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, 2008
   6    Free Software Foundation, Inc.
   7
   8    This file is part of GDB.
   9
  10    This program is free software; you can redistribute it and/or modify
  11    it under the terms of the GNU General Public License as published by
  12    the Free Software Foundation; either version 3 of the License, or
  13    (at your option) any later version.
  14
  15    This program 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
  18    GNU General Public License for more details.
  19
  20    You should have received a copy of the GNU General Public License
  21    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  22
  23 #include "defs.h"
  24 #include "gdb_obstack.h"
  25 #include "bcache.h"
  26 #include "gdb_string.h"         /* For memcpy declaration */
  27 #include "gdb_assert.h"
  28
  29 #include <stddef.h>
  30 #include <stdlib.h>
  31
  32 /* The type used to hold a single bcache string.  The user data is
  33    stored in d.data.  Since it can be any type, it needs to have the
  34    same alignment as the most strict alignment of any type on the host
  35    machine.  I don't know of any really correct way to do this in
  36    stock ANSI C, so just do it the same way obstack.h does.  */
  37
  38 struct bstring
  39 {
  40   /* Hash chain.  */
  41   struct bstring *next;
  42   /* Assume the data length is no more than 64k.  */
  43   unsigned short length;
  44   /* The half hash hack.  This contains the upper 16 bits of the hash
  45      value and is used as a pre-check when comparing two strings and
  46      avoids the need to do length or memcmp calls.  It proves to be
  47      roughly 100% effective.  */
  48   unsigned short half_hash;
  49
  50   union
  51   {
  52     char data[1];
  53     double dummy;
  54   }
  55   d;
  56 };
  57
  58
  59 /* The structure for a bcache itself.  The bcache is initialized, in
  60    bcache_xmalloc(), by filling it with zeros and then setting the
  61    corresponding obstack's malloc() and free() methods.  */
  62
  63 struct bcache
  64 {
  65   /* All the bstrings are allocated here.  */
  66   struct obstack cache;
  67
  68   /* How many hash buckets we're using.  */
  69   unsigned int num_buckets;
  70
  71   /* Hash buckets.  This table is allocated using malloc, so when we
  72      grow the table we can return the old table to the system.  */
  73   struct bstring **bucket;
  74
  75   /* Statistics.  */
  76   unsigned long unique_count;   /* number of unique strings */
  77   long total_count;     /* total number of strings cached, including dups */
  78   long unique_size;     /* size of unique strings, in bytes */
  79   long total_size;      /* total number of bytes cached, including dups */
  80   long structure_size;  /* total size of bcache, including infrastructure */
  81   /* Number of times that the hash table is expanded and hence
  82      re-built, and the corresponding number of times that a string is
  83      [re]hashed as part of entering it into the expanded table.  The
  84      total number of hashes can be computed by adding TOTAL_COUNT to
  85      expand_hash_count.  */
  86   unsigned long expand_count;
  87   unsigned long expand_hash_count;
  88   /* Number of times that the half-hash compare hit (compare the upper
  89      16 bits of hash values) hit, but the corresponding combined
  90      length/data compare missed.  */
  91   unsigned long half_hash_miss_count;
  92 };
  93
  94 /* The old hash function was stolen from SDBM. This is what DB 3.0 uses now,
  95  * and is better than the old one.
