lib/memcmp.c

   1 /* Copyright (C) 1991, 1993, 1995, 1997-1998, 2003, 2006, 2009-2018 Free
   2    Software Foundation, Inc.
   3
   4    Contributed by Torbjorn Granlund (tege@sics.se).
   5
   6    NOTE: The canonical source of this file is maintained with the GNU C Library.
   7    Bugs can be reported to bug-glibc@prep.ai.mit.edu.
   8
   9    This program is free software: you can redistribute it and/or modify it
  10    under the terms of the GNU General Public License as published by the
  11    Free Software Foundation; either version 3 of the License, or any
  12    later version.
  13
  14    This program is distributed in the hope that it will be useful,
  15    but WITHOUT ANY WARRANTY; without even the implied warranty of
  16    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  17    GNU General Public License for more details.
  18
  19    You should have received a copy of the GNU General Public License
  20    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  21
  22 #ifndef _LIBC
  23 # include <config.h>
  24 #endif
  25
  26 #include <string.h>
  27
  28 #include <stdint.h>
  29
  30 #undef memcmp
  31
  32 #ifdef _LIBC
  33
  34 # include <memcopy.h>
  35 # include <endian.h>
  36
  37 # if __BYTE_ORDER == __BIG_ENDIAN
  38 #  define WORDS_BIGENDIAN
  39 # endif
  40
  41 #else   /* Not in the GNU C library.  */
  42
  43 # include <sys/types.h>
  44
  45 /* Type to use for aligned memory operations.
  46    This should normally be the biggest type supported by a single load
  47    and store.  Must be an unsigned type.  */
  48 # define op_t   unsigned long int
  49 # define OPSIZ  (sizeof(op_t))
  50
  51 /* Threshold value for when to enter the unrolled loops.  */
  52 # define OP_T_THRES     16
  53
  54 /* Type to use for unaligned operations.  */
  55 typedef unsigned char byte;
  56
  57 # ifndef WORDS_BIGENDIAN
  58 #  define MERGE(w0, sh_1, w1, sh_2) (((w0) >> (sh_1)) | ((w1) << (sh_2)))
  59 # else
  60 #  define MERGE(w0, sh_1, w1, sh_2) (((w0) << (sh_1)) | ((w1) >> (sh_2)))
  61 # endif
  62
  63 #endif  /* In the GNU C library.  */
  64
  65 #ifdef WORDS_BIGENDIAN
  66 # define CMP_LT_OR_GT(a, b) ((a) > (b) ? 1 : -1)
  67 #else
  68 # define CMP_LT_OR_GT(a, b) memcmp_bytes (a, b)
  69 #endif
  70
  71 /* BE VERY CAREFUL IF YOU CHANGE THIS CODE!  */
  72
  73 /* The strategy of this memcmp is:
  74
  75    1. Compare bytes until one of the block pointers is aligned.
  76
  77    2. Compare using memcmp_common_alignment or
  78       memcmp_not_common_alignment, regarding the alignment of the other
  79       block after the initial byte operations.  The maximum number of
  80       full words (of type op_t) are compared in this way.
  81
  82    3. Compare the few remaining bytes.  */
  83
  84 #ifndef WORDS_BIGENDIAN
  85 /* memcmp_bytes -- Compare A and B bytewise in the byte order of the machine.
