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/* Include file