  96  */
  97 \f
  98 unsigned long
  99 hash(const void *addr, int length)
 100 {
 101                 const unsigned char *k, *e;
 102                 unsigned long h;
 103
 104                 k = (const unsigned char *)addr;
 105                 e = k+length;
 106                 for (h=0; k< e;++k)
 107                 {
 108                                 h *=16777619;
 109                                 h ^= *k;
 110                 }
 111                 return (h);
 112 }
 113 \f
 114 /* Growing the bcache's hash table.  */
 115
 116 /* If the average chain length grows beyond this, then we want to
 117    resize our hash table.  */
 118 #define CHAIN_LENGTH_THRESHOLD (5)
 119
 120 static void
 121 expand_hash_table (struct bcache *bcache)
 122 {
 123   /* A table of good hash table sizes.  Whenever we grow, we pick the
 124      next larger size from this table.  sizes[i] is close to 1 << (i+10),
 125      so we roughly double the table size each time.  After we fall off
 126      the end of this table, we just double.  Don't laugh --- there have
 127      been executables sighted with a gigabyte of debug info.  */
 128   static unsigned long sizes[] = {
 129     1021, 2053, 4099, 8191, 16381, 32771,
 130     65537, 131071, 262144, 524287, 1048573, 2097143,
 131     4194301, 8388617, 16777213, 33554467, 67108859, 134217757,
 132     268435459, 536870923, 1073741827, 2147483659UL
 133   };
 134   unsigned int new_num_buckets;
 135   struct bstring **new_buckets;
 136   unsigned int i;
 137
 138   /* Count the stats.  Every unique item needs to be re-hashed and
 139      re-entered.  */
 140   bcache->expand_count++;
 141   bcache->expand_hash_count += bcache->unique_count;
 142
 143   /* Find the next size.  */
 144   new_num_buckets = bcache->num_buckets * 2;
 145   for (i = 0; i < (sizeof (sizes) / sizeof (sizes[0])); i++)
 146     if (sizes[i] > bcache->num_buckets)
 147       {
 148         new_num_buckets = sizes[i];
 149         break;
 150       }
 151
 152   /* Allocate the new table.  */
 153   {
 154     size_t new_size = new_num_buckets * sizeof (new_buckets[0]);
 155     new_buckets = (struct bstring **) xmalloc (new_size);
 156     memset (new_buckets, 0, new_size);
 157
 158     bcache->structure_size -= (bcache->num_buckets
 159                                * sizeof (bcache->bucket[0]));
 160     bcache->structure_size += new_size;
 161   }
 162
 163   /* Rehash all existing strings.  */
 164   for (i = 0; i < bcache->num_buckets; i++)
 165     {
 166       struct bstring *s, *next;
 167
 168       for (s = bcache->bucket[i]; s; s = next)
 169         {
 170           struct bstring **new_bucket;
 171           next = s->next;
 172
 173           new_bucket = &new_buckets[(hash (&s->d.data, s->length)
 174                                      % new_num_buckets)];
 175           s->next = *new_bucket;
 176           *new_bucket = s;
 177         }
 178     }
 179
 180   /* Plug in the new table.  */
 181   if (bcache->bucket)
 182     xfree (bcache->bucket);
 183   bcache->bucket = new_buckets;
 184   bcache->num_buckets = new_num_buckets;
 185 }
 186
 187 \f
 188 /* Looking up things in the bcache.  */
 189
 190 /* The number of bytes needed to allocate a struct bstring whose data
 191    is N bytes long.  */
 192 #define BSTRING_SIZE(n) (offsetof (struct bstring, d.data) + (n))
 193
 194 /* Find a copy of the LENGTH bytes at ADDR in BCACHE.  If BCACHE has
 195    never seen those bytes before, add a copy of them to BCACHE.  In
 196    either case, return a pointer to BCACHE's copy of that string.  */
 197 const void *
 198 bcache (const void *addr, int length, struct bcache *bcache)
 199 {
 200   return bcache_full (addr, length, bcache, NULL);
 201 }
 202
 203 /* Find a copy of the LENGTH bytes at ADDR in BCACHE.  If BCACHE has
 204    never seen those bytes before, add a copy of them to BCACHE.  In
 205    either case, return a pointer to BCACHE's copy of that string.  If
 206    optional ADDED is not NULL, return 1 in case of new entry or 0 if
 207    returning an old entry.  */
 208
 209 const void *
 210 bcache_full (const void *addr, int length, struct bcache *bcache, int *added)
 211 {
 212   unsigned long full_hash;
 213   unsigned short half_hash;
 214   int hash_index;
 215   struct bstring *s;
 216
 217   if (added)
 218     *added = 0;
 219
 220   /* If our average chain length is too high, expand the hash table.  */
 221   if (bcache->unique_count >= bcache->num_buckets * CHAIN_LENGTH_THRESHOLD)
 222     expand_hash_table (bcache);
 223
 224   bcache->total_count++;
 225   bcache->total_size += length;
 226
 227   full_hash = hash (addr, length);
 228   half_hash = (full_hash >> 16);
 229   hash_index = full_hash % bcache->num_buckets;
 230
 231   /* Search the hash bucket for a string identical to the caller's.
