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) 1999-2013 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 #include "gdb_string.h" /* For memcpy declaration */
26 #include "gdb_assert.h"
27
28 #include <stddef.h>
29 #include <stdlib.h>
30
31 /* The type used to hold a single bcache string. The user data is
32 stored in d.data. Since it can be any type, it needs to have the
33 same alignment as the most strict alignment of any type on the host
34 machine. I don't know of any really correct way to do this in
35 stock ANSI C, so just do it the same way obstack.h does. */
36
37 struct bstring
38 {
39 /* Hash chain. */
40 struct bstring *next;
41 /* Assume the data length is no more than 64k. */
42 unsigned short length;
43 /* The half hash hack. This contains the upper 16 bits of the hash
44 value and is used as a pre-check when comparing two strings and
45 avoids the need to do length or memcmp calls. It proves to be
46 roughly 100% effective. */
47 unsigned short half_hash;
48
49 union
50 {
51 char data[1];
52 double dummy;
53 }
54 d;
55 };
56
57
58 /* The structure for a bcache itself. The bcache is initialized, in
59 bcache_xmalloc(), by filling it with zeros and then setting the
60 corresponding obstack's malloc() and free() methods. */
61
62 struct bcache
63 {
64 /* All the bstrings are allocated here. */
65 struct obstack cache;
66
67 /* How many hash buckets we're using. */
68 unsigned int num_buckets;
69
70 /* Hash buckets. This table is allocated using malloc, so when we
71 grow the table we can return the old table to the system. */
72 struct bstring **bucket;
73
74 /* Statistics. */
75 unsigned long unique_count; /* number of unique strings */
76 long total_count; /* total number of strings cached, including dups */
77 long unique_size; /* size of unique strings, in bytes */
78 long total_size; /* total number of bytes cached, including dups */
79 long structure_size; /* total size of bcache, including infrastructure */
80 /* Number of times that the hash table is expanded and hence
81 re-built, and the corresponding number of times that a string is
82 [re]hashed as part of entering it into the expanded table. The
83 total number of hashes can be computed by adding TOTAL_COUNT to
84 expand_hash_count. */
85 unsigned long expand_count;
86 unsigned long expand_hash_count;
87 /* Number of times that the half-hash compare hit (compare the upper
88 16 bits of hash values) hit, but the corresponding combined
89 length/data compare missed. */
90 unsigned long half_hash_miss_count;
91
92 /* Hash function to be used for this bcache object. */
93 unsigned long (*hash_function)(const void *addr, int length);
94
95 /* Compare function to be used for this bcache object. */
96 int (*compare_function)(const void *, const void *, int length);
97 };
98
99 /* The old hash function was stolen from SDBM. This is what DB 3.0
100 uses now, and is better than the old one. */
101
102 unsigned long
hash(const void * addr,int length)103 hash(const void *addr, int length)
104 {
105 return hash_continue (addr, length, 0);
106 }
107
108 /* Continue the calculation of the hash H at the given address. */
109
110 unsigned long
hash_continue(const void * addr,int length,unsigned long h)111 hash_continue (const void *addr, int length, unsigned long h)
112 {
113 const unsigned char *k, *e;
114
115 k = (const unsigned char *)addr;
116 e = k+length;
117 for (; k< e;++k)
118 {
119 h *=16777619;
120 h ^= *k;
121 }
122 return (h);
123 }
124
125 /* Growing the bcache's hash table. */
126
127 /* If the average chain length grows beyond this, then we want to
128 resize our hash table. */
129 #define CHAIN_LENGTH_THRESHOLD (5)
130
131 static void
expand_hash_table(struct bcache * bcache)132 expand_hash_table (struct bcache *bcache)
133 {
134 /* A table of good hash table sizes. Whenever we grow, we pick the
135 next larger size from this table. sizes[i] is close to 1 << (i+10),
