include/inline-hashtab.h

   1 /* Fully-inline hash table, used mainly for managing TLS descriptors.
   2    Copyright (C) 1999-2021 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    This file is derived from a 2003's version of libiberty's
   6    hashtab.c, contributed by Vladimir Makarov (vmakarov@cygnus.com),
   7    but with most adaptation points and support for deleting elements
   8    removed.
   9
  10    The GNU C Library is free software; you can redistribute it and/or
  11    modify it under the terms of the GNU Lesser General Public
  12    License as published by the Free Software Foundation; either
  13    version 2.1 of the License, or (at your option) any later version.
  14
  15    The GNU C Library is distributed in the hope that it will be useful,
  16    but WITHOUT ANY WARRANTY; without even the implied warranty of
  17    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  18    Lesser General Public License for more details.
  19
  20    You should have received a copy of the GNU Lesser General Public
  21    License along with the GNU C Library; if not, see
  22    <https://www.gnu.org/licenses/>.  */
  23
  24 #ifndef INLINE_HASHTAB_H
  25 # define INLINE_HASHTAB_H 1
  26
  27 struct hashtab
  28 {
  29   /* Table itself.  */
  30   void **entries;
  31
  32   /* Current size (in entries) of the hash table */
  33   size_t size;
  34
  35   /* Current number of elements.  */
  36   size_t n_elements;
  37
  38   /* Free function for the entries array.  This may vary depending on
  39      how early the array was allocated.  If it is NULL, then the array
  40      can't be freed.  */
  41   void (*free) (void *ptr);
  42 };
  43
  44 inline static struct hashtab *
  45 htab_create (void)
  46 {
  47   struct hashtab *ht = malloc (sizeof (struct hashtab));
  48
  49   if (! ht)
  50     return NULL;
  51   ht->size = 3;
  52   ht->entries = malloc (sizeof (void *) * ht->size);
  53   ht->free = __rtld_free;
  54   if (! ht->entries)
  55     {
  56       free (ht);
  57       return NULL;
  58     }
  59
  60   ht->n_elements = 0;
  61
  62   memset (ht->entries, 0, sizeof (void *) * ht->size);
  63
  64   return ht;
  65 }
  66
  67 /* This is only called from _dl_unmap, so it's safe to call
  68    free().  */
  69 inline static void
  70 htab_delete (struct hashtab *htab)
  71 {
  72   int i;
  73
  74   for (i = htab->size - 1; i >= 0; i--)
  75     free (htab->entries[i]);
  76
  77   htab->free (htab->entries);
  78   free (htab);
  79 }
  80
  81 /* Similar to htab_find_slot, but without several unwanted side effects:
  82     - Does not call htab->eq_f when it finds an existing entry.
  83     - Does not change the count of elements/searches/collisions in the
  84       hash table.
  85    This function also assumes there are no deleted entries in the table.
  86    HASH is the hash value for the element to be inserted.  */
  87
  88 inline static void **
  89 find_empty_slot_for_expand (struct hashtab *htab, int hash)
  90 {
  91   size_t size = htab->size;
  92   unsigned int index = hash % size;
  93   void **slot = htab->entries + index;
  94   int hash2;
  95
  96   if (! *slot)
  97     return slot;
  98
  99   hash2 = 1 + hash % (size - 2);
 100   for (;;)
 101     {
 102       index += hash2;
 103       if (index >= size)
 104         index -= size;
 105
 106       slot = htab->entries + index;
 107       if (! *slot)
 108         return slot;
 109     }
 110 }
 111
 112 /* The following function changes size of memory allocated for the
 113    entries and repeatedly inserts the table elements.  The occupancy
 114    of the table after the call will be about 50%.  Naturally the hash
 115    table must already exist.  Remember also that the place of the
 116    table entries is changed.  If memory allocation failures are allowed,
 117    this function will return zero, indicating that the table could not be
 118    expanded.  If all goes well, it will return a non-zero value.  */
 119
 120 inline static int
 121 htab_expand (struct hashtab *htab, int (*hash_fn) (void *))
 122 {
 123   void **oentries;
 124   void **olimit;
 125   void **p;
 126   void **nentries;
 127   size_t nsize;
 128
 129   oentries = htab->entries;
 130   olimit = oentries + htab->size;
 131
 132   /* Resize only when table after removal of unused elements is either
 133      too full or too empty.  */
 134   if (htab->n_elements * 2 > htab->size)
 135     nsize = _dl_higher_prime_number (htab->n_elements * 2);
 136   else
 137     nsize = htab->size;
 138
 139   nentries = calloc (sizeof (void *), nsize);
 140   if (nentries == NULL)
 141     return 0;
 142   htab->entries = nentries;
 143   htab->size = nsize;
 144
 145   p = oentries;
 146   do
 147     {
 148       if (*p)
 149         *find_empty_slot_for_expand (htab, hash_fn (*p))
 150           = *p;
 151
 152       p++;
 153     }
 154   while (p < olimit);
 155
 156   /* Without recording the free corresponding to the malloc used to
 157      allocate the table, we couldn't tell whether this was allocated
 158      by the malloc() built into ld.so or the one in the main
 159      executable or libc.  Calling free() for something that was
 160      allocated by the early malloc(), rather than the final run-time
 161      malloc() could do Very Bad Things (TM).  We will waste memory
 162      allocated early as long as there's no corresponding free(), but
 163      this isn't so much memory as to be significant.  */
 164
 165   htab->free (oentries);
 166
 167   /* Use the free() corresponding to the malloc() above to free this
 168      up.  */
 169   htab->free = __rtld_free;
 170
 171   return 1;
 172 }
 173
 174 /* This function searches for a hash table slot containing an entry
 175    equal to the given element.  To delete an entry, call this with
 176    INSERT = 0, then call htab_clear_slot on the slot returned (possibly
 177    after doing some checks).  To insert an entry, call this with
 178    INSERT = 1, then write the value you want into the returned slot.
 179    When inserting an entry, NULL may be returned if memory allocation
 180    fails.  */
 181
 182 inline static void **
 183 htab_find_slot (struct hashtab *htab, void *ptr, int insert,
 184                 int (*hash_fn)(void *), int (*eq_fn)(void *, void *))
 185 {
 186   unsigned int index;
 187   int hash, hash2;
 188   size_t size;
 189   void **entry;
 190
 191   if (htab->size * 3 <= htab->n_elements * 4
 192       && htab_expand (htab, hash_fn) == 0)
 193     return NULL;
 194
 195   hash = hash_fn (ptr);
 196
 197   size = htab->size;
 198   index = hash % size;
 199
 200   entry = &htab->entries[index];
 201   if (!*entry)
 202     goto empty_entry;
 203   else if (eq_fn (*entry, ptr))
 204     return entry;
 205
 206   hash2 = 1 + hash % (size - 2);
 207   for (;;)
 208     {
 209       index += hash2;
 210       if (index >= size)
 211         index -= size;
 212
 213       entry = &htab->entries[index];
 214       if (!*entry)
 215         goto empty_entry;
 216       else if (eq_fn (*entry, ptr))
 217         return entry;
 218     }
 219
 220  empty_entry:
 221   if (!insert)
 222     return NULL;
 223
 224   htab->n_elements++;
 225   return entry;
 226 }
 227
 228 #endif /* INLINE_HASHTAB_H */