 232      As a short-circuit first compare the upper part of each hash
 233      values.  */
 234   for (s = bcache->bucket[hash_index]; s; s = s->next)
 235     {
 236       if (s->half_hash == half_hash)
 237         {
 238           if (s->length == length
 239               && ! memcmp (&s->d.data, addr, length))
 240             return &s->d.data;
 241           else
 242             bcache->half_hash_miss_count++;
 243         }
 244     }
 245
 246   /* The user's string isn't in the list.  Insert it after *ps.  */
 247   {
 248     struct bstring *new
 249       = obstack_alloc (&bcache->cache, BSTRING_SIZE (length));
 250     memcpy (&new->d.data, addr, length);
 251     new->length = length;
 252     new->next = bcache->bucket[hash_index];
 253     new->half_hash = half_hash;
 254     bcache->bucket[hash_index] = new;
 255
 256     bcache->unique_count++;
 257     bcache->unique_size += length;
 258     bcache->structure_size += BSTRING_SIZE (length);
 259
 260     if (added)
 261       *added = 1;
 262
 263     return &new->d.data;
 264   }
 265 }
 266 \f
 267 /* Allocating and freeing bcaches.  */
 268
 269 struct bcache *
 270 bcache_xmalloc (void)
 271 {
 272   /* Allocate the bcache pre-zeroed.  */
 273   struct bcache *b = XCALLOC (1, struct bcache);
 274   /* We could use obstack_specify_allocation here instead, but
 275      gdb_obstack.h specifies the allocation/deallocation
 276      functions.  */
 277   obstack_init (&b->cache);
 278   return b;
 279 }
 280
 281 /* Free all the storage associated with BCACHE.  */
 282 void
 283 bcache_xfree (struct bcache *bcache)
 284 {
 285   if (bcache == NULL)
 286     return;
 287   obstack_free (&bcache->cache, 0);
 288   xfree (bcache->bucket);
 289   xfree (bcache);
 290 }
 291
 292
 293 \f
 294 /* Printing statistics.  */
 295
 296 static int
 297 compare_ints (const void *ap, const void *bp)
 298 {
 299   /* Because we know we're comparing two ints which are positive,
 300      there's no danger of overflow here.  */
 301   return * (int *) ap - * (int *) bp;
 302 }
 303
 304
 305 static void
 306 print_percentage (int portion, int total)
 307 {
 308   if (total == 0)
 309     /* i18n: Like "Percentage of duplicates, by count: (not applicable)" */
 310     printf_filtered (_("(not applicable)\n"));
 311   else
 312     printf_filtered ("%3d%%\n", (int) (portion * 100.0 / total));
 313 }
 314
 315
 316 /* Print statistics on BCACHE's memory usage and efficacity at
 317    eliminating duplication.  NAME should describe the kind of data
 318    BCACHE holds.  Statistics are printed using `printf_filtered' and
 319    its ilk.  */
 320 void
 321 print_bcache_statistics (struct bcache *c, char *type)
 322 {
 323   int occupied_buckets;
 324   int max_chain_length;
 325   int median_chain_length;
 326   int max_entry_size;
 327   int median_entry_size;
 328
 329   /* Count the number of occupied buckets, tally the various string
 330      lengths, and measure chain lengths.  */
 331   {
 332     unsigned int b;
 333     int *chain_length = XCALLOC (c->num_buckets + 1, int);
 334     int *entry_size = XCALLOC (c->unique_count + 1, int);
 335     int stringi = 0;
 336
 337     occupied_buckets = 0;
 338
 339     for (b = 0; b < c->num_buckets; b++)
 340       {
 341         struct bstring *s = c->bucket[b];
 342
 343         chain_length[b] = 0;
 344
 345         if (s)
 346           {
 347             occupied_buckets++;
 348
 349             while (s)
 350               {
 351                 gdb_assert (b < c->num_buckets);
 352                 chain_length[b]++;
 353                 gdb_assert (stringi < c->unique_count);
 354                 entry_size[stringi++] = s->length;
 355                 s = s->next;
 356               }
 357           }
 358       }
 359
