/* Implement a cached obstack. | |

Written by Fred Fish <fnf@cygnus.com> | |

Rewritten by Jim Blandy <jimb@cygnus.com> | |

Copyright (C) 1999-2021 Free Software Foundation, Inc. | |

This file is part of GDB. | |

This program is free software; you can redistribute it and/or modify | |

it under the terms of the GNU General Public License as published by | |

the Free Software Foundation; either version 3 of the License, or | |

(at your option) any later version. | |

This program is distributed in the hope that it will be useful, | |

but WITHOUT ANY WARRANTY; without even the implied warranty of | |

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |

GNU General Public License for more details. | |

You should have received a copy of the GNU General Public License | |

along with this program. If not, see <http://www.gnu.org/licenses/>. */ | |

#include "defs.h" | |

#include "gdb_obstack.h" | |

#include "bcache.h" | |

#include <algorithm> | |

namespace gdb { | |

/* The type used to hold a single bcache string. The user data is | |

stored in d.data. Since it can be any type, it needs to have the | |

same alignment as the most strict alignment of any type on the host | |

machine. I don't know of any really correct way to do this in | |

stock ANSI C, so just do it the same way obstack.h does. */ | |

struct bstring | |

{ | |

/* Hash chain. */ | |

struct bstring *next; | |

/* Assume the data length is no more than 64k. */ | |

unsigned short length; | |

/* The half hash hack. This contains the upper 16 bits of the hash | |

value and is used as a pre-check when comparing two strings and | |

avoids the need to do length or memcmp calls. It proves to be | |

roughly 100% effective. */ | |

unsigned short half_hash; | |

union | |

{ | |

char data[1]; | |

double dummy; | |

} | |

d; | |

}; | |

/* Growing the bcache's hash table. */ | |

/* If the average chain length grows beyond this, then we want to | |

resize our hash table. */ | |

#define CHAIN_LENGTH_THRESHOLD (5) | |

void | |

bcache::expand_hash_table () | |

{ | |

/* A table of good hash table sizes. Whenever we grow, we pick the | |

next larger size from this table. sizes[i] is close to 1 << (i+10), | |

so we roughly double the table size each time. After we fall off | |

the end of this table, we just double. Don't laugh --- there have | |

been executables sighted with a gigabyte of debug info. */ | |

static unsigned long sizes[] = { | |

1021, 2053, 4099, 8191, 16381, 32771, | |

65537, 131071, 262144, 524287, 1048573, 2097143, | |

4194301, 8388617, 16777213, 33554467, 67108859, 134217757, | |

268435459, 536870923, 1073741827, 2147483659UL | |

}; | |

unsigned int new_num_buckets; | |

struct bstring **new_buckets; | |

unsigned int i; | |

/* Count the stats. Every unique item needs to be re-hashed and | |

re-entered. */ | |

m_expand_count++; | |

m_expand_hash_count += m_unique_count; | |

/* Find the next size. */ | |

new_num_buckets = m_num_buckets * 2; | |

for (i = 0; i < (sizeof (sizes) / sizeof (sizes[0])); i++) | |

if (sizes[i] > m_num_buckets) | |

{ | |

new_num_buckets = sizes[i]; | |

break; | |

} | |

/* Allocate the new table. */ | |

{ | |

size_t new_size = new_num_buckets * sizeof (new_buckets[0]); | |

new_buckets = (struct bstring **) xmalloc (new_size); | |

memset (new_buckets, 0, new_size); | |

m_structure_size -= m_num_buckets * sizeof (m_bucket[0]); | |

m_structure_size += new_size; | |

} | |

/* Rehash all existing strings. */ | |

for (i = 0; i < m_num_buckets; i++) | |

{ | |

struct bstring *s, *next; | |

for (s = m_bucket[i]; s; s = next) | |

{ | |

struct bstring **new_bucket; | |

next = s->next; | |

new_bucket = &new_buckets[(this->hash (&s->d.data, s->length) | |

% new_num_buckets)]; | |

s->next = *new_bucket; | |

*new_bucket = s; | |

} | |

} | |

/* Plug in the new table. */ | |

