385 lines
11 KiB
C
385 lines
11 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) 1999-2020 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 <algorithm>
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namespace gdb {
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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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/* 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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void
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bcache::expand_hash_table ()
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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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m_expand_count++;
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m_expand_hash_count += m_unique_count;
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/* Find the next size. */
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new_num_buckets = m_num_buckets * 2;
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for (i = 0; i < (sizeof (sizes) / sizeof (sizes[0])); i++)
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if (sizes[i] > m_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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m_structure_size -= m_num_buckets * sizeof (m_bucket[0]);
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m_structure_size += new_size;
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}
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/* Rehash all existing strings. */
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for (i = 0; i < m_num_buckets; i++)
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{
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struct bstring *s, *next;
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for (s = m_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[(m_hash_function (&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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xfree (m_bucket);
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m_bucket = new_buckets;
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m_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. 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::insert (const void *addr, int length, 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 (m_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 (&m_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 (m_unique_count >= m_num_buckets * CHAIN_LENGTH_THRESHOLD)
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expand_hash_table ();
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m_total_count++;
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m_total_size += length;
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full_hash = m_hash_function (addr, length);
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half_hash = (full_hash >> 16);
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hash_index = full_hash % m_num_buckets;
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/* Search the hash m_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 = m_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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&& m_compare_function (&s->d.data, addr, length))
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return &s->d.data;
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else
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m_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 *newobj
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= (struct bstring *) obstack_alloc (&m_cache,
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BSTRING_SIZE (length));
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memcpy (&newobj->d.data, addr, length);
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newobj->length = length;
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newobj->next = m_bucket[hash_index];
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newobj->half_hash = half_hash;
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m_bucket[hash_index] = newobj;
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m_unique_count++;
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m_unique_size += length;
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m_structure_size += BSTRING_SIZE (length);
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if (added)
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*added = 1;
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return &newobj->d.data;
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}
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}
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/* Compare the byte string at ADDR1 of lenght LENGHT to the
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string at ADDR2. Return 1 if they are equal. */
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int
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bcache::compare (const void *addr1, const void *addr2, int length)
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{
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return memcmp (addr1, addr2, length) == 0;
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}
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/* Free all the storage associated with BCACHE. */
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bcache::~bcache ()
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{
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/* Only free the obstack if we actually initialized it. */
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if (m_total_count > 0)
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obstack_free (&m_cache, 0);
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xfree (m_bucket);
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}
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/* Printing statistics. */
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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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bcache::print_statistics (const 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 = XCNEWVEC (int, m_num_buckets + 1);
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int *entry_size = XCNEWVEC (int, m_unique_count + 1);
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int stringi = 0;
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occupied_buckets = 0;
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for (b = 0; b < m_num_buckets; b++)
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{
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struct bstring *s = m_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 < m_num_buckets);
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chain_length[b]++;
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gdb_assert (stringi < m_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
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sorted. */
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std::sort (chain_length, chain_length + m_num_buckets);
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std::sort (entry_size, entry_size + m_unique_count);
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if (m_num_buckets > 0)
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{
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max_chain_length = chain_length[m_num_buckets - 1];
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median_chain_length = chain_length[m_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 (m_unique_count > 0)
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{
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max_entry_size = entry_size[m_unique_count - 1];
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median_entry_size = entry_size[m_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 (_(" M_Cached '%s' statistics:\n"), type);
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printf_filtered (_(" Total object count: %ld\n"), m_total_count);
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printf_filtered (_(" Unique object count: %lu\n"), m_unique_count);
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printf_filtered (_(" Percentage of duplicates, by count: "));
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print_percentage (m_total_count - m_unique_count, m_total_count);
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printf_filtered ("\n");
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printf_filtered (_(" Total object size: %ld\n"), m_total_size);
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printf_filtered (_(" Unique object size: %ld\n"), m_unique_size);
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printf_filtered (_(" Percentage of duplicates, by size: "));
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print_percentage (m_total_size - m_unique_size, m_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 (m_unique_count > 0)
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printf_filtered ("%ld\n", m_unique_size / m_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 (_(" \
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Total memory used by bcache, including overhead: %ld\n"),
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m_structure_size);
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printf_filtered (_(" Percentage memory overhead: "));
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print_percentage (m_structure_size - m_unique_size, m_unique_size);
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printf_filtered (_(" Net memory savings: "));
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print_percentage (m_total_size - m_structure_size, m_total_size);
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printf_filtered ("\n");
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printf_filtered (_(" Hash table size: %3d\n"),
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m_num_buckets);
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printf_filtered (_(" Hash table expands: %lu\n"),
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m_expand_count);
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printf_filtered (_(" Hash table hashes: %lu\n"),
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m_total_count + m_expand_hash_count);
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printf_filtered (_(" Half hash misses: %lu\n"),
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m_half_hash_miss_count);
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printf_filtered (_(" Hash table population: "));
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print_percentage (occupied_buckets, m_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 (m_num_buckets > 0)
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printf_filtered ("%3lu\n", m_unique_count / m_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"),
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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 ()
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{
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if (m_total_count == 0)
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return 0;
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return obstack_memory_used (&m_cache);
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}
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} /* namespace gdb */
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