += 8;
 118             hold += (unsigned long)(PUP(in)) << bits;
 119             bits += 8;
 120         }
 121         this = lcode[hold & lmask];
 122       dolen:
 123         op = (unsigned)(this.bits);
 124         hold >>= op;
 125         bits -= op;
 126         op = (unsigned)(this.op);
 127         if (op == 0) {                          /* literal */
 128             Tracevv((stderr, this.val >= 0x20 && this.val < 0x7f ?
 129                     "inflate:         literal '%c'\n" :
 130                     "inflate:         literal 0x%02x\n", this.val));
 131             PUP(out) = (unsigned char)(this.val);
 132         }
 133         else if (op & 16) {                     /* length base */
 134             len = (unsigned)(this.val);
 135             op &= 15;                           /* number of extra bits */
 136             if (op) {
 137                 if (bits < op) {
 138                     hold += (unsigned long)(PUP(in)) << bits;
 139                     bits += 8;
 140                 }
 141                 len += (unsigned)hold & ((1U << op) - 1);
 142                 hold >>= op;
 143                 bits -= op;
 144             }
 145             Tracevv((stderr, "inflate:         length %u\n", len));
 146             if (bits < 15) {
 147                 hold += (unsigned long)(PUP(in)) << bits;
 148                 bits += 8;
 149                 hold += (unsigned long)(PUP(in)) << bits;
 150                 bits += 8;
 151             }
 152             this = dcode[hold & dmask];
 153           dodist:
 154             op = (unsigned)(this.bits);
 155             hold >>= op;
 156             bits -= op;
 157             op = (unsigned)(this.op);
 158             if (op & 16) {                      /* distance base */
 159                 dist = (unsigned)(this.val);
 160                 op &= 15;                       /* number of extra bits */
 161                 if (bits < op) {
 162                     hold += (unsigned long)(PUP(in)) << bits;
 163                     bits += 8;
 164                     if (bits < op) {
 165                         hold += (unsigned long)(PUP(in)) << bits;
 166                         bits += 8;
 167                     }
 168                 }
 169                 dist += (unsigned)hold & ((1U << op) - 1);
 170                 hold >>= op;
 171                 bits -= op;
 172                 Tracevv((stderr, "inflate:         distance %u\n", dist));
 173                 op = (unsigned)(out - beg);     /* max distance in output */
 174                 if (dist > op) {                /* see if copy from window */
 175                     op = dist - op;             /* distance back in window */
 176                     if (op > whave) {
 177                         strm->msg = (char *)"invalid distance too far back";
 178                         state->mode = BAD;
 179                         break;
 180                     }
 181                     from = window - OFF;
 182                     if (write == 0) {           /* very common case */
 183                         from += wsize - op;
 184                         if (op < len) {         /* some from window */
 185                             len -= op;
 186                             do {
 187                                 PUP(out) = PUP(from);
 188                             } while (--op);
 189                             from = out - dist;  /* rest from output */
 190                         }
 191                     }
 192                     else if (write < op) {      /* wrap around window */
 193                         from += wsize + write - op;
 194                         op -= write;
 195                         if (op < len) {         /* some from end of window */
 196                             len -= op;
 197                             do {
 198                                 PUP(out) = PUP(from);
 199                             } while (--op);
 200                             from = window - OFF;
 201                             if (write < len) {  /* some from start of window */
 202                                 op = write;
 203                                 len -= op;
 204                                 do {
 205                                     PUP(out) = PUP(from);
 206                                 } while (--op);
 207                                 from = out - dist;      /* rest from output */
 208                             }
 209                         }
 210                     }
 211                     else {                      /* contiguous in window */
 212                         from += write - op;
 213                         if (op < len) {         /* some from window */
 214                             len -= op;
 215                             do {
 216                                 PUP(out) = PUP(from);
 217                             } while (--op);
 218                             from = out - dist;  /* rest from output */
 219                         }
 220                     }
 221                     while (len > 2) {
 222                         PUP(out) = PUP(from);
 223                         PUP(out) = PUP(from);
 224                         PUP(out) = PUP(from);
 225                         len -= 3;
 226                     }
 227                     if (len) {
 228                         PUP(out) = PUP(from);
 229                         if (len > 1)
 230                             PUP(out) = PUP(from);
 231                     }
 232                 }
 233                 else {
 234                     from = out - dist;          /* copy direct from output */
 235                     do {                        /* minimum length is three */
 236                         PUP(out) = PUP(from);
 237                         PUP(out) = PUP(from);
 238                         PUP(out) = PUP(from);
 239                         len -= 3;
 240                     } while (len > 2);
 241                     if (len) {
 242                         PUP(out) = PUP(from);
 243                         if (len > 1)
 244                             PUP(out) = PUP(from);
 245                     }
 246                 }
 247             }
 248             else if ((op & 64) == 0) {          /* 2nd level distance code */
 249                 this = dcode[this.val + (hold & ((1U << op) - 1))];
 250                 goto dodist;
 251             }
 252             else {
 253                 strm->msg = (char *)"invalid distance code";
 254                 state->mode = BAD;
 255                 break;
 256             }
 257         }
 258         else if ((op & 64) == 0) {              /* 2nd level length code */
 259             this = lcode[this.val + (hold & ((1U << op) - 1))];
 260             goto dolen;
 261         }
 262         else if (op & 32) {                     /* end-of-block */
 263             Tracevv((stderr, "inflate:         end of block\n"));
 264             state->mode = TYPE;
 265             break;
 266         }
 267         else {
 268             strm->msg = (char *)"invalid literal/length code";
 269             state->mode = BAD;
 270             break;
 271         }
 272     } while (in < last && out < end);
 273
 274     /* return unused bytes (on entry, bits < 8, so in won't go too far back) */
 275     len = bits >> 3;
 276     in -= len;
 277     bits -= len << 3;
 278     hold &= (1U << bits) - 1;
 279
 280     /* update state and return */
 281     strm->next_in = in + OFF;
 282     strm->next_out = out + OFF;
 283     strm->avail_in = (unsigned)(in < last ? 5 + (last - in) : 5 - (in - last));
 284     strm->avail_out = (unsigned)(out < end ?
 285                                  257 + (end - out) : 257 - (out - end));
 286     state->hold = hold;
 287     state->bits = bits;
 288     return;
 289 }
 290
 291 /*
 292    inflate_fast() speedups that turned out slower (on a PowerPC G3 750CXe):
 293    - Using bit fields for code structure
 294    - Different op definition to avoid & for extra bits (do & for table bits)
 295    - Three separate decoding do-loops for direct, window, and write == 0
 296    - Special case for distance > 1 copies to do overlapped load and store copy
 297    - Explicit branch predictions (based on measured branch probabilities)
 298    - Deferring match copy and interspersed it with decoding subsequent codes
 299    - Swapping literal/length else
 300    - Swapping window/direct else
 301    - Larger unrolled copy loops (three is about right)
 302    - Moving len -= 3 statement into middle of loop
 303  */
 304
 305 #endif /* !ASMINF */