string/memcmp.c

   1 /* Copyright (C) 1991-2020 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    <https://www.gnu.org/licenses/>.  */
  18
  19 #ifdef HAVE_CONFIG_H
  20 # include "config.h"
  21 #endif
  22
  23 #if defined HAVE_STRING_H || defined _LIBC
  24 # include <string.h>
  25 #endif
  26
  27 #undef memcmp
  28
  29 #ifndef MEMCMP
  30 # define MEMCMP memcmp
  31 #endif
  32
  33 #ifdef _LIBC
  34
  35 # include <memcopy.h>
  36 # include <endian.h>
  37
  38 # if __BYTE_ORDER == __BIG_ENDIAN
  39 #  define WORDS_BIGENDIAN
  40 # endif
  41
  42 #else   /* Not in the GNU C library.  */
  43
  44 # include <sys/types.h>
  45
  46 /* Type to use for aligned memory operations.
  47    This should normally be the biggest type supported by a single load
  48    and store.  Must be an unsigned type.  */
  49 # define op_t   unsigned long int
  50 # define OPSIZ  (sizeof (op_t))
  51
  52 /* Threshold value for when to enter the unrolled loops.  */
  53 # define OP_T_THRES     16
  54
  55 /* Type to use for unaligned operations.  */
  56 typedef unsigned char byte;
  57
  58 # ifndef WORDS_BIGENDIAN
  59 #  define MERGE(w0, sh_1, w1, sh_2) (((w0) >> (sh_1)) | ((w1) << (sh_2)))
  60 # else
  61 #  define MERGE(w0, sh_1, w1, sh_2) (((w0) << (sh_1)) | ((w1) >> (sh_2)))
  62 # endif
  63
  64 #endif  /* In the GNU C library.  */
  65
  66 #ifdef WORDS_BIGENDIAN
  67 # define CMP_LT_OR_GT(a, b) ((a) > (b) ? 1 : -1)
  68 #else
  69 # define CMP_LT_OR_GT(a, b) memcmp_bytes ((a), (b))
  70 #endif
  71
  72 /* BE VERY CAREFUL IF YOU CHANGE THIS CODE!  */
  73
  74 /* The strategy of this memcmp is:
  75
  76    1. Compare bytes until one of the block pointers is aligned.
  77
  78    2. Compare using memcmp_common_alignment or
  79       memcmp_not_common_alignment, regarding the alignment of the other
  80       block after the initial byte operations.  The maximum number of
  81       full words (of type op_t) are compared in this way.
  82
  83    3. Compare the few remaining bytes.  */
  84
  85 #ifndef WORDS_BIGENDIAN
  86 /* memcmp_bytes -- Compare A and B bytewise in the byte order of the machine.
  87    A and B are known to be different.
  88    This is needed only on little-endian machines.  */
  89
  90 static int memcmp_bytes (op_t, op_t) __THROW;
  91
  92 static int
  93 memcmp_bytes (op_t a, op_t b)
  94 {
  95   long int srcp1 = (long int) &a;
  96   long int srcp2 = (long int) &b;
  97   op_t a0, b0;
  98
  99   do
 100     {
 101       a0 = ((byte *) srcp1)[0];
 102       b0 = ((byte *) srcp2)[0];
 103       srcp1 += 1;
 104       srcp2 += 1;
 105     }
 106   while (a0 == b0);
 107   return a0 - b0;
 108 }
 109 #endif
 110
 111 static int memcmp_common_alignment (long, long, size_t) __THROW;
 112
 113 /* memcmp_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN `op_t'
 114    objects (not LEN bytes!).  Both SRCP1 and SRCP2 should be aligned for
 115    memory operations on `op_t's.  */
 116 static int
 117 memcmp_common_alignment (long int srcp1, long int srcp2, size_t len)
 118 {
 119   op_t a0, a1;
 120   op_t b0, b1;
 121
 122   switch (len % 4)
 123     {
 124     default: /* Avoid warning about uninitialized local variables.  */
 125     case 2:
 126       a0 = ((op_t *) srcp1)[0];
 127       b0 = ((op_t *) srcp2)[0];
 128       srcp1 -= 2 * OPSIZ;
