string/memcmp.c

   1 /* Copyright (C) 1991-2015 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Torbjorn Granlund (tege@sics.se).
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #ifdef HAVE_CONFIG_H
  20 # include "config.h"
  21 #endif
  22
  23 #undef  __ptr_t
  24 #define __ptr_t void *
  25
  26 #if defined HAVE_STRING_H || defined _LIBC
  27 # include <string.h>
  28 #endif
  29
  30 #undef memcmp
  31
  32 #ifndef MEMCMP
  33 # define MEMCMP memcmp
  34 #endif
  35
  36 #ifdef _LIBC
  37
  38 # include <memcopy.h>
  39 # include <endian.h>
  40
  41 # if __BYTE_ORDER == __BIG_ENDIAN
  42 #  define WORDS_BIGENDIAN
  43 # endif
  44
  45 #else   /* Not in the GNU C library.  */
  46
  47 # include <sys/types.h>
  48
  49 /* Type to use for aligned memory operations.
  50    This should normally be the biggest type supported by a single load
  51    and store.  Must be an unsigned type.  */
  52 # define op_t   unsigned long int
  53 # define OPSIZ  (sizeof(op_t))
  54
  55 /* Threshold value for when to enter the unrolled loops.  */
  56 # define OP_T_THRES     16
  57
  58 /* Type to use for unaligned operations.  */
  59 typedef unsigned char byte;
  60
  61 # ifndef WORDS_BIGENDIAN
  62 #  define MERGE(w0, sh_1, w1, sh_2) (((w0) >> (sh_1)) | ((w1) << (sh_2)))
  63 # else
  64 #  define MERGE(w0, sh_1, w1, sh_2) (((w0) << (sh_1)) | ((w1) >> (sh_2)))
  65 # endif
  66
  67 #endif  /* In the GNU C library.  */
  68
  69 #ifdef WORDS_BIGENDIAN
  70 # define CMP_LT_OR_GT(a, b) ((a) > (b) ? 1 : -1)
  71 #else
  72 # define CMP_LT_OR_GT(a, b) memcmp_bytes ((a), (b))
  73 #endif
  74
  75 /* BE VERY CAREFUL IF YOU CHANGE THIS CODE!  */
  76
  77 /* The strategy of this memcmp is:
  78
  79    1. Compare bytes until one of the block pointers is aligned.
  80
  81    2. Compare using memcmp_common_alignment or
  82       memcmp_not_common_alignment, regarding the alignment of the other
  83       block after the initial byte operations.  The maximum number of
  84       full words (of type op_t) are compared in this way.
  85
  86    3. Compare the few remaining bytes.  */
  87
  88 #ifndef WORDS_BIGENDIAN
  89 /* memcmp_bytes -- Compare A and B bytewise in the byte order of the machine.
  90    A and B are known to be different.
  91    This is needed only on little-endian machines.  */
  92
  93 static int memcmp_bytes (op_t, op_t) __THROW;
  94
  95 static int
  96 memcmp_bytes (a, b)
  97      op_t a, b;
  98 {
  99   long int srcp1 = (long int) &a;
 100   long int srcp2 = (long int) &b;
 101   op_t a0, b0;
 102
 103   do
 104     {
 105       a0 = ((byte *) srcp1)[0];
 106       b0 = ((byte *) srcp2)[0];
 107       srcp1 += 1;
 108       srcp2 += 1;
 109     }
 110   while (a0 == b0);
 111   return a0 - b0;
 112 }
 113 #endif
 114
 115 static int memcmp_common_alignment (long, long, size_t) __THROW;
 116
 117 /* memcmp_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN `op_t'
 118    objects (not LEN bytes!).  Both SRCP1 and SRCP2 should be aligned for
 119    memory operations on `op_t's.  */
 120 static int
 121 memcmp_common_alignment (srcp1, srcp2, len)
 122      long int srcp1;
 123      long int srcp2;
 124      size_t len;
 125 {
 126   op_t a0, a1;
 127   op_t b0, b1;
 128
 129   switch (len % 4)
 130     {
 131     default: /* Avoid warning about uninitialized local variables.  */
 132     case 2:
 133       a0 = ((op_t *) srcp1)[0];
