458 lines
14 KiB
C
458 lines
14 KiB
C
/* Implement a cached obstack.
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Written by Fred Fish <fnf@cygnus.com>
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Rewritten by Jim Blandy <jimb@cygnus.com>
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Copyright (C) 1999, 2000, 2002, 2003, 2007, 2008
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Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#include "defs.h"
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#include "gdb_obstack.h"
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#include "bcache.h"
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#include "gdb_string.h" /* For memcpy declaration */
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#include "gdb_assert.h"
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#include <stddef.h>
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#include <stdlib.h>
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/* The type used to hold a single bcache string. The user data is
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stored in d.data. Since it can be any type, it needs to have the
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same alignment as the most strict alignment of any type on the host
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machine. I don't know of any really correct way to do this in
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stock ANSI C, so just do it the same way obstack.h does. */
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struct bstring
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{
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/* Hash chain. */
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struct bstring *next;
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/* Assume the data length is no more than 64k. */
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unsigned short length;
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/* The half hash hack. This contains the upper 16 bits of the hash
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value and is used as a pre-check when comparing two strings and
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avoids the need to do length or memcmp calls. It proves to be
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roughly 100% effective. */
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unsigned short half_hash;
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union
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{
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char data[1];
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double dummy;
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}
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d;
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};
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/* The structure for a bcache itself. The bcache is initialized, in
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bcache_xmalloc(), by filling it with zeros and then setting the
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corresponding obstack's malloc() and free() methods. */
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struct bcache
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{
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/* All the bstrings are allocated here. */
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struct obstack cache;
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/* How many hash buckets we're using. */
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unsigned int num_buckets;
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/* Hash buckets. This table is allocated using malloc, so when we
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grow the table we can return the old table to the system. */
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struct bstring **bucket;
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/* Statistics. */
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unsigned long unique_count; /* number of unique strings */
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long total_count; /* total number of strings cached, including dups */
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long unique_size; /* size of unique strings, in bytes */
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long total_size; /* total number of bytes cached, including dups */
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long structure_size; /* total size of bcache, including infrastructure */
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/* Number of times that the hash table is expanded and hence
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re-built, and the corresponding number of times that a string is
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[re]hashed as part of entering it into the expanded table. The
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total number of hashes can be computed by adding TOTAL_COUNT to
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expand_hash_count. */
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unsigned long expand_count;
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unsigned long expand_hash_count;
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/* Number of times that the half-hash compare hit (compare the upper
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16 bits of hash values) hit, but the corresponding combined
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length/data compare missed. */
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unsigned long half_hash_miss_count;
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};
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/* The old hash function was stolen from SDBM. This is what DB 3.0 uses now,
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* and is better than the old one.
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*/
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unsigned long
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hash(const void *addr, int length)
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{
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const unsigned char *k, *e;
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unsigned long h;
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k = (const unsigned char *)addr;
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e = k+length;
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for (h=0; k< e;++k)
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{
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h *=16777619;
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h ^= *k;
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}
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return (h);
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}
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/* Growing the bcache's hash table. */
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/* If the average chain length grows beyond this, then we want to
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resize our hash table. */
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#define CHAIN_LENGTH_THRESHOLD (5)
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static void
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expand_hash_table (struct bcache *bcache)
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{
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/* A table of good hash table sizes. Whenever we grow, we pick the
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next larger size from this table. sizes[i] is close to 1 << (i+10),
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so we roughly double the table size each time. After we fall off
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the end of this table, we just double. Don't laugh --- there have
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been executables sighted with a gigabyte of debug info. */
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static unsigned long sizes[] = {
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1021, 2053, 4099, 8191, 16381, 32771,
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65537, 131071, 262144, 524287, 1048573, 2097143,
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4194301, 8388617, 16777213, 33554467, 67108859, 134217757,
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268435459, 536870923, 1073741827, 2147483659UL
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};
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unsigned int new_num_buckets;
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struct bstring **new_buckets;
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unsigned int i;
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/* Count the stats. Every unique item needs to be re-hashed and
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re-entered. */
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bcache->expand_count++;
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bcache->expand_hash_count += bcache->unique_count;
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/* Find the next size. */
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new_num_buckets = bcache->num_buckets * 2;
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for (i = 0; i < (sizeof (sizes) / sizeof (sizes[0])); i++)
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if (sizes[i] > bcache->num_buckets)
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{
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new_num_buckets = sizes[i];
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break;
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}
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/* Allocate the new table. */
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{
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size_t new_size = new_num_buckets * sizeof (new_buckets[0]);
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new_buckets = (struct bstring **) xmalloc (new_size);
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memset (new_buckets, 0, new_size);
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bcache->structure_size -= (bcache->num_buckets
