sysdeps/generic/dl-hash.h

   1 /* Compute hash value for given string according to ELF standard.
   2    Copyright (C) 1995,1996,1997,1998,2003,2005 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 _dl_elf_hash (const char *name_arg)
  29 {
  30   const unsigned char *name = (const unsigned char *) name_arg;
  31   unsigned long int hash = 0;
  32   if (*name != '\0')
  33     {
  34       hash = *name++;
  35       if (*name != '\0')
  36         {
  37           hash = (hash << 4) + *name++;
  38           if (*name != '\0')
  39             {
  40               hash = (hash << 4) + *name++;
  41               if (*name != '\0')
  42                 {
  43                   hash = (hash << 4) + *name++;
  44                   if (*name != '\0')
  45                     {
  46                       hash = (hash << 4) + *name++;
  47                       while (*name != '\0')
  48                         {
  49                           unsigned long int hi;
  50                           hash = (hash << 4) + *name++;
  51                           hi = hash & 0xf0000000;
  52
  53                           /* The algorithm specified in the ELF ABI is as
  54                              follows:
  55
  56                              if (hi != 0)
  57                                hash ^= hi >> 24;
  58
  59                              hash &= ~hi;
  60
  61                              But the following is equivalent and a lot
  62                              faster, especially on modern processors.  */
  63
  64                           hash ^= hi;
  65                           hash ^= hi >> 24;
  66                         }
  67                     }
  68                 }
  69             }
  70         }
  71     }
  72   return hash;
  73 }
  74
  75 #endif /* dl-hash.h */