 134       b0 = ((op_t *) srcp2)[0];
 135       srcp1 -= 2 * OPSIZ;
 136       srcp2 -= 2 * OPSIZ;
 137       len += 2;
 138       goto do1;
 139     case 3:
 140       a1 = ((op_t *) srcp1)[0];
 141       b1 = ((op_t *) srcp2)[0];
 142       srcp1 -= OPSIZ;
 143       srcp2 -= OPSIZ;
 144       len += 1;
 145       goto do2;
 146     case 0:
 147       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 148         return 0;
 149       a0 = ((op_t *) srcp1)[0];
 150       b0 = ((op_t *) srcp2)[0];
 151       goto do3;
 152     case 1:
 153       a1 = ((op_t *) srcp1)[0];
 154       b1 = ((op_t *) srcp2)[0];
 155       srcp1 += OPSIZ;
 156       srcp2 += OPSIZ;
 157       len -= 1;
 158       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 159         goto do0;
 160       /* Fall through.  */
 161     }
 162
 163   do
 164     {
 165       a0 = ((op_t *) srcp1)[0];
 166       b0 = ((op_t *) srcp2)[0];
 167       if (a1 != b1)
 168         return CMP_LT_OR_GT (a1, b1);
 169
 170     do3:
 171       a1 = ((op_t *) srcp1)[1];
 172       b1 = ((op_t *) srcp2)[1];
 173       if (a0 != b0)
 174         return CMP_LT_OR_GT (a0, b0);
 175
 176     do2:
 177       a0 = ((op_t *) srcp1)[2];
 178       b0 = ((op_t *) srcp2)[2];
 179       if (a1 != b1)
 180         return CMP_LT_OR_GT (a1, b1);
 181
 182     do1:
 183       a1 = ((op_t *) srcp1)[3];
 184       b1 = ((op_t *) srcp2)[3];
 185       if (a0 != b0)
 186         return CMP_LT_OR_GT (a0, b0);
 187
 188       srcp1 += 4 * OPSIZ;
 189       srcp2 += 4 * OPSIZ;
 190       len -= 4;
 191     }
 192   while (len != 0);
 193
 194   /* This is the right position for do0.  Please don't move
 195      it into the loop.  */
 196  do0:
 197   if (a1 != b1)
 198     return CMP_LT_OR_GT (a1, b1);
 199   return 0;
 200 }
 201
 202 static int memcmp_not_common_alignment (long, long, size_t) __THROW;
 203
 204 /* memcmp_not_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN
 205    `op_t' objects (not LEN bytes!).  SRCP2 should be aligned for memory
 206    operations on `op_t', but SRCP1 *should be unaligned*.  */
 207 static int
 208 memcmp_not_common_alignment (srcp1, srcp2, len)
 209      long int srcp1;
 210      long int srcp2;
 211      size_t len;
 212 {
 213   op_t a0, a1, a2, a3;
 214   op_t b0, b1, b2, b3;
 215   op_t x;
 216   int shl, shr;
 217
 218   /* Calculate how to shift a word read at the memory operation
 219      aligned srcp1 to make it aligned for comparison.  */
 220
 221   shl = 8 * (srcp1 % OPSIZ);
 222   shr = 8 * OPSIZ - shl;
 223
 224   /* Make SRCP1 aligned by rounding it down to the beginning of the `op_t'
 225      it points in the middle of.  */
 226   srcp1 &= -OPSIZ;
 227
 228   switch (len % 4)
 229     {
 230     default: /* Avoid warning about uninitialized local variables.  */
 231     case 2:
 232       a1 = ((op_t *) srcp1)[0];
 233       a2 = ((op_t *) srcp1)[1];
 234       b2 = ((op_t *) srcp2)[0];
 235       srcp1 -= 1 * OPSIZ;
 236       srcp2 -= 2 * OPSIZ;
 237       len += 2;
 238       goto do1;
 239     case 3:
 240       a0 = ((op_t *) srcp1)[0];
 241       a1 = ((op_t *) srcp1)[1];
 242       b1 = ((op_t *) srcp2)[0];
 243       srcp2 -= 1 * OPSIZ;
 244       len += 1;
 245       goto do2;
 246     case 0:
 247       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 248         return 0;
 249       a3 = ((op_t *) srcp1)[0];
 250       a0 = ((op_t *) srcp1)[1];
 251       b0 = ((op_t *) srcp2)[0];