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* sizeof (bcache->bucket[0]));
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bcache->structure_size += new_size;
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}
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/* Rehash all existing strings. */
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for (i = 0; i < bcache->num_buckets; i++)
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{
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struct bstring *s, *next;
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for (s = bcache->bucket[i]; s; s = next)
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{
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struct bstring **new_bucket;
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next = s->next;
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new_bucket = &new_buckets[(hash (&s->d.data, s->length)
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% new_num_buckets)];
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s->next = *new_bucket;
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*new_bucket = s;
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}
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}
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/* Plug in the new table. */
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if (bcache->bucket)
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xfree (bcache->bucket);
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bcache->bucket = new_buckets;
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bcache->num_buckets = new_num_buckets;
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}
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/* Looking up things in the bcache. */
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/* The number of bytes needed to allocate a struct bstring whose data
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is N bytes long. */
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#define BSTRING_SIZE(n) (offsetof (struct bstring, d.data) + (n))
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/* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
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never seen those bytes before, add a copy of them to BCACHE. In
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either case, return a pointer to BCACHE's copy of that string. */
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const void *
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bcache (const void *addr, int length, struct bcache *bcache)
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{
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return bcache_full (addr, length, bcache, NULL);
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}
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/* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has
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never seen those bytes before, add a copy of them to BCACHE. In
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either case, return a pointer to BCACHE's copy of that string. If
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optional ADDED is not NULL, return 1 in case of new entry or 0 if
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returning an old entry. */
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const void *
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bcache_full (const void *addr, int length, struct bcache *bcache, int *added)
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{
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unsigned long full_hash;
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unsigned short half_hash;
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int hash_index;
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struct bstring *s;
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if (added)
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*added = 0;
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/* Lazily initialize the obstack. This can save quite a bit of
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memory in some cases. */
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if (bcache->total_count == 0)
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{
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/* We could use obstack_specify_allocation here instead, but
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gdb_obstack.h specifies the allocation/deallocation
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functions. */
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obstack_init (&bcache->cache);
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}
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/* If our average chain length is too high, expand the hash table. */
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if (bcache->unique_count >= bcache->num_buckets * CHAIN_LENGTH_THRESHOLD)
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expand_hash_table (bcache);
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bcache->total_count++;
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bcache->total_size += length;
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full_hash = hash (addr, length);
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half_hash = (full_hash >> 16);
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hash_index = full_hash % bcache->num_buckets;
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/* Search the hash bucket for a string identical to the caller's.
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As a short-circuit first compare the upper part of each hash
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values. */
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for (s = bcache->bucket[hash_index]; s; s = s->next)
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{
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if (s->half_hash == half_hash)
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{
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if (s->length == length
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&& ! memcmp (&s->d.data, addr, length))
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return &s->d.data;
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else
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bcache->half_hash_miss_count++;
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}
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}
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/* The user's string isn't in the list. Insert it after *ps. */
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{
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struct bstring *new
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= obstack_alloc (&bcache->cache, BSTRING_SIZE (length));
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memcpy (&new->d.data, addr, length);
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new->length = length;
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new->next = bcache->bucket[hash_index];
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new->half_hash = half_hash;
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bcache->bucket[hash_index] = new;
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bcache->unique_count++;
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bcache->unique_size += length;
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bcache->structure_size += BSTRING_SIZE (length);
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if (added)
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*added = 1;
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return &new->d.data;
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}
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}
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/* Allocating and freeing bcaches. */
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struct bcache *
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bcache_xmalloc (void)
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{
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/* Allocate the bcache pre-zeroed. */
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struct bcache *b = XCALLOC (1, struct bcache);
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return b;
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}
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/* Free all the storage associated with BCACHE. */
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void
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bcache_xfree (struct bcache *bcache)
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{
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if (bcache == NULL)
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return;
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/* Only free the obstack if we actually initialized it. */
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if (bcache->total_count > 0)
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obstack_free (&bcache->cache, 0);
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xfree (bcache->bucket);
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xfree (bcache);
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}
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/* Printing statistics. */
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static int
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compare_ints (const void *ap, const void *bp)
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{
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/* Because we know we're comparing two ints which are positive,
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there's no danger of overflow here. */
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return * (int *) ap - * (int *) bp;
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}
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static void
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print_percentage (int portion, int total)
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{
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if (total == 0)
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/* i18n: Like "Percentage of duplicates, by count: (not applicable)" */
