sysdeps/generic/dl-hash.h

   1 /* Compute hash value for given string according to ELF standard.
   2    Copyright (C) 1995-1998,2003,2005,2011 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, write to the Free
  17    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  18    02111-1307 USA.  */
  19
  20 #ifndef _DL_HASH_H
  21 #define _DL_HASH_H      1
  22
  23
  24 /* This is the hashing function specified by the ELF ABI.  In the
  25    first five operations no overflow is possible so we optimized it a
  26    bit.  */
  27 static unsigned int
  28 __attribute__ ((used))
  29 _dl_elf_hash (const char *name_arg)
  30 {
  31   const unsigned char *name = (const unsigned char *) name_arg;
  32   unsigned long int hash = 0;
  33   if (*name != '\0')
  34     {
  35       hash = *name++;
  36       if (*name != '\0')
  37         {
  38           hash = (hash << 4) + *name++;
  39           if (*name != '\0')
  40             {
  41               hash = (hash << 4) + *name++;
  42               if (*name != '\0')
  43                 {
  44                   hash = (hash << 4) + *name++;
  45                   if (*name != '\0')
  46                     {
  47                       hash = (hash << 4) + *name++;
  48                       while (*name != '\0')
  49                         {
  50                           unsigned long int hi;
  51                           hash = (hash << 4) + *name++;
  52                           hi = hash & 0xf0000000;
  53
  54                           /* The algorithm specified in the ELF ABI is as
  55                              follows:
  56
  57                              if (hi != 0)
  58                                hash ^= hi >> 24;
  59
  60                              hash &= ~hi;
  61
  62                              But the following is equivalent and a lot
  63                              faster, especially on modern processors.  */
  64
  65                           hash ^= hi;
  66                           hash ^= hi >> 24;
  67                         }
  68                     }
  69                 }
  70             }
  71         }
  72     }
  73   return hash;
  74 }
  75
  76 #endif /* dl-hash.h */