xfree (m_bucket); | |

m_bucket = new_buckets; | |

m_num_buckets = new_num_buckets; | |

} | |

/* Looking up things in the bcache. */ | |

/* The number of bytes needed to allocate a struct bstring whose data | |

is N bytes long. */ | |

#define BSTRING_SIZE(n) (offsetof (struct bstring, d.data) + (n)) | |

/* Find a copy of the LENGTH bytes at ADDR in BCACHE. If BCACHE has | |

never seen those bytes before, add a copy of them to BCACHE. In | |

either case, return a pointer to BCACHE's copy of that string. If | |

optional ADDED is not NULL, return 1 in case of new entry or 0 if | |

returning an old entry. */ | |

const void * | |

bcache::insert (const void *addr, int length, bool *added) | |

{ | |

unsigned long full_hash; | |

unsigned short half_hash; | |

int hash_index; | |

struct bstring *s; | |

if (added != nullptr) | |

*added = false; | |

/* Lazily initialize the obstack. This can save quite a bit of | |

memory in some cases. */ | |

if (m_total_count == 0) | |

{ | |

/* We could use obstack_specify_allocation here instead, but | |

gdb_obstack.h specifies the allocation/deallocation | |

functions. */ | |

obstack_init (&m_cache); | |

} | |

/* If our average chain length is too high, expand the hash table. */ | |

if (m_unique_count >= m_num_buckets * CHAIN_LENGTH_THRESHOLD) | |

expand_hash_table (); | |

m_total_count++; | |

m_total_size += length; | |

full_hash = this->hash (addr, length); | |

half_hash = (full_hash >> 16); | |

hash_index = full_hash % m_num_buckets; | |

/* Search the hash m_bucket for a string identical to the caller's. | |

As a short-circuit first compare the upper part of each hash | |

values. */ | |

for (s = m_bucket[hash_index]; s; s = s->next) | |

{ | |

if (s->half_hash == half_hash) | |

{ | |

if (s->length == length | |

&& this->compare (&s->d.data, addr, length)) | |

return &s->d.data; | |

else | |

m_half_hash_miss_count++; | |

} | |

} | |

/* The user's string isn't in the list. Insert it after *ps. */ | |

{ | |

struct bstring *newobj | |

= (struct bstring *) obstack_alloc (&m_cache, | |

BSTRING_SIZE (length)); | |

memcpy (&newobj->d.data, addr, length); | |

newobj->length = length; | |

newobj->next = m_bucket[hash_index]; | |

newobj->half_hash = half_hash; | |

m_bucket[hash_index] = newobj; | |

m_unique_count++; | |

m_unique_size += length; | |

m_structure_size += BSTRING_SIZE (length); | |

if (added != nullptr) | |

*added = true; | |

return &newobj->d.data; | |

} | |

} | |

/* See bcache.h. */ | |

unsigned long | |

bcache::hash (const void *addr, int length) | |

{ | |

return fast_hash (addr, length, 0); | |

} | |

/* See bcache.h. */ | |

int | |

bcache::compare (const void *left, const void *right, int length) | |

{ | |

return memcmp (left, right, length) == 0; | |

} | |

/* Free all the storage associated with BCACHE. */ | |

bcache::~bcache () | |

{ | |

/* Only free the obstack if we actually initialized it. */ | |

if (m_total_count > 0) | |

obstack_free (&m_cache, 0); | |

xfree (m_bucket); | |

} | |

/* Printing statistics. */ | |

static void | |

print_percentage (int portion, int total) | |

{ | |

if (total == 0) | |

/* i18n: Like "Percentage of duplicates, by count: (not applicable)". */ | |

printf_filtered (_("(not applicable)\n")); | |

else | |

printf_filtered ("%3d%%\n", (int) (portion * 100.0 / total)); | |

} | |

/* Print statistics on BCACHE's memory usage and efficacity at | |

eliminating duplication. NAME should describe the kind of data | |

BCACHE holds. Statistics are printed using `printf_filtered' and | |

its ilk. */ | |

void | |

bcache::print_statistics (const char *type) | |

{ | |

int occupied_buckets; | |

int max_chain_length; | |

int median_chain_length; | |

int max_entry_size; | |

int median_entry_size; | |

/* Count the number of occupied buckets, tally the various string | |