 129       srcp2 -= 2 * OPSIZ;
 130       len += 2;
 131       goto do1;
 132     case 3:
 133       a1 = ((op_t *) srcp1)[0];
 134       b1 = ((op_t *) srcp2)[0];
 135       srcp1 -= OPSIZ;
 136       srcp2 -= OPSIZ;
 137       len += 1;
 138       goto do2;
 139     case 0:
 140       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 141         return 0;
 142       a0 = ((op_t *) srcp1)[0];
 143       b0 = ((op_t *) srcp2)[0];
 144       goto do3;
 145     case 1:
 146       a1 = ((op_t *) srcp1)[0];
 147       b1 = ((op_t *) srcp2)[0];
 148       srcp1 += OPSIZ;
 149       srcp2 += OPSIZ;
 150       len -= 1;
 151       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 152         goto do0;
 153       /* Fall through.  */
 154     }
 155
 156   do
 157     {
 158       a0 = ((op_t *) srcp1)[0];
 159       b0 = ((op_t *) srcp2)[0];
 160       if (a1 != b1)
 161         return CMP_LT_OR_GT (a1, b1);
 162
 163     do3:
 164       a1 = ((op_t *) srcp1)[1];
 165       b1 = ((op_t *) srcp2)[1];
 166       if (a0 != b0)
 167         return CMP_LT_OR_GT (a0, b0);
 168
 169     do2:
 170       a0 = ((op_t *) srcp1)[2];
 171       b0 = ((op_t *) srcp2)[2];
 172       if (a1 != b1)
 173         return CMP_LT_OR_GT (a1, b1);
 174
 175     do1:
 176       a1 = ((op_t *) srcp1)[3];
 177       b1 = ((op_t *) srcp2)[3];
 178       if (a0 != b0)
 179         return CMP_LT_OR_GT (a0, b0);
 180
 181       srcp1 += 4 * OPSIZ;
 182       srcp2 += 4 * OPSIZ;
 183       len -= 4;
 184     }
 185   while (len != 0);
 186
 187   /* This is the right position for do0.  Please don't move
 188      it into the loop.  */
 189  do0:
 190   if (a1 != b1)
 191     return CMP_LT_OR_GT (a1, b1);
 192   return 0;
 193 }
 194
 195 static int memcmp_not_common_alignment (long, long, size_t) __THROW;
 196
 197 /* memcmp_not_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN
 198    `op_t' objects (not LEN bytes!).  SRCP2 should be aligned for memory
 199    operations on `op_t', but SRCP1 *should be unaligned*.  */
 200 static int
 201 memcmp_not_common_alignment (long int srcp1, long int srcp2, size_t len)
 202 {
 203   op_t a0, a1, a2, a3;
 204   op_t b0, b1, b2, b3;
 205   op_t x;
 206   int shl, shr;
 207
 208   /* Calculate how to shift a word read at the memory operation
 209      aligned srcp1 to make it aligned for comparison.  */
 210
 211   shl = 8 * (srcp1 % OPSIZ);
 212   shr = 8 * OPSIZ - shl;
 213
 214   /* Make SRCP1 aligned by rounding it down to the beginning of the `op_t'
 215      it points in the middle of.  */
 216   srcp1 &= -OPSIZ;
 217
 218   switch (len % 4)
 219     {
 220     default: /* Avoid warning about uninitialized local variables.  */
 221     case 2:
 222       a1 = ((op_t *) srcp1)[0];
 223       a2 = ((op_t *) srcp1)[1];
 224       b2 = ((op_t *) srcp2)[0];
 225       srcp1 -= 1 * OPSIZ;
 226       srcp2 -= 2 * OPSIZ;
 227       len += 2;
 228       goto do1;
 229     case 3:
 230       a0 = ((op_t *) srcp1)[0];
 231       a1 = ((op_t *) srcp1)[1];
 232       b1 = ((op_t *) srcp2)[0];
 233       srcp2 -= 1 * OPSIZ;
 234       len += 1;
 235       goto do2;
 236     case 0:
 237       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 238         return 0;
 239       a3 = ((op_t *) srcp1)[0];
 240       a0 = ((op_t *) srcp1)[1];
 241       b0 = ((op_t *) srcp2)[0];
 242       srcp1 += 1 * OPSIZ;
