string/memcmp.c

   1 /* Copyright (C) 1991-2016 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 (op_t a, op_t b)
  97 {
  98   long int srcp1 = (long int) &a;
  99   long int srcp2 = (long int) &b;
 100   op_t a0, b0;
 101
 102   do
 103     {
 104       a0 = ((byte *) srcp1)[0];
 105       b0 = ((byte *) srcp2)[0];
 106       srcp1 += 1;
 107       srcp2 += 1;
 108     }
 109   while (a0 == b0);
 110   return a0 - b0;
 111 }
 112 #endif
 113
 114 static int memcmp_common_alignment (long, long, size_t) __THROW;
 115
 116 /* memcmp_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN `op_t'
 117    objects (not LEN bytes!).  Both SRCP1 and SRCP2 should be aligned for
 118    memory operations on `op_t's.  */
 119 static int
 120 memcmp_common_alignment (long int srcp1, long int srcp2, size_t len)
 121 {
 122   op_t a0, a1;
 123   op_t b0, b1;
 124
 125   switch (len % 4)
 126     {
 127     default: /* Avoid warning about uninitialized local variables.  */
 128     case 2:
 129       a0 = ((op_t *) srcp1)[0];
 130       b0 = ((op_t *) srcp2)[0];
 131       srcp1 -= 2 * OPSIZ;
 132       srcp2 -= 2 * OPSIZ;
 133       len += 2;
 134       goto do1;
 135     case 3:
 136       a1 = ((op_t *) srcp1)[0];
 137       b1 = ((op_t *) srcp2)[0];
 138       srcp1 -= OPSIZ;
 139       srcp2 -= OPSIZ;
 140       len += 1;
 141       goto do2;
 142     case 0:
 143       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 144         return 0;
 145       a0 = ((op_t *) srcp1)[0];
 146       b0 = ((op_t *) srcp2)[0];
 147       goto do3;
 148     case 1:
 149       a1 = ((op_t *) srcp1)[0];
 150       b1 = ((op_t *) srcp2)[0];
 151       srcp1 += OPSIZ;
 152       srcp2 += OPSIZ;
 153       len -= 1;
 154       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 155         goto do0;
 156       /* Fall through.  */
 157     }
 158
 159   do
 160     {
 161       a0 = ((op_t *) srcp1)[0];
 162       b0 = ((op_t *) srcp2)[0];
 163       if (a1 != b1)
 164         return CMP_LT_OR_GT (a1, b1);
 165
 166     do3:
 167       a1 = ((op_t *) srcp1)[1];
 168       b1 = ((op_t *) srcp2)[1];
 169       if (a0 != b0)
 170         return CMP_LT_OR_GT (a0, b0);
 171
 172     do2:
 173       a0 = ((op_t *) srcp1)[2];
 174       b0 = ((op_t *) srcp2)[2];
 175       if (a1 != b1)
 176         return CMP_LT_OR_GT (a1, b1);
 177
 178     do1:
 179       a1 = ((op_t *) srcp1)[3];
 180       b1 = ((op_t *) srcp2)[3];
 181       if (a0 != b0)
 182         return CMP_LT_OR_GT (a0, b0);
 183
 184       srcp1 += 4 * OPSIZ;
 185       srcp2 += 4 * OPSIZ;
 186       len -= 4;
 187     }
 188   while (len != 0);
 189
 190   /* This is the right position for do0.  Please don't move
 191      it into the loop.  */
 192  do0:
 193   if (a1 != b1)
 194     return CMP_LT_OR_GT (a1, b1);
 195   return 0;
 196 }
 197
 198 static int memcmp_not_common_alignment (long, long, size_t) __THROW;
 199
 200 /* memcmp_not_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN
 201    `op_t' objects (not LEN bytes!).  SRCP2 should be aligned for memory
 202    operations on `op_t', but SRCP1 *should be unaligned*.  */
 203 static int
 204 memcmp_not_common_alignment (long int srcp1, long int srcp2, size_t len)
 205 {
 206   op_t a0, a1, a2, a3;
 207   op_t b0, b1, b2, b3;
 208   op_t x;
 209   int shl, shr;
 210
 211   /* Calculate how to shift a word read at the memory operation
 212      aligned srcp1 to make it aligned for comparison.  */
 213
 214   shl = 8 * (srcp1 % OPSIZ);
 215   shr = 8 * OPSIZ - shl;
 216
 217   /* Make SRCP1 aligned by rounding it down to the beginning of the `op_t'
 218      it points in the middle of.  */
 219   srcp1 &= -OPSIZ;
 220
 221   switch (len % 4)
 222     {
 223     default: /* Avoid warning about uninitialized local variables.  */
 224     case 2:
 225       a1 = ((op_t *) srcp1)[0];
 226       a2 = ((op_t *) srcp1)[1];
 227       b2 = ((op_t *) srcp2)[0];