136 so we roughly double the table size each time. After we fall off
137 the end of this table, we just double. Don't laugh --- there have
138 been executables sighted with a gigabyte of debug info. */
139 static unsigned long sizes[] = {
140 1021, 2053, 4099, 8191, 16381, 32771,
141 65537, 131071, 262144, 524287, 1048573, 2097143,
142 4194301, 8388617, 16777213, 33554467, 67108859, 134217757,
143 268435459, 536870923, 1073741827, 2147483659UL
144 };
145 unsigned int new_num_buckets;
146 struct bstring **new_buckets;
147 unsigned int i;
148
149 /* Count the stats. Every unique item needs to be re-hashed and
150 re-entered. */
151 bcache->expand_count++;
152 bcache->expand_hash_count += bcache->unique_count;
153
154 /* Find the next size. */
155 new_num_buckets = bcache->num_buckets * 2;
156 for (i = 0; i < (sizeof (sizes) / sizeof (sizes[0])); i++)
157 if (sizes[i] > bcache->num_buckets)
158 {
159 new_num_buckets = sizes[i];
160 break;
161 }
162
163 /* Allocate the new table. */
164 {
165 size_t new_size = new_num_buckets * sizeof (new_buckets[0]);
166
167 new_buckets = (struct bstring **) xmalloc (new_size);
168 memset (new_buckets, 0, new_size);
169
170 bcache->structure_size -= (bcache->num_buckets
171 * sizeof (bcache->bucket[0]));
172 bcache->structure_size += new_size;
173 }
174
175 /* Rehash all existing strings. */
176 for (i = 0; i < bcache->num_buckets; i++)
177 {
178 struct bstring *s, *next;
179
180 for (s = bcache->bucket[i]; s; s = next)
181 {
182 struct bstring **new_bucket;
183 next = s->next;
184
185 new_bucket = &new_buckets[(bcache->hash_function (&s->d.data,
186 s->length)
187 % new_num_buckets)];
188 s->next = *new_bucket;
189 *new_bucket = s;
190 }
191 }
192
193 /* Plug in the new table. */
194 if (bcache->bucket)
195 xfree (bcache->bucket);
196 bcache->bucket = new_buckets;
197 bcache->num_buckets = new_num_buckets;
198 }
199
200
201 /* Looking up things in the bcache. */
202
203 /* The number of bytes needed to allocate a struct bstring whose data
204 is N bytes long. */
205 #define BSTRING_SIZE(n) (offsetof (struct bstring, d.data) + (n))
206
207 /* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
208 never seen those bytes before, add a copy of them to BCACHE. In
209 either case, return a pointer to BCACHE's copy of that string. */
210 const void *
bcache(const void * addr,int length,struct bcache * cache)211 bcache (const void *addr, int length, struct bcache *cache)
212 {
213 return bcache_full (addr, length, cache, NULL);
214 }
215
216 /* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
217 never seen those bytes before, add a copy of them to BCACHE. In
218 either case, return a pointer to BCACHE's copy of that string. If
219 optional ADDED is not NULL, return 1 in case of new entry or 0 if
220 returning an old entry. */
221
222 const void *
bcache_full(const void * addr,int length,struct bcache * bcache,int * added)223 bcache_full (const void *addr, int length, struct bcache *bcache, int *added)
224 {
225 unsigned long full_hash;
226 unsigned short half_hash;
227 int hash_index;
228 struct bstring *s;
229
230 if (added)
231 *added = 0;
232
233 /* Lazily initialize the obstack. This can save quite a bit of
234 memory in some cases. */
235 if (bcache->total_count == 0)
236 {
237 /* We could use obstack_specify_allocation here instead, but
238 gdb_obstack.h specifies the allocation/deallocation
239 functions. */
240 obstack_init (&bcache->cache);
241 }
242
243 /* If our average chain length is too high, expand the hash table. */
244 if (bcache->unique_count >= bcache->num_buckets * CHAIN_LENGTH_THRESHOLD)
245 expand_hash_table (bcache);
246
247 bcache->total_count++;
248 bcache->total_size += length;
249
250 full_hash = bcache->hash_function (addr, length);
251
252 half_hash = (full_hash >> 16);
253 hash_index = full_hash % bcache->num_buckets;
254
255 /* Search the hash bucket for a string identical to the caller's.