 360     /* To compute the median, we need the set of chain lengths sorted.  */
 361     qsort (chain_length, c->num_buckets, sizeof (chain_length[0]),
 362            compare_ints);
 363     qsort (entry_size, c->unique_count, sizeof (entry_size[0]),
 364            compare_ints);
 365
 366     if (c->num_buckets > 0)
 367       {
 368         max_chain_length = chain_length[c->num_buckets - 1];
 369         median_chain_length = chain_length[c->num_buckets / 2];
 370       }
 371     else
 372       {
 373         max_chain_length = 0;
 374         median_chain_length = 0;
 375       }
 376     if (c->unique_count > 0)
 377       {
 378         max_entry_size = entry_size[c->unique_count - 1];
 379         median_entry_size = entry_size[c->unique_count / 2];
 380       }
 381     else
 382       {
 383         max_entry_size = 0;
 384         median_entry_size = 0;
 385       }
 386
 387     xfree (chain_length);
 388     xfree (entry_size);
 389   }
 390
 391   printf_filtered (_("  Cached '%s' statistics:\n"), type);
 392   printf_filtered (_("    Total object count:  %ld\n"), c->total_count);
 393   printf_filtered (_("    Unique object count: %lu\n"), c->unique_count);
 394   printf_filtered (_("    Percentage of duplicates, by count: "));
 395   print_percentage (c->total_count - c->unique_count, c->total_count);
 396   printf_filtered ("\n");
 397
 398   printf_filtered (_("    Total object size:   %ld\n"), c->total_size);
 399   printf_filtered (_("    Unique object size:  %ld\n"), c->unique_size);
 400   printf_filtered (_("    Percentage of duplicates, by size:  "));
 401   print_percentage (c->total_size - c->unique_size, c->total_size);
 402   printf_filtered ("\n");
 403
 404   printf_filtered (_("    Max entry size:     %d\n"), max_entry_size);
 405   printf_filtered (_("    Average entry size: "));
 406   if (c->unique_count > 0)
 407     printf_filtered ("%ld\n", c->unique_size / c->unique_count);
 408   else
 409     /* i18n: "Average entry size: (not applicable)" */
 410     printf_filtered (_("(not applicable)\n"));
 411   printf_filtered (_("    Median entry size:  %d\n"), median_entry_size);
 412   printf_filtered ("\n");
 413
 414   printf_filtered (_("    Total memory used by bcache, including overhead: %ld\n"),
 415                    c->structure_size);
 416   printf_filtered (_("    Percentage memory overhead: "));
 417   print_percentage (c->structure_size - c->unique_size, c->unique_size);
 418   printf_filtered (_("    Net memory savings:         "));
 419   print_percentage (c->total_size - c->structure_size, c->total_size);
 420   printf_filtered ("\n");
 421
 422   printf_filtered (_("    Hash table size:           %3d\n"), c->num_buckets);
 423   printf_filtered (_("    Hash table expands:        %lu\n"),
 424                    c->expand_count);
 425   printf_filtered (_("    Hash table hashes:         %lu\n"),
 426                    c->total_count + c->expand_hash_count);
 427   printf_filtered (_("    Half hash misses:          %lu\n"),
 428                    c->half_hash_miss_count);
 429   printf_filtered (_("    Hash table population:     "));
 430   print_percentage (occupied_buckets, c->num_buckets);
 431   printf_filtered (_("    Median hash chain length:  %3d\n"),
 432                    median_chain_length);
 433   printf_filtered (_("    Average hash chain length: "));
 434   if (c->num_buckets > 0)
 435     printf_filtered ("%3lu\n", c->unique_count / c->num_buckets);
 436   else
 437     /* i18n: "Average hash chain length: (not applicable)" */
 438     printf_filtered (_("(not applicable)\n"));
 439   printf_filtered (_("    Maximum hash chain length: %3d\n"), max_chain_length);
 440   printf_filtered ("\n");
 441 }
 442
 443 int
 444 bcache_memory_used (struct bcache *bcache)
 445 {
 446   return obstack_memory_used (&bcache->cache);
 447 }