  86    A and B are known to be different.
  87    This is needed only on little-endian machines.  */
  88
  89 # ifdef  __GNUC__
  90 __inline
  91 # endif
  92 static int
  93 memcmp_bytes (op_t a, op_t b)
  94 {
  95   const byte *srcp1 = (const byte *) &a;
  96   const byte *srcp2 = (const byte *) &b;
  97   op_t a0, b0;
  98
  99   do
 100     {
 101       a0 = srcp1[0];
 102       b0 = srcp2[0];
 103       srcp1 += 1;
 104       srcp2 += 1;
 105     }
 106   while (a0 == b0);
 107   return a0 - b0;
 108 }
 109 #endif
 110
 111 /* memcmp_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN 'op_t'
 112    objects (not LEN bytes!).  Both SRCP1 and SRCP2 should be aligned for
 113    memory operations on 'op_t's.  */
 114 #ifdef __GNUC__
 115 __inline
 116 #endif
 117 static int
 118 memcmp_common_alignment (uintptr_t srcp1, uintptr_t srcp2, size_t len)
 119 {
 120   op_t a0, a1;
 121   op_t b0, b1;
 122
 123   switch (len % 4)
 124     {
 125     default: /* Avoid warning about uninitialized local variables.  */
 126     case 2:
 127       a0 = ((op_t *) srcp1)[0];
 128       b0 = ((op_t *) srcp2)[0];
 129       srcp1 -= 2 * OPSIZ;
 130       srcp2 -= 2 * OPSIZ;
 131       len += 2;
 132       goto do1;
 133     case 3:
 134       a1 = ((op_t *) srcp1)[0];
 135       b1 = ((op_t *) srcp2)[0];
 136       srcp1 -= OPSIZ;
 137       srcp2 -= OPSIZ;
 138       len += 1;
 139       goto do2;
 140     case 0:
 141       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 142         return 0;
 143       a0 = ((op_t *) srcp1)[0];
 144       b0 = ((op_t *) srcp2)[0];
 145       goto do3;
 146     case 1:
 147       a1 = ((op_t *) srcp1)[0];
 148       b1 = ((op_t *) srcp2)[0];
 149       srcp1 += OPSIZ;
 150       srcp2 += OPSIZ;
 151       len -= 1;
 152       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 153         goto do0;
 154       /* Fall through.  */
 155     }
 156
 157   do
 158     {
 159       a0 = ((op_t *) srcp1)[0];
 160       b0 = ((op_t *) srcp2)[0];
 161       if (a1 != b1)
 162         return CMP_LT_OR_GT (a1, b1);
 163
 164     do3:
 165       a1 = ((op_t *) srcp1)[1];
 166       b1 = ((op_t *) srcp2)[1];
 167       if (a0 != b0)
 168         return CMP_LT_OR_GT (a0, b0);
 169
 170     do2:
 171       a0 = ((op_t *) srcp1)[2];
 172       b0 = ((op_t *) srcp2)[2];
 173       if (a1 != b1)
 174         return CMP_LT_OR_GT (a1, b1);
 175
 176     do1:
 177       a1 = ((op_t *) srcp1)[3];
 178       b1 = ((op_t *) srcp2)[3];
 179       if (a0 != b0)
 180         return CMP_LT_OR_GT (a0, b0);
 181
 182       srcp1 += 4 * OPSIZ;
 183       srcp2 += 4 * OPSIZ;
 184       len -= 4;
 185     }
 186   while (len != 0);
 187
 188   /* This is the right position for do0.  Please don't move
 189      it into the loop.  */
 190  do0:
 191   if (a1 != b1)
 192     return CMP_LT_OR_GT (a1, b1);
 193   return 0;
 194 }
 195
 196 /* memcmp_not_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN
 197    'op_t' objects (not LEN bytes!).  SRCP2 should be aligned for memory
 198    operations on 'op_t', but SRCP1 *should be unaligned*.  */
 199 #ifdef __GNUC__
 200 __inline
 201 #endif
 202 static int
 203 memcmp_not_common_alignment (uintptr_t srcp1, uintptr_t srcp2, size_t len)
 204 {
 205   op_t a0, a1, a2, a3;
 206   op_t b0, b1, b2, b3;
 207   op_t x;
 208   int shl, shr;
 209
 210   /* Calculate how to shift a word read at the memory operation
 211      aligned srcp1 to make it aligned for comparison.  */
 212
 213   shl = 8 * (srcp1 % OPSIZ);
 214   shr = 8 * OPSIZ - shl;
 215
 216   /* Make SRCP1 aligned by rounding it down to the beginning of the 'op_t'
 217      it points in the middle of.  */
 218   srcp1 &= -OPSIZ;
 219
 220   switch (len % 4)
 221     {
 222     default: /* Avoid warning about uninitialized local variables.  */
 223     case 2:
 224       a1 = ((op_t *) srcp1)[0];
 225       a2 = ((op_t *) srcp1)[1];