256 As a short-circuit first compare the upper part of each hash
257 values. */
258 for (s = bcache->bucket[hash_index]; s; s = s->next)
259 {
260 if (s->half_hash == half_hash)
261 {
262 if (s->length == length
263 && bcache->compare_function (&s->d.data, addr, length))
264 return &s->d.data;
265 else
266 bcache->half_hash_miss_count++;
267 }
268 }
269
270 /* The user's string isn't in the list. Insert it after *ps. */
271 {
272 struct bstring *new
273 = obstack_alloc (&bcache->cache, BSTRING_SIZE (length));
274
275 memcpy (&new->d.data, addr, length);
276 new->length = length;
277 new->next = bcache->bucket[hash_index];
278 new->half_hash = half_hash;
279 bcache->bucket[hash_index] = new;
280
281 bcache->unique_count++;
282 bcache->unique_size += length;
283 bcache->structure_size += BSTRING_SIZE (length);
284
285 if (added)
286 *added = 1;
287
288 return &new->d.data;
289 }
290 }
291
292
293 /* Compare the byte string at ADDR1 of lenght LENGHT to the
294 string at ADDR2. Return 1 if they are equal. */
295
296 static int
bcache_compare(const void * addr1,const void * addr2,int length)297 bcache_compare (const void *addr1, const void *addr2, int length)
298 {
299 return memcmp (addr1, addr2, length) == 0;
300 }
301
302 /* Allocating and freeing bcaches. */
303
304 /* Allocated a bcache. HASH_FUNCTION and COMPARE_FUNCTION can be used
305 to pass in custom hash, and compare functions to be used by this
306 bcache. If HASH_FUNCTION is NULL hash() is used and if
307 COMPARE_FUNCTION is NULL memcmp() is used. */
308
309 struct bcache *
bcache_xmalloc(unsigned long (* hash_function)(const void *,int length),int (* compare_function)(const void *,const void *,int length))310 bcache_xmalloc (unsigned long (*hash_function)(const void *, int length),
311 int (*compare_function)(const void *,
312 const void *,
313 int length))
314 {
315 /* Allocate the bcache pre-zeroed. */
316 struct bcache *b = XCALLOC (1, struct bcache);
317
318 if (hash_function)
319 b->hash_function = hash_function;
320 else
321 b->hash_function = hash;
322
323 if (compare_function)
324 b->compare_function = compare_function;
325 else
326 b->compare_function = bcache_compare;
327 return b;
328 }
329
330 /* Free all the storage associated with BCACHE. */
331 void
bcache_xfree(struct bcache * bcache)332 bcache_xfree (struct bcache *bcache)
333 {
334 if (bcache == NULL)
335 return;
336 /* Only free the obstack if we actually initialized it. */
337 if (bcache->total_count > 0)
338 obstack_free (&bcache->cache, 0);
339 xfree (bcache->bucket);
340 xfree (bcache);
341 }
342
343
344
345 /* Printing statistics. */
346
347 static void
print_percentage(int portion,int total)348 print_percentage (int portion, int total)
349 {
350 if (total == 0)
351 /* i18n: Like "Percentage of duplicates, by count: (not applicable)". */
352 printf_filtered (_("(not applicable)\n"));
353 else
354 printf_filtered ("%3d%%\n", (int) (portion * 100.0 / total));
355 }
356
357
358 /* Print statistics on BCACHE's memory usage and efficacity at
359 eliminating duplication. NAME should describe the kind of data
360 BCACHE holds. Statistics are printed using `printf_filtered' and
361 its ilk. */
362 void
print_bcache_statistics(struct bcache * c,char * type)363 print_bcache_statistics (struct bcache *c, char *type)
364 {
365 int occupied_buckets;
366 int max_chain_length;
367 int median_chain_length;
368 int max_entry_size;
369 int median_entry_size;
370
371 /* Count the number of occupied buckets, tally the various string
372 lengths, and measure chain lengths. */
373 {
374 unsigned int b;
375 int *chain_length = XCALLOC (c->num_buckets + 1, int);
376 int *entry_size = XCALLOC (c->unique_count + 1, int);
377 int stringi = 0;
378
379 occupied_buckets = 0;
380
