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, 2000, 2002, 2003, 2007 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 2 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, write to the Free Software
  21    Foundation, Inc., 51 Franklin Street, Fifth Floor,
  22    Boston, MA 02110-1301, USA.  */
  23
  24 #include "defs.h"
  25 #include "gdb_obstack.h"
  26 #include "bcache.h"
  27 #include "gdb_string.h"         /* For memcpy declaration */
  28 #include "gdb_assert.h"
  29
  30 #include <stddef.h>
  31 #include <stdlib.h>
  32
  33 /* The type used to hold a single bcache string.  The user data is
  34    stored in d.data.  Since it can be any type, it needs to have the
  35    same alignment as the most strict alignment of any type on the host
  36    machine.  I don't know of any really correct way to do this in
  37    stock ANSI C, so just do it the same way obstack.h does.  */
  38
  39 struct bstring
  40 {
  41   /* Hash chain.  */
  42   struct bstring *next;
  43   /* Assume the data length is no more than 64k.  */
  44   unsigned short length;
  45   /* The half hash hack.  This contains the upper 16 bits of the hash
  46      value and is used as a pre-check when comparing two strings and
  47      avoids the need to do length or memcmp calls.  It proves to be
  48      roughly 100% effective.  */
  49   unsigned short half_hash;
  50
  51   union
  52   {
  53     char data[1];
  54     double dummy;
  55   }
  56   d;
  57 };
  58
  59
  60 /* The structure for a bcache itself.  The bcache is initialized, in
  61    bcache_xmalloc(), by filling it with zeros and then setting the
  62    corresponding obstack's malloc() and free() methods.  */
  63
  64 struct bcache
  65 {
  66   /* All the bstrings are allocated here.  */
  67   struct obstack cache;
  68
  69   /* How many hash buckets we're using.  */
  70   unsigned int num_buckets;
  71
  72   /* Hash buckets.  This table is allocated using malloc, so when we
  73      grow the table we can return the old table to the system.  */
  74   struct bstring **bucket;
  75
  76   /* Statistics.  */
  77   unsigned long unique_count;   /* number of unique strings */
  78   long total_count;     /* total number of strings cached, including dups */
  79   long unique_size;     /* size of unique strings, in bytes */
  80   long total_size;      /* total number of bytes cached, including dups */
  81   long structure_size;  /* total size of bcache, including infrastructure */
  82   /* Number of times that the hash table is expanded and hence
  83      re-built, and the corresponding number of times that a string is
  84      [re]hashed as part of entering it into the expanded table.  The
  85      total number of hashes can be computed by adding TOTAL_COUNT to
  86      expand_hash_count.  */
  87   unsigned long expand_count;
  88   unsigned long expand_hash_count;
  89   /* Number of times that the half-hash compare hit (compare the upper
  90      16 bits of hash values) hit, but the corresponding combined
  91      length/data compare missed.  */
  92   unsigned long half_hash_miss_count;
  93 };
  94
  95 /* The old hash function was stolen from SDBM. This is what DB 3.0 uses now,
  96  * and is better than the old one.