lengths, and measure chain lengths. */ | |

{ | |

unsigned int b; | |

int *chain_length = XCNEWVEC (int, m_num_buckets + 1); | |

int *entry_size = XCNEWVEC (int, m_unique_count + 1); | |

int stringi = 0; | |

occupied_buckets = 0; | |

for (b = 0; b < m_num_buckets; b++) | |

{ | |

struct bstring *s = m_bucket[b]; | |

chain_length[b] = 0; | |

if (s) | |

{ | |

occupied_buckets++; | |

while (s) | |

{ | |

gdb_assert (b < m_num_buckets); | |

chain_length[b]++; | |

gdb_assert (stringi < m_unique_count); | |

entry_size[stringi++] = s->length; | |

s = s->next; | |

} | |

} | |

} | |

/* To compute the median, we need the set of chain lengths | |

sorted. */ | |

std::sort (chain_length, chain_length + m_num_buckets); | |

std::sort (entry_size, entry_size + m_unique_count); | |

if (m_num_buckets > 0) | |

{ | |

max_chain_length = chain_length[m_num_buckets - 1]; | |

median_chain_length = chain_length[m_num_buckets / 2]; | |

} | |

else | |

{ | |

max_chain_length = 0; | |

median_chain_length = 0; | |

} | |

if (m_unique_count > 0) | |

{ | |

max_entry_size = entry_size[m_unique_count - 1]; | |

median_entry_size = entry_size[m_unique_count / 2]; | |

} | |

else | |

{ | |

max_entry_size = 0; | |

median_entry_size = 0; | |

} | |

xfree (chain_length); | |

xfree (entry_size); | |

} | |

printf_filtered (_(" M_Cached '%s' statistics:\n"), type); | |

printf_filtered (_(" Total object count: %ld\n"), m_total_count); | |

printf_filtered (_(" Unique object count: %lu\n"), m_unique_count); | |

printf_filtered (_(" Percentage of duplicates, by count: ")); | |

print_percentage (m_total_count - m_unique_count, m_total_count); | |

printf_filtered ("\n"); | |

printf_filtered (_(" Total object size: %ld\n"), m_total_size); | |

printf_filtered (_(" Unique object size: %ld\n"), m_unique_size); | |

printf_filtered (_(" Percentage of duplicates, by size: ")); | |

print_percentage (m_total_size - m_unique_size, m_total_size); | |

printf_filtered ("\n"); | |

printf_filtered (_(" Max entry size: %d\n"), max_entry_size); | |

printf_filtered (_(" Average entry size: ")); | |

if (m_unique_count > 0) | |

printf_filtered ("%ld\n", m_unique_size / m_unique_count); | |

else | |

/* i18n: "Average entry size: (not applicable)". */ | |

printf_filtered (_("(not applicable)\n")); | |

printf_filtered (_(" Median entry size: %d\n"), median_entry_size); | |

printf_filtered ("\n"); | |

printf_filtered (_(" \ | |

Total memory used by bcache, including overhead: %ld\n"), | |

m_structure_size); | |

printf_filtered (_(" Percentage memory overhead: ")); | |

print_percentage (m_structure_size - m_unique_size, m_unique_size); | |

printf_filtered (_(" Net memory savings: ")); | |

print_percentage (m_total_size - m_structure_size, m_total_size); | |

printf_filtered ("\n"); | |

printf_filtered (_(" Hash table size: %3d\n"), | |

m_num_buckets); | |

printf_filtered (_(" Hash table expands: %lu\n"), | |

m_expand_count); | |

printf_filtered (_(" Hash table hashes: %lu\n"), | |

m_total_count + m_expand_hash_count); | |

printf_filtered (_(" Half hash misses: %lu\n"), | |

m_half_hash_miss_count); | |

printf_filtered (_(" Hash table population: ")); | |

print_percentage (occupied_buckets, m_num_buckets); | |

printf_filtered (_(" Median hash chain length: %3d\n"), | |

median_chain_length); | |

printf_filtered (_(" Average hash chain length: ")); | |

if (m_num_buckets > 0) | |

printf_filtered ("%3lu\n", m_unique_count / m_num_buckets); | |

else | |

/* i18n: "Average hash chain length: (not applicable)". */ | |

printf_filtered (_("(not applicable)\n")); | |

printf_filtered (_(" Maximum hash chain length: %3d\n"), | |

max_chain_length); | |

printf_filtered ("\n"); | |

} | |

int | |

bcache::memory_used () | |

{ | |

if (m_total_count == 0) | |

return 0; | |

return obstack_memory_used (&m_cache); | |

} | |

} /* namespace gdb */ |