381 for (b = 0; b < c->num_buckets; b++)
382 {
383 struct bstring *s = c->bucket[b];
384
385 chain_length[b] = 0;
386
387 if (s)
388 {
389 occupied_buckets++;
390
391 while (s)
392 {
393 gdb_assert (b < c->num_buckets);
394 chain_length[b]++;
395 gdb_assert (stringi < c->unique_count);
396 entry_size[stringi++] = s->length;
397 s = s->next;
398 }
399 }
400 }
401
402 /* To compute the median, we need the set of chain lengths
403 sorted. */
404 qsort (chain_length, c->num_buckets, sizeof (chain_length[0]),
405 compare_positive_ints);
406 qsort (entry_size, c->unique_count, sizeof (entry_size[0]),
407 compare_positive_ints);
408
409 if (c->num_buckets > 0)
410 {
411 max_chain_length = chain_length[c->num_buckets - 1];
412 median_chain_length = chain_length[c->num_buckets / 2];
413 }
414 else
415 {
416 max_chain_length = 0;
417 median_chain_length = 0;
418 }
419 if (c->unique_count > 0)
420 {
421 max_entry_size = entry_size[c->unique_count - 1];
422 median_entry_size = entry_size[c->unique_count / 2];
423 }
424 else
425 {
426 max_entry_size = 0;
427 median_entry_size = 0;
428 }
429
430 xfree (chain_length);
431 xfree (entry_size);
432 }
433
434 printf_filtered (_(" Cached '%s' statistics:\n"), type);
435 printf_filtered (_(" Total object count: %ld\n"), c->total_count);
436 printf_filtered (_(" Unique object count: %lu\n"), c->unique_count);
437 printf_filtered (_(" Percentage of duplicates, by count: "));
438 print_percentage (c->total_count - c->unique_count, c->total_count);
439 printf_filtered ("\n");
440
441 printf_filtered (_(" Total object size: %ld\n"), c->total_size);
442 printf_filtered (_(" Unique object size: %ld\n"), c->unique_size);
443 printf_filtered (_(" Percentage of duplicates, by size: "));
444 print_percentage (c->total_size - c->unique_size, c->total_size);
445 printf_filtered ("\n");
446
447 printf_filtered (_(" Max entry size: %d\n"), max_entry_size);
448 printf_filtered (_(" Average entry size: "));
449 if (c->unique_count > 0)
450 printf_filtered ("%ld\n", c->unique_size / c->unique_count);
451 else
452 /* i18n: "Average entry size: (not applicable)". */
453 printf_filtered (_("(not applicable)\n"));
454 printf_filtered (_(" Median entry size: %d\n"), median_entry_size);
455 printf_filtered ("\n");
456
457 printf_filtered (_(" \
458 Total memory used by bcache, including overhead: %ld\n"),
459 c->structure_size);
460 printf_filtered (_(" Percentage memory overhead: "));
461 print_percentage (c->structure_size - c->unique_size, c->unique_size);
462 printf_filtered (_(" Net memory savings: "));
463 print_percentage (c->total_size - c->structure_size, c->total_size);
464 printf_filtered ("\n");
465
466 printf_filtered (_(" Hash table size: %3d\n"),
467 c->num_buckets);
468 printf_filtered (_(" Hash table expands: %lu\n"),
469 c->expand_count);
470 printf_filtered (_(" Hash table hashes: %lu\n"),
471 c->total_count + c->expand_hash_count);
472 printf_filtered (_(" Half hash misses: %lu\n"),
473 c->half_hash_miss_count);
474 printf_filtered (_(" Hash table population: "));
475 print_percentage (occupied_buckets, c->num_buckets);
476 printf_filtered (_(" Median hash chain length: %3d\n"),
477 median_chain_length);
478 printf_filtered (_(" Average hash chain length: "));
479 if (c->num_buckets > 0)
480 printf_filtered ("%3lu\n", c->unique_count / c->num_buckets);
481 else
482 /* i18n: "Average hash chain length: (not applicable)". */
483 printf_filtered (_("(not applicable)\n"));
484 printf_filtered (_(" Maximum hash chain length: %3d\n"),
485 max_chain_length);
486 printf_filtered ("\n");
487 }
488
489 int
bcache_memory_used(struct bcache * bcache)490 bcache_memory_used (struct bcache *bcache)
491 {
492 if (bcache->total_count == 0)
493 return 0;
494 return obstack_memory_used (&bcache->cache);
495 }
496