 243       goto do3;
 244     case 1:
 245       a2 = ((op_t *) srcp1)[0];
 246       a3 = ((op_t *) srcp1)[1];
 247       b3 = ((op_t *) srcp2)[0];
 248       srcp1 += 2 * OPSIZ;
 249       srcp2 += 1 * OPSIZ;
 250       len -= 1;
 251       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 252         goto do0;
 253       /* Fall through.  */
 254     }
 255
 256   do
 257     {
 258       a0 = ((op_t *) srcp1)[0];
 259       b0 = ((op_t *) srcp2)[0];
 260       x = MERGE(a2, shl, a3, shr);
 261       if (x != b3)
 262         return CMP_LT_OR_GT (x, b3);
 263
 264     do3:
 265       a1 = ((op_t *) srcp1)[1];
 266       b1 = ((op_t *) srcp2)[1];
 267       x = MERGE(a3, shl, a0, shr);
 268       if (x != b0)
 269         return CMP_LT_OR_GT (x, b0);
 270
 271     do2:
 272       a2 = ((op_t *) srcp1)[2];
 273       b2 = ((op_t *) srcp2)[2];
 274       x = MERGE(a0, shl, a1, shr);
 275       if (x != b1)
 276         return CMP_LT_OR_GT (x, b1);
 277
 278     do1:
 279       a3 = ((op_t *) srcp1)[3];
 280       b3 = ((op_t *) srcp2)[3];
 281       x = MERGE(a1, shl, a2, shr);
 282       if (x != b2)
 283         return CMP_LT_OR_GT (x, b2);
 284
 285       srcp1 += 4 * OPSIZ;
 286       srcp2 += 4 * OPSIZ;
 287       len -= 4;
 288     }
 289   while (len != 0);
 290
 291   /* This is the right position for do0.  Please don't move
 292      it into the loop.  */
 293  do0:
 294   x = MERGE(a2, shl, a3, shr);
 295   if (x != b3)
 296     return CMP_LT_OR_GT (x, b3);
 297   return 0;
 298 }
 299
 300 int
 301 MEMCMP (const void *s1, const void *s2, size_t len)
 302 {
 303   op_t a0;
 304   op_t b0;
 305   long int srcp1 = (long int) s1;
 306   long int srcp2 = (long int) s2;
 307   op_t res;
 308
 309   if (len >= OP_T_THRES)
 310     {
 311       /* There are at least some bytes to compare.  No need to test
 312          for LEN == 0 in this alignment loop.  */
 313       while (srcp2 % OPSIZ != 0)
 314         {
 315           a0 = ((byte *) srcp1)[0];
 316           b0 = ((byte *) srcp2)[0];
 317           srcp1 += 1;
 318           srcp2 += 1;
 319           res = a0 - b0;
 320           if (res != 0)
 321             return res;
 322           len -= 1;
 323         }
 324
 325       /* SRCP2 is now aligned for memory operations on `op_t'.
 326          SRCP1 alignment determines if we can do a simple,
 327          aligned compare or need to shuffle bits.  */
 328
 329       if (srcp1 % OPSIZ == 0)
 330         res = memcmp_common_alignment (srcp1, srcp2, len / OPSIZ);
 331       else
 332         res = memcmp_not_common_alignment (srcp1, srcp2, len / OPSIZ);
 333       if (res != 0)
 334         return res;
 335
 336       /* Number of bytes remaining in the interval [0..OPSIZ-1].  */
 337       srcp1 += len & -OPSIZ;
 338       srcp2 += len & -OPSIZ;
 339       len %= OPSIZ;
 340     }
 341
 342   /* There are just a few bytes to compare.  Use byte memory operations.  */
 343   while (len != 0)
 344     {
 345       a0 = ((byte *) srcp1)[0];
 346       b0 = ((byte *) srcp2)[0];
 347       srcp1 += 1;
 348       srcp2 += 1;
 349       res = a0 - b0;
 350       if (res != 0)
 351         return res;
 352       len -= 1;
 353     }
 354
 355   return 0;
 356 }
 357 libc_hidden_builtin_def(memcmp)
 358 #ifdef weak_alias
 359 # undef bcmp
 360 weak_alias (memcmp, bcmp)
 361 #endif