  97  */
  98 \f
  99 unsigned long
 100 hash(const void *addr, int length)
 101 {
 102                 const unsigned char *k, *e;
 103                 unsigned long h;
 104
 105                 k = (const unsigned char *)addr;
 106                 e = k+length;
 107                 for (h=0; k< e;++k)
 108                 {
 109                                 h *=16777619;
 110                                 h ^= *k;
 111                 }
 112                 return (h);
 113 }
 114 \f
 115 /* Growing the bcache's hash table.  */
 116
 117 /* If the average chain length grows beyond this, then we want to
 118    resize our hash table.  */
 119 #define CHAIN_LENGTH_THRESHOLD (5)
 120
 121 static void
 122 expand_hash_table (struct bcache *bcache)
 123 {
 124   /* A table of good hash table sizes.  Whenever we grow, we pick the
 125      next larger size from this table.  sizes[i] is close to 1 << (i+10),
 126      so we roughly double the table size each time.  After we fall off
 127      the end of this table, we just double.  Don't laugh --- there have
 128      been executables sighted with a gigabyte of debug info.  */
 129   static unsigned long sizes[] = {
 130     1021, 2053, 4099, 8191, 16381, 32771,
 131     65537, 131071, 262144, 524287, 1048573, 2097143,
 132     4194301, 8388617, 16777213, 33554467, 67108859, 134217757,
 133     268435459, 536870923, 1073741827, 2147483659UL
 134   };
 135   unsigned int new_num_buckets;
 136   struct bstring **new_buckets;
 137   unsigned int i;
 138
 139   /* Count the stats.  Every unique item needs to be re-hashed and
 140      re-entered.  */
 141   bcache->expand_count++;
 142   bcache->expand_hash_count += bcache->unique_count;
 143
 144   /* Find the next size.  */
 145   new_num_buckets = bcache->num_buckets * 2;
 146   for (i = 0; i < (sizeof (sizes) / sizeof (sizes[0])); i++)
 147     if (sizes[i] > bcache->num_buckets)
 148       {
 149         new_num_buckets = sizes[i];
 150         break;
 151       }
 152
 153   /* Allocate the new table.  */
 154   {
 155     size_t new_size = new_num_buckets * sizeof (new_buckets[0]);
 156     new_buckets = (struct bstring **) xmalloc (new_size);
 157     memset (new_buckets, 0, new_size);
 158
 159     bcache->structure_size -= (bcache->num_buckets
 160                                * sizeof (bcache->bucket[0]));
 161     bcache->structure_size += new_size;
 162   }
 163
 164   /* Rehash all existing strings.  */
 165   for (i = 0; i < bcache->num_buckets; i++)
 166     {
 167       struct bstring *s, *next;
 168
 169       for (s = bcache->bucket[i]; s; s = next)
 170         {
 171           struct bstring **new_bucket;
 172           next = s->next;
 173
 174           new_bucket = &new_buckets[(hash (&s->d.data, s->length)
 175                                      % new_num_buckets)];
 176           s->next = *new_bucket;
 177           *new_bucket = s;
 178         }
 179     }
 180
 181   /* Plug in the new table.  */
 182   if (bcache->bucket)
 183     xfree (bcache->bucket);
 184   bcache->bucket = new_buckets;
 185   bcache->num_buckets = new_num_buckets;
 186 }
 187
 188 \f
 189 /* Looking up things in the bcache.  */
 190
 191 /* The number of bytes needed to allocate a struct bstring whose data
 192    is N bytes long.  */
 193 #define BSTRING_SIZE(n) (offsetof (struct bstring, d.data) + (n))
 194
 195 /* Find a copy of the LENGTH bytes at ADDR in BCACHE.  If BCACHE has
 196    never seen those bytes before, add a copy of them to BCACHE.  In
 197    either case, return a pointer to BCACHE's copy of that string.  */
 198 static void *
 199 bcache_data (const void *addr, int length, struct bcache *bcache)
 200 {
 201   unsigned long full_hash;
 202   unsigned short half_hash;
 203   int hash_index;
 204   struct bstring *s;
 205
 206   /* If our average chain length is too high, expand the hash table.  */
 207   if (bcache->unique_count >= bcache->num_buckets * CHAIN_LENGTH_THRESHOLD)
 208     expand_hash_table (bcache);
 209
 210   bcache->total_count++;
 211   bcache->total_size += length;
 212
 213   full_hash = hash (addr, length);
 214   half_hash = (full_hash >> 16);
 215   hash_index = full_hash % bcache->num_buckets;
 216
 217   /* Search the hash bucket for a string identical to the caller's.