 252       srcp1 += 1 * OPSIZ;
 253       goto do3;
 254     case 1:
 255       a2 = ((op_t *) srcp1)[0];
 256       a3 = ((op_t *) srcp1)[1];
 257       b3 = ((op_t *) srcp2)[0];
 258       srcp1 += 2 * OPSIZ;
 259       srcp2 += 1 * OPSIZ;
 260       len -= 1;
 261       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 262         goto do0;
 263       /* Fall through.  */
 264     }
 265
 266   do
 267     {
 268       a0 = ((op_t *) srcp1)[0];
 269       b0 = ((op_t *) srcp2)[0];
 270       x = MERGE(a2, shl, a3, shr);
 271       if (x != b3)
 272         return CMP_LT_OR_GT (x, b3);
 273
 274     do3:
 275       a1 = ((op_t *) srcp1)[1];
 276       b1 = ((op_t *) srcp2)[1];
 277       x = MERGE(a3, shl, a0, shr);
 278       if (x != b0)
 279         return CMP_LT_OR_GT (x, b0);
 280
 281     do2:
 282       a2 = ((op_t *) srcp1)[2];
 283       b2 = ((op_t *) srcp2)[2];
 284       x = MERGE(a0, shl, a1, shr);
 285       if (x != b1)
 286         return CMP_LT_OR_GT (x, b1);
 287
 288     do1:
 289       a3 = ((op_t *) srcp1)[3];
 290       b3 = ((op_t *) srcp2)[3];
 291       x = MERGE(a1, shl, a2, shr);
 292       if (x != b2)
 293         return CMP_LT_OR_GT (x, b2);
 294
 295       srcp1 += 4 * OPSIZ;
 296       srcp2 += 4 * OPSIZ;
 297       len -= 4;
 298     }
 299   while (len != 0);
 300
 301   /* This is the right position for do0.  Please don't move
 302      it into the loop.  */
 303  do0:
 304   x = MERGE(a2, shl, a3, shr);
 305   if (x != b3)
 306     return CMP_LT_OR_GT (x, b3);
 307   return 0;
 308 }
 309
 310 int
 311 MEMCMP (s1, s2, len)
 312      const __ptr_t s1;
 313      const __ptr_t s2;
 314      size_t len;
 315 {
 316   op_t a0;
 317   op_t b0;
 318   long int srcp1 = (long int) s1;
 319   long int srcp2 = (long int) s2;
 320   op_t res;
 321
 322   if (len >= OP_T_THRES)
 323     {
 324       /* There are at least some bytes to compare.  No need to test
 325          for LEN == 0 in this alignment loop.  */
 326       while (srcp2 % OPSIZ != 0)
 327         {
 328           a0 = ((byte *) srcp1)[0];
 329           b0 = ((byte *) srcp2)[0];
 330           srcp1 += 1;
 331           srcp2 += 1;
 332           res = a0 - b0;
 333           if (res != 0)
 334             return res;
 335           len -= 1;
 336         }
 337
 338       /* SRCP2 is now aligned for memory operations on `op_t'.
 339          SRCP1 alignment determines if we can do a simple,
 340          aligned compare or need to shuffle bits.  */
 341
 342       if (srcp1 % OPSIZ == 0)
 343         res = memcmp_common_alignment (srcp1, srcp2, len / OPSIZ);
 344       else
 345         res = memcmp_not_common_alignment (srcp1, srcp2, len / OPSIZ);
 346       if (res != 0)
 347         return res;
 348
 349       /* Number of bytes remaining in the interval [0..OPSIZ-1].  */
 350       srcp1 += len & -OPSIZ;
 351       srcp2 += len & -OPSIZ;
 352       len %= OPSIZ;
 353     }
 354
 355   /* There are just a few bytes to compare.  Use byte memory operations.  */
 356   while (len != 0)
 357     {
 358       a0 = ((byte *) srcp1)[0];
 359       b0 = ((byte *) srcp2)[0];
 360       srcp1 += 1;
 361       srcp2 += 1;
 362       res = a0 - b0;
 363       if (res != 0)
 364         return res;
 365       len -= 1;
 366     }
 367
 368   return 0;
 369 }
 370 libc_hidden_builtin_def(memcmp)
 371 #ifdef weak_alias
 372 # undef bcmp
 373 weak_alias (memcmp, bcmp)
 374 #endif