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printf_filtered (_("(not applicable)\n"));
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else
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printf_filtered ("%3d%%\n", (int) (portion * 100.0 / total));
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}
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/* Print statistics on BCACHE's memory usage and efficacity at
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eliminating duplication. NAME should describe the kind of data
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BCACHE holds. Statistics are printed using `printf_filtered' and
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its ilk. */
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void
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print_bcache_statistics (struct bcache *c, char *type)
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{
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int occupied_buckets;
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int max_chain_length;
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int median_chain_length;
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int max_entry_size;
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int median_entry_size;
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/* Count the number of occupied buckets, tally the various string
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lengths, and measure chain lengths. */
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{
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unsigned int b;
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int *chain_length = XCALLOC (c->num_buckets + 1, int);
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int *entry_size = XCALLOC (c->unique_count + 1, int);
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int stringi = 0;
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occupied_buckets = 0;
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for (b = 0; b < c->num_buckets; b++)
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{
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struct bstring *s = c->bucket[b];
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chain_length[b] = 0;
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if (s)
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{
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occupied_buckets++;
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while (s)
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{
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gdb_assert (b < c->num_buckets);
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chain_length[b]++;
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gdb_assert (stringi < c->unique_count);
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entry_size[stringi++] = s->length;
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s = s->next;
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}
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}
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}
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/* To compute the median, we need the set of chain lengths sorted. */
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qsort (chain_length, c->num_buckets, sizeof (chain_length[0]),
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compare_ints);
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qsort (entry_size, c->unique_count, sizeof (entry_size[0]),
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compare_ints);
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if (c->num_buckets > 0)
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{
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max_chain_length = chain_length[c->num_buckets - 1];
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median_chain_length = chain_length[c->num_buckets / 2];
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}
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else
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{
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max_chain_length = 0;
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median_chain_length = 0;
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}
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if (c->unique_count > 0)
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{
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max_entry_size = entry_size[c->unique_count - 1];
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median_entry_size = entry_size[c->unique_count / 2];
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}
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else
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{
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max_entry_size = 0;
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median_entry_size = 0;
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}
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xfree (chain_length);
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xfree (entry_size);
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}
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printf_filtered (_(" Cached '%s' statistics:\n"), type);
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printf_filtered (_(" Total object count: %ld\n"), c->total_count);
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printf_filtered (_(" Unique object count: %lu\n"), c->unique_count);
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printf_filtered (_(" Percentage of duplicates, by count: "));
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print_percentage (c->total_count - c->unique_count, c->total_count);
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printf_filtered ("\n");
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printf_filtered (_(" Total object size: %ld\n"), c->total_size);
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printf_filtered (_(" Unique object size: %ld\n"), c->unique_size);
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printf_filtered (_(" Percentage of duplicates, by size: "));
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print_percentage (c->total_size - c->unique_size, c->total_size);
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printf_filtered ("\n");
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printf_filtered (_(" Max entry size: %d\n"), max_entry_size);
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printf_filtered (_(" Average entry size: "));
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if (c->unique_count > 0)
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printf_filtered ("%ld\n", c->unique_size / c->unique_count);
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else
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/* i18n: "Average entry size: (not applicable)" */
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printf_filtered (_("(not applicable)\n"));
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printf_filtered (_(" Median entry size: %d\n"), median_entry_size);
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printf_filtered ("\n");
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printf_filtered (_(" Total memory used by bcache, including overhead: %ld\n"),
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c->structure_size);
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printf_filtered (_(" Percentage memory overhead: "));
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print_percentage (c->structure_size - c->unique_size, c->unique_size);
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printf_filtered (_(" Net memory savings: "));
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print_percentage (c->total_size - c->structure_size, c->total_size);
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printf_filtered ("\n");
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printf_filtered (_(" Hash table size: %3d\n"), c->num_buckets);
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printf_filtered (_(" Hash table expands: %lu\n"),
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c->expand_count);
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printf_filtered (_(" Hash table hashes: %lu\n"),
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c->total_count + c->expand_hash_count);
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printf_filtered (_(" Half hash misses: %lu\n"),
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c->half_hash_miss_count);
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printf_filtered (_(" Hash table population: "));
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print_percentage (occupied_buckets, c->num_buckets);
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printf_filtered (_(" Median hash chain length: %3d\n"),
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median_chain_length);
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printf_filtered (_(" Average hash chain length: "));
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if (c->num_buckets > 0)
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printf_filtered ("%3lu\n", c->unique_count / c->num_buckets);
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else
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/* i18n: "Average hash chain length: (not applicable)" */
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printf_filtered (_("(not applicable)\n"));
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printf_filtered (_(" Maximum hash chain length: %3d\n"), max_chain_length);
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printf_filtered ("\n");
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}
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int
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bcache_memory_used (struct bcache *bcache)
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{
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if (bcache->total_count == 0)
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return 0;
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return obstack_memory_used (&bcache->cache);
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}
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