string/memrchr.c

   1 /* memrchr -- find the last occurrence of a byte in a memory block
   2    Copyright (C) 1991-2024 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   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 #include <string-fzb.h>
  20 #include <string-fzc.h>
  21 #include <string-fzi.h>
  22 #include <string-shift.h>
  23 #include <string.h>
  24 #include <libc-pointer-arith.h>
  25
  26 #undef __memrchr
  27 #undef memrchr
  28
  29 #ifdef MEMRCHR
  30 # define __memrchr MEMRCHR
  31 #endif
  32
  33 void *
  34 __memrchr (const void *s, int c_in, size_t n)
  35 {
  36   if (__glibc_unlikely (n == 0))
  37     return NULL;
  38
  39   const op_t *word_ptr = (const op_t *) PTR_ALIGN_UP (s + n, sizeof (op_t));
  40   uintptr_t s_int = (uintptr_t) s + n;
  41
  42   op_t word = *--word_ptr;
  43   op_t repeated_c = repeat_bytes (c_in);
  44
  45   /* Compute the address of the word containing the initial byte. */
  46   const op_t *sword = (const op_t *) PTR_ALIGN_DOWN (s, sizeof (op_t));
  47
  48   /* If the end of buffer is not op_t aligned, mask off the undesirable bits
  49      before find the last byte position.  */
  50   find_t mask = shift_find_last (find_eq_all (word, repeated_c), s_int);
  51   if (mask != 0)
  52     {
  53       char *ret = (char *) word_ptr + index_last (mask);
  54       return ret >= (char *) s ? ret : NULL;
  55     }
  56   if (word_ptr == sword)
  57     return NULL;
  58   word = *--word_ptr;
  59
  60   while (word_ptr != sword)
  61     {
  62       if (has_eq (word, repeated_c))
  63         return (char *) word_ptr + index_last_eq (word, repeated_c);
  64       word = *--word_ptr;
  65     }
  66
  67   if (has_eq (word, repeated_c))
  68     {
  69       /* We found a match, but it might be in a byte past the end of the
  70          array.  */
  71       char *ret = (char *) word_ptr + index_last_eq (word, repeated_c);
  72       if (ret >= (char *) s)
  73         return ret;
  74     }
  75   return NULL;
  76 }
  77 #ifndef MEMRCHR
  78 libc_hidden_def (__memrchr)
  79 weak_alias (__memrchr, memrchr)
  80 #endif