 228       srcp1 -= 1 * OPSIZ;
 229       srcp2 -= 2 * OPSIZ;
 230       len += 2;
 231       goto do1;
 232     case 3:
 233       a0 = ((op_t *) srcp1)[0];
 234       a1 = ((op_t *) srcp1)[1];
 235       b1 = ((op_t *) srcp2)[0];
 236       srcp2 -= 1 * OPSIZ;
 237       len += 1;
 238       goto do2;
 239     case 0:
 240       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 241         return 0;
 242       a3 = ((op_t *) srcp1)[0];
 243       a0 = ((op_t *) srcp1)[1];
 244       b0 = ((op_t *) srcp2)[0];
 245       srcp1 += 1 * OPSIZ;
 246       goto do3;
 247     case 1:
 248       a2 = ((op_t *) srcp1)[0];
 249       a3 = ((op_t *) srcp1)[1];
 250       b3 = ((op_t *) srcp2)[0];
 251       srcp1 += 2 * OPSIZ;
 252       srcp2 += 1 * OPSIZ;
 253       len -= 1;
 254       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 255         goto do0;
 256       /* Fall through.  */
 257     }
 258
 259   do
 260     {
 261       a0 = ((op_t *) srcp1)[0];
 262       b0 = ((op_t *) srcp2)[0];
 263       x = MERGE(a2, shl, a3, shr);
 264       if (x != b3)
 265         return CMP_LT_OR_GT (x, b3);
 266
 267     do3:
 268       a1 = ((op_t *) srcp1)[1];
 269       b1 = ((op_t *) srcp2)[1];
 270       x = MERGE(a3, shl, a0, shr);
 271       if (x != b0)
 272         return CMP_LT_OR_GT (x, b0);
 273
 274     do2:
 275       a2 = ((op_t *) srcp1)[2];
 276       b2 = ((op_t *) srcp2)[2];
 277       x = MERGE(a0, shl, a1, shr);
 278       if (x != b1)
 279         return CMP_LT_OR_GT (x, b1);
 280
 281     do1:
 282       a3 = ((op_t *) srcp1)[3];
 283       b3 = ((op_t *) srcp2)[3];
 284       x = MERGE(a1, shl, a2, shr);
 285       if (x != b2)
 286         return CMP_LT_OR_GT (x, b2);
 287
 288       srcp1 += 4 * OPSIZ;
 289       srcp2 += 4 * OPSIZ;
 290       len -= 4;
 291     }
 292   while (len != 0);
 293
 294   /* This is the right position for do0.  Please don't move
 295      it into the loop.  */
 296  do0:
 297   x = MERGE(a2, shl, a3, shr);
 298   if (x != b3)
 299     return CMP_LT_OR_GT (x, b3);
 300   return 0;
 301 }
 302
 303 int
 304 MEMCMP (const __ptr_t s1, const __ptr_t s2, size_t len)
 305 {
 306   op_t a0;
 307   op_t b0;
 308   long int srcp1 = (long int) s1;
 309   long int srcp2 = (long int) s2;
 310   op_t res;
 311
 312   if (len >= OP_T_THRES)
 313     {
 314       /* There are at least some bytes to compare.  No need to test
 315          for LEN == 0 in this alignment loop.  */
 316       while (srcp2 % OPSIZ != 0)
 317         {
 318           a0 = ((byte *) srcp1)[0];
 319           b0 = ((byte *) srcp2)[0];
 320           srcp1 += 1;
 321           srcp2 += 1;
 322           res = a0 - b0;
 323           if (res != 0)
 324             return res;
 325           len -= 1;
 326         }
 327
 328       /* SRCP2 is now aligned for memory operations on `op_t'.
 329          SRCP1 alignment determines if we can do a simple,
 330          aligned compare or need to shuffle bits.  */
 331
 332       if (srcp1 % OPSIZ == 0)
 333         res = memcmp_common_alignment (srcp1, srcp2, len / OPSIZ);
 334       else
 335         res = memcmp_not_common_alignment (srcp1, srcp2, len / OPSIZ);
 336       if (res != 0)
 337         return res;
 338
 339       /* Number of bytes remaining in the interval [0..OPSIZ-1].  */
 340       srcp1 += len & -OPSIZ;
 341       srcp2 += len & -OPSIZ;
 342       len %= OPSIZ;
 343     }
 344
 345   /* There are just a few bytes to compare.  Use byte memory operations.  */
 346   while (len != 0)
 347     {
 348       a0 = ((byte *) srcp1)[0];
 349       b0 = ((byte *) srcp2)[0];
 350       srcp1 += 1;
 351       srcp2 += 1;
 352       res = a0 - b0;
 353       if (res != 0)
 354         return res;
 355       len -= 1;
 356     }
 357
 358   return 0;
 359 }
 360 libc_hidden_builtin_def(memcmp)
 361 #ifdef weak_alias
 362 # undef bcmp
 363 weak_alias (memcmp, bcmp)
 364 #endif