 218      As a short-circuit first compare the upper part of each hash
 219      values.  */
 220   for (s = bcache->bucket[hash_index]; s; s = s->next)
 221     {
 222       if (s->half_hash == half_hash)
 223         {
 224           if (s->length == length
 225               && ! memcmp (&s->d.data, addr, length))
 226             return &s->d.data;
 227           else
 228             bcache->half_hash_miss_count++;
 229         }
 230     }
 231
 232   /* The user's string isn't in the list.  Insert it after *ps.  */
 233   {
 234     struct bstring *new
 235       = obstack_alloc (&bcache->cache, BSTRING_SIZE (length));
 236     memcpy (&new->d.data, addr, length);
 237     new->length = length;
 238     new->next = bcache->bucket[hash_index];
 239     new->half_hash = half_hash;
 240     bcache->bucket[hash_index] = new;
 241
 242     bcache->unique_count++;
 243     bcache->unique_size += length;
 244     bcache->structure_size += BSTRING_SIZE (length);
 245
 246     return &new->d.data;
 247   }
 248 }
 249
 250 void *
 251 deprecated_bcache (const void *addr, int length, struct bcache *bcache)
 252 {
 253   return bcache_data (addr, length, bcache);
 254 }
 255
 256 const void *
 257 bcache (const void *addr, int length, struct bcache *bcache)
 258 {
 259   return bcache_data (addr, length, bcache);
 260 }
 261 \f
 262 /* Allocating and freeing bcaches.  */
 263
 264 struct bcache *
 265 bcache_xmalloc (void)
 266 {
 267   /* Allocate the bcache pre-zeroed.  */
 268   struct bcache *b = XCALLOC (1, struct bcache);
 269   /* We could use obstack_specify_allocation here instead, but
 270      gdb_obstack.h specifies the allocation/deallocation
 271      functions.  */
 272   obstack_init (&b->cache);
 273   return b;
 274 }
 275
 276 /* Free all the storage associated with BCACHE.  */
 277 void
 278 bcache_xfree (struct bcache *bcache)
 279 {
 280   if (bcache == NULL)
 281     return;
 282   obstack_free (&bcache->cache, 0);
 283   xfree (bcache->bucket);
 284   xfree (bcache);
 285 }
 286
 287
 288 \f
 289 /* Printing statistics.  */
 290
 291 static int
 292 compare_ints (const void *ap, const void *bp)
 293 {
 294   /* Because we know we're comparing two ints which are positive,
 295      there's no danger of overflow here.  */
 296   return * (int *) ap - * (int *) bp;
 297 }
 298
 299
 300 static void
 301 print_percentage (int portion, int total)
 302 {
 303   if (total == 0)
 304     /* i18n: Like "Percentage of duplicates, by count: (not applicable)" */
 305     printf_filtered (_("(not applicable)\n"));
 306   else
 307     printf_filtered ("%3d%%\n", (int) (portion * 100.0 / total));
 308 }
 309
 310
 311 /* Print statistics on BCACHE's memory usage and efficacity at
 312    eliminating duplication.  NAME should describe the kind of data
 313    BCACHE holds.  Statistics are printed using `printf_filtered' and
 314    its ilk.  */
 315 void
 316 print_bcache_statistics (struct bcache *c, char *type)
 317 {
 318   int occupied_buckets;
 319   int max_chain_length;
 320   int median_chain_length;
 321   int max_entry_size;
 322   int median_entry_size;
 323
 324   /* Count the number of occupied buckets, tally the various string
 325      lengths, and measure chain lengths.  */
 326   {
 327     unsigned int b;
 328     int *chain_length = XCALLOC (c->num_buckets + 1, int);
 329     int *entry_size = XCALLOC (c->unique_count + 1, int);
 330     int stringi = 0;
 331
 332     occupied_buckets = 0;
 333
 334     for (b = 0; b < c->num_buckets; b++)
 335       {
 336         struct bstring *s = c->bucket[b];
 337
 338         chain_length[b] = 0;
 339
 340         if (s)
 341           {
 342             occupied_buckets++;
 343
 344             while (s)
 345               {
 346                 gdb_assert (b < c->num_buckets);
 347                 chain_length[b]++;
 348                 gdb_assert (stringi < c->unique_count);
 349                 entry_size[stringi++] = s->length;
