gdb/bcache.c

   1 /* Implement a cached obstack.
   2    Written by Fred Fish <fnf@cygnus.com>
   3    Rewritten by Jim Blandy <jimb@cygnus.com>
   4
   5    Copyright (C) 1999-2019 Free Software Foundation, Inc.
   6
   7    This file is part of GDB.
   8
   9    This program is free software; you can redistribute it and/or modify
  10    it under the terms of the GNU General Public License as published by
  11    the Free Software Foundation; either version 3 of the License, or
  12    (at your option) any 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 <http://www.gnu.org/licenses/>.  */
  21
  22 #include "defs.h"
  23 #include "gdb_obstack.h"
  24 #include "bcache.h"
  25
  26 #include <algorithm>
  27
  28 namespace gdb {
  29
  30 /* The type used to hold a single bcache string.  The user data is
  31    stored in d.data.  Since it can be any type, it needs to have the
  32    same alignment as the most strict alignment of any type on the host
  33    machine.  I don't know of any really correct way to do this in
  34    stock ANSI C, so just do it the same way obstack.h does.  */
  35
  36 struct bstring
  37 {
  38   /* Hash chain.  */
  39   struct bstring *next;
  40   /* Assume the data length is no more than 64k.  */
  41   unsigned short length;
  42   /* The half hash hack.  This contains the upper 16 bits of the hash
  43      value and is used as a pre-check when comparing two strings and
  44      avoids the need to do length or memcmp calls.  It proves to be
  45      roughly 100% effective.  */
  46   unsigned short half_hash;
  47
  48   union
  49   {
  50     char data[1];
  51     double dummy;
  52   }
  53   d;
  54 };
  55
  56 \f
  57 /* Growing the bcache's hash table.  */
  58
  59 /* If the average chain length grows beyond this, then we want to
  60    resize our hash table.  */
  61 #define CHAIN_LENGTH_THRESHOLD (5)
  62
  63 void
  64 bcache::expand_hash_table ()
  65 {
  66   /* A table of good hash table sizes.  Whenever we grow, we pick the
  67      next larger size from this table.  sizes[i] is close to 1 << (i+10),
  68      so we roughly double the table size each time.  After we fall off
  69      the end of this table, we just double.  Don't laugh --- there have
  70      been executables sighted with a gigabyte of debug info.  */
  71   static unsigned long sizes[] = {
  72     1021, 2053, 4099, 8191, 16381, 32771,
  73     65537, 131071, 262144, 524287, 1048573, 2097143,
  74     4194301, 8388617, 16777213, 33554467, 67108859, 134217757,
  75     268435459, 536870923, 1073741827, 2147483659UL
  76   };
  77   unsigned int new_num_buckets;
  78   struct bstring **new_buckets;
  79   unsigned int i;
  80
  81   /* Count the stats.  Every unique item needs to be re-hashed and
  82      re-entered.  */
  83   m_expand_count++;
  84   m_expand_hash_count += m_unique_count;
  85
  86   /* Find the next size.  */
  87   new_num_buckets = m_num_buckets * 2;
  88   for (i = 0; i < (sizeof (sizes) / sizeof (sizes[0])); i++)
  89     if (sizes[i] > m_num_buckets)
  90       {
  91         new_num_buckets = sizes[i];
  92         break;
  93       }
  94
  95   /* Allocate the new table.  */
  96   {
  97     size_t new_size = new_num_buckets * sizeof (new_buckets[0]);
  98
  99     new_buckets = (struct bstring **) xmalloc (new_size);
 100     memset (new_buckets, 0, new_size);
 101
 102     m_structure_size -= m_num_buckets * sizeof (m_bucket[0]);
 103     m_structure_size += new_size;
 104   }
 105
 106   /* Rehash all existing strings.  */
 107   for (i = 0; i < m_num_buckets; i++)
 108     {
 109       struct bstring *s, *next;
 110
 111       for (s = m_bucket[i]; s; s = next)
 112         {
 113           struct bstring **new_bucket;
 114           next = s->next;
 115
 116           new_bucket = &new_buckets[(m_hash_function (&s->d.data, s->length)
 117                                      % new_num_buckets)];
 118           s->next = *new_bucket;
 119           *new_bucket = s;
 120         }
 121     }
 122
 123   /* Plug in the new table.  */
 124   xfree (m_bucket);
 125   m_bucket = new_buckets;
 126   m_num_buckets = new_num_buckets;
 127 }
 128
 129 \f
 130 /* Looking up things in the bcache.  */
 131
 132 /* The number of bytes needed to allocate a struct bstring whose data
 133    is N bytes long.  */
 134 #define BSTRING_SIZE(n) (offsetof (struct bstring, d.data) + (n))