 226       b2 = ((op_t *) srcp2)[0];
 227       srcp1 -= 1 * OPSIZ;
 228       srcp2 -= 2 * OPSIZ;
 229       len += 2;
 230       goto do1;
 231     case 3:
 232       a0 = ((op_t *) srcp1)[0];
 233       a1 = ((op_t *) srcp1)[1];
 234       b1 = ((op_t *) srcp2)[0];
 235       srcp2 -= 1 * OPSIZ;
 236       len += 1;
 237       goto do2;
 238     case 0:
 239       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 240         return 0;
 241       a3 = ((op_t *) srcp1)[0];
 242       a0 = ((op_t *) srcp1)[1];
 243       b0 = ((op_t *) srcp2)[0];
 244       srcp1 += 1 * OPSIZ;
 245       goto do3;
 246     case 1:
 247       a2 = ((op_t *) srcp1)[0];
 248       a3 = ((op_t *) srcp1)[1];
 249       b3 = ((op_t *) srcp2)[0];
 250       srcp1 += 2 * OPSIZ;
 251       srcp2 += 1 * OPSIZ;
 252       len -= 1;
 253       if (OP_T_THRES <= 3 * OPSIZ && len == 0)
 254         goto do0;
 255       /* Fall through.  */
 256     }
 257
 258   do
 259     {
 260       a0 = ((op_t *) srcp1)[0];
 261       b0 = ((op_t *) srcp2)[0];
 262       x = MERGE (a2, shl, a3, shr);
 263       if (x != b3)
 264         return CMP_LT_OR_GT (x, b3);
 265
 266     do3:
 267       a1 = ((op_t *) srcp1)[1];
 268       b1 = ((op_t *) srcp2)[1];
 269       x = MERGE (a3, shl, a0, shr);
 270       if (x != b0)
 271         return CMP_LT_OR_GT (x, b0);
 272
 273     do2:
 274       a2 = ((op_t *) srcp1)[2];
 275       b2 = ((op_t *) srcp2)[2];
 276       x = MERGE (a0, shl, a1, shr);
 277       if (x != b1)
 278         return CMP_LT_OR_GT (x, b1);
 279
 280     do1:
 281       a3 = ((op_t *) srcp1)[3];
 282       b3 = ((op_t *) srcp2)[3];
 283       x = MERGE (a1, shl, a2, shr);
 284       if (x != b2)
 285         return CMP_LT_OR_GT (x, b2);
 286
 287       srcp1 += 4 * OPSIZ;
 288       srcp2 += 4 * OPSIZ;
 289       len -= 4;
 290     }
 291   while (len != 0);
 292
 293   /* This is the right position for do0.  Please don't move
 294      it into the loop.  */
 295  do0:
 296   x = MERGE (a2, shl, a3, shr);
 297   if (x != b3)
 298     return CMP_LT_OR_GT (x, b3);
 299   return 0;
 300 }
 301
 302 int
 303 rpl_memcmp (const void *s1, const void *s2, size_t len)
 304 {
 305   op_t a0;
 306   op_t b0;
 307   uintptr_t srcp1 = (uintptr_t) s1;
 308   uintptr_t srcp2 = (uintptr_t) s2;
 309   op_t res;
 310
 311   if (len >= OP_T_THRES)
 312     {
 313       /* There are at least some bytes to compare.  No need to test
 314          for LEN == 0 in this alignment loop.  */
 315       while (srcp2 % OPSIZ != 0)
 316         {
 317           a0 = ((byte *) srcp1)[0];
 318           b0 = ((byte *) srcp2)[0];
 319           srcp1 += 1;
 320           srcp2 += 1;
 321           res = a0 - b0;
 322           if (res != 0)
 323             return res;
 324           len -= 1;
 325         }
 326
 327       /* SRCP2 is now aligned for memory operations on 'op_t'.
 328          SRCP1 alignment determines if we can do a simple,
 329          aligned compare or need to shuffle bits.  */
 330
 331       if (srcp1 % OPSIZ == 0)
 332         res = memcmp_common_alignment (srcp1, srcp2, len / OPSIZ);
 333       else
 334         res = memcmp_not_common_alignment (srcp1, srcp2, len / OPSIZ);
 335       if (res != 0)
 336         return res;
 337
 338       /* Number of bytes remaining in the interval [0..OPSIZ-1].  */
 339       srcp1 += len & -OPSIZ;
 340       srcp2 += len & -OPSIZ;
 341       len %= OPSIZ;
 342     }
 343
 344   /* There are just a few bytes to compare.  Use byte memory operations.  */
 345   while (len != 0)
 346     {
 347       a0 = ((byte *) srcp1)[0];
 348       b0 = ((byte *) srcp2)[0];
 349       srcp1 += 1;
 350       srcp2 += 1;
 351       res = a0 - b0;
 352       if (res != 0)
 353         return res;
 354       len -= 1;
 355     }
 356
 357   return 0;
 358 }
 359
 360 #ifdef weak_alias
 361 # undef bcmp
 362 weak_alias (memcmp, bcmp)
 363 #endif