 350                 s = s->next;
 351               }
 352           }
 353       }
 354
 355     /* To compute the median, we need the set of chain lengths sorted.  */
 356     qsort (chain_length, c->num_buckets, sizeof (chain_length[0]),
 357            compare_ints);
 358     qsort (entry_size, c->unique_count, sizeof (entry_size[0]),
 359            compare_ints);
 360
 361     if (c->num_buckets > 0)
 362       {
 363         max_chain_length = chain_length[c->num_buckets - 1];
 364         median_chain_length = chain_length[c->num_buckets / 2];
 365       }
 366     else
 367       {
 368         max_chain_length = 0;
 369         median_chain_length = 0;
 370       }
 371     if (c->unique_count > 0)
 372       {
 373         max_entry_size = entry_size[c->unique_count - 1];
 374         median_entry_size = entry_size[c->unique_count / 2];
 375       }
 376     else
 377       {
 378         max_entry_size = 0;
 379         median_entry_size = 0;
 380       }
 381
 382     xfree (chain_length);
 383     xfree (entry_size);
 384   }
 385
 386   printf_filtered (_("  Cached '%s' statistics:\n"), type);
 387   printf_filtered (_("    Total object count:  %ld\n"), c->total_count);
 388   printf_filtered (_("    Unique object count: %lu\n"), c->unique_count);
 389   printf_filtered (_("    Percentage of duplicates, by count: "));
 390   print_percentage (c->total_count - c->unique_count, c->total_count);
 391   printf_filtered ("\n");
 392
 393   printf_filtered (_("    Total object size:   %ld\n"), c->total_size);
 394   printf_filtered (_("    Unique object size:  %ld\n"), c->unique_size);
 395   printf_filtered (_("    Percentage of duplicates, by size:  "));
 396   print_percentage (c->total_size - c->unique_size, c->total_size);
 397   printf_filtered ("\n");
 398
 399   printf_filtered (_("    Max entry size:     %d\n"), max_entry_size);
 400   printf_filtered (_("    Average entry size: "));
 401   if (c->unique_count > 0)
 402     printf_filtered ("%ld\n", c->unique_size / c->unique_count);
 403   else
 404     /* i18n: "Average entry size: (not applicable)" */
 405     printf_filtered (_("(not applicable)\n"));
 406   printf_filtered (_("    Median entry size:  %d\n"), median_entry_size);
 407   printf_filtered ("\n");
 408
 409   printf_filtered (_("    Total memory used by bcache, including overhead: %ld\n"),
 410                    c->structure_size);
 411   printf_filtered (_("    Percentage memory overhead: "));
 412   print_percentage (c->structure_size - c->unique_size, c->unique_size);
 413   printf_filtered (_("    Net memory savings:         "));
 414   print_percentage (c->total_size - c->structure_size, c->total_size);
 415   printf_filtered ("\n");
 416
 417   printf_filtered (_("    Hash table size:           %3d\n"), c->num_buckets);
 418   printf_filtered (_("    Hash table expands:        %lu\n"),
 419                    c->expand_count);
 420   printf_filtered (_("    Hash table hashes:         %lu\n"),
 421                    c->total_count + c->expand_hash_count);
 422   printf_filtered (_("    Half hash misses:          %lu\n"),
 423                    c->half_hash_miss_count);
 424   printf_filtered (_("    Hash table population:     "));
 425   print_percentage (occupied_buckets, c->num_buckets);
 426   printf_filtered (_("    Median hash chain length:  %3d\n"),
 427                    median_chain_length);
 428   printf_filtered (_("    Average hash chain length: "));
 429   if (c->num_buckets > 0)
 430     printf_filtered ("%3lu\n", c->unique_count / c->num_buckets);
 431   else
 432     /* i18n: "Average hash chain length: (not applicable)" */
 433     printf_filtered (_("(not applicable)\n"));
 434   printf_filtered (_("    Maximum hash chain length: %3d\n"), max_chain_length);
 435   printf_filtered ("\n");
 436 }
 437
 438 int
 439 bcache_memory_used (struct bcache *bcache)
 440 {
 441   return obstack_memory_used (&bcache->cache);
 442 }