 135
 136 /* Find a copy of the LENGTH bytes at ADDR in BCACHE.  If BCACHE has
 137    never seen those bytes before, add a copy of them to BCACHE.  In
 138    either case, return a pointer to BCACHE's copy of that string.  If
 139    optional ADDED is not NULL, return 1 in case of new entry or 0 if
 140    returning an old entry.  */
 141
 142 const void *
 143 bcache::insert (const void *addr, int length, int *added)
 144 {
 145   unsigned long full_hash;
 146   unsigned short half_hash;
 147   int hash_index;
 148   struct bstring *s;
 149
 150   if (added)
 151     *added = 0;
 152
 153   /* Lazily initialize the obstack.  This can save quite a bit of
 154      memory in some cases.  */
 155   if (m_total_count == 0)
 156     {
 157       /* We could use obstack_specify_allocation here instead, but
 158          gdb_obstack.h specifies the allocation/deallocation
 159          functions.  */
 160       obstack_init (&m_cache);
 161     }
 162
 163   /* If our average chain length is too high, expand the hash table.  */
 164   if (m_unique_count >= m_num_buckets * CHAIN_LENGTH_THRESHOLD)
 165     expand_hash_table ();
 166
 167   m_total_count++;
 168   m_total_size += length;
 169
 170   full_hash = m_hash_function (addr, length);
 171
 172   half_hash = (full_hash >> 16);
 173   hash_index = full_hash % m_num_buckets;
 174
 175   /* Search the hash m_bucket for a string identical to the caller's.
 176      As a short-circuit first compare the upper part of each hash
 177      values.  */
 178   for (s = m_bucket[hash_index]; s; s = s->next)
 179     {
 180       if (s->half_hash == half_hash)
 181         {
 182           if (s->length == length
 183               && m_compare_function (&s->d.data, addr, length))
 184             return &s->d.data;
 185           else
 186             m_half_hash_miss_count++;
 187         }
 188     }
 189
 190   /* The user's string isn't in the list.  Insert it after *ps.  */
 191   {
 192     struct bstring *newobj
 193       = (struct bstring *) obstack_alloc (&m_cache,
 194                                           BSTRING_SIZE (length));
 195
 196     memcpy (&newobj->d.data, addr, length);
 197     newobj->length = length;
 198     newobj->next = m_bucket[hash_index];
 199     newobj->half_hash = half_hash;
 200     m_bucket[hash_index] = newobj;
 201
 202     m_unique_count++;
 203     m_unique_size += length;
 204     m_structure_size += BSTRING_SIZE (length);
 205
 206     if (added)
 207       *added = 1;
 208
 209     return &newobj->d.data;
 210   }
 211 }
 212 \f
 213
 214 /* Compare the byte string at ADDR1 of lenght LENGHT to the
 215    string at ADDR2.  Return 1 if they are equal.  */
 216
 217 int
 218 bcache::compare (const void *addr1, const void *addr2, int length)
 219 {
 220   return memcmp (addr1, addr2, length) == 0;
 221 }
 222
 223 /* Free all the storage associated with BCACHE.  */
 224 bcache::~bcache ()
 225 {
 226   /* Only free the obstack if we actually initialized it.  */
 227   if (m_total_count > 0)
 228     obstack_free (&m_cache, 0);
 229   xfree (m_bucket);
 230 }
 231
 232
 233 \f
 234 /* Printing statistics.  */
 235
 236 static void
 237 print_percentage (int portion, int total)
 238 {
 239   if (total == 0)
 240     /* i18n: Like "Percentage of duplicates, by count: (not applicable)".  */
 241     printf_filtered (_("(not applicable)\n"));
 242   else
 243     printf_filtered ("%3d%%\n", (int) (portion * 100.0 / total));
 244 }
 245
 246
 247 /* Print statistics on BCACHE's memory usage and efficacity at
 248    eliminating duplication.  NAME should describe the kind of data
 249    BCACHE holds.  Statistics are printed using `printf_filtered' and
 250    its ilk.  */
 251 void
 252 bcache::print_statistics (const char *type)
 253 {
 254   int occupied_buckets;
 255   int max_chain_length;
 256   int median_chain_length;
 257   int max_entry_size;
 258   int median_entry_size;
 259
 260   /* Count the number of occupied buckets, tally the various string
 261      lengths, and measure chain lengths.  */
 262   {
 263     unsigned int b;
 264     int *chain_length = XCNEWVEC (int, m_num_buckets + 1);
 265     int *entry_size = XCNEWVEC (int, m_unique_count + 1);
 266     int stringi = 0;
 267
 268     occupied_buckets = 0;
 269
 270     for (b = 0; b < m_num_buckets; b++)
 271       {
 272         struct bstring *s = m_bucket[b];
 273
 274         chain_length[b] = 0;
 275
 276         if (s)
 277           {
 278             occupied_buckets++;
 279
 280             while (s)
 281               {
 282                 gdb_assert (b < m_num_buckets);
 283                 chain_length[b]++;
 284                 gdb_assert (stringi < m_unique_count);
 285                 entry_size[stringi++] = s->length;
 286                 s = s->next;
 287               }
 288           }
 289       }
 290
 291     /* To compute the median, we need the set of chain lengths
 292        sorted.  */
 293     std::sort (chain_length, chain_length + m_num_buckets);
 294     std::sort (entry_size, entry_size + m_unique_count);
 295
 296     if (m_num_buckets > 0)
 297       {
 298         max_chain_length = chain_length[m_num_buckets - 1];
 299         median_chain_length = chain_length[m_num_buckets / 2];
 300       }
 301     else
 302       {
 303         max_chain_length = 0;
 304         median_chain_length = 0;
 305       }
 306     if (m_unique_count > 0)
 307       {
 308         max_entry_size = entry_size[m_unique_count - 1];
 309         median_entry_size = entry_size[m_unique_count / 2];
 310       }
 311     else
 312       {
 313         max_entry_size = 0;
 314         median_entry_size = 0;
 315       }
 316
 317     xfree (chain_length);
 318     xfree (entry_size);
 319   }
 320
 321   printf_filtered (_("  M_Cached '%s' statistics:\n"), type);
 322   printf_filtered (_("    Total object count:  %ld\n"), m_total_count);
 323   printf_filtered (_("    Unique object count: %lu\n"), m_unique_count);
 324   printf_filtered (_("    Percentage of duplicates, by count: "));
 325   print_percentage (m_total_count - m_unique_count, m_total_count);
 326   printf_filtered ("\n");
 327
 328   printf_filtered (_("    Total object size:   %ld\n"), m_total_size);
 329   printf_filtered (_("    Unique object size:  %ld\n"), m_unique_size);
 330   printf_filtered (_("    Percentage of duplicates, by size:  "));
 331   print_percentage (m_total_size - m_unique_size, m_total_size);
 332   printf_filtered ("\n");
 333
 334   printf_filtered (_("    Max entry size:     %d\n"), max_entry_size);
 335   printf_filtered (_("    Average entry size: "));
 336   if (m_unique_count > 0)
 337     printf_filtered ("%ld\n", m_unique_size / m_unique_count);
 338   else
 339     /* i18n: "Average entry size: (not applicable)".  */
 340     printf_filtered (_("(not applicable)\n"));
 341   printf_filtered (_("    Median entry size:  %d\n"), median_entry_size);
 342   printf_filtered ("\n");
 343
 344   printf_filtered (_("    \
 345 Total memory used by bcache, including overhead: %ld\n"),
 346                    m_structure_size);
 347   printf_filtered (_("    Percentage memory overhead: "));
 348   print_percentage (m_structure_size - m_unique_size, m_unique_size);
 349   printf_filtered (_("    Net memory savings:         "));
 350   print_percentage (m_total_size - m_structure_size, m_total_size);
 351   printf_filtered ("\n");
 352
 353   printf_filtered (_("    Hash table size:           %3d\n"),
 354                    m_num_buckets);
 355   printf_filtered (_("    Hash table expands:        %lu\n"),
 356                    m_expand_count);
 357   printf_filtered (_("    Hash table hashes:         %lu\n"),
 358                    m_total_count + m_expand_hash_count);
 359   printf_filtered (_("    Half hash misses:          %lu\n"),
 360                    m_half_hash_miss_count);
 361   printf_filtered (_("    Hash table population:     "));
 362   print_percentage (occupied_buckets, m_num_buckets);
 363   printf_filtered (_("    Median hash chain length:  %3d\n"),
 364                    median_chain_length);
 365   printf_filtered (_("    Average hash chain length: "));
 366   if (m_num_buckets > 0)
 367     printf_filtered ("%3lu\n", m_unique_count / m_num_buckets);
 368   else
 369     /* i18n: "Average hash chain length: (not applicable)".  */
 370     printf_filtered (_("(not applicable)\n"));
 371   printf_filtered (_("    Maximum hash chain length: %3d\n"),
 372                    max_chain_length);
 373   printf_filtered ("\n");
 374 }
 375
 376 int
 377 bcache::memory_used ()
 378 {
 379   if (m_total_count == 0)
 380     return 0;
 381   return obstack_memory_used (&m_cache);
 382 }
 383
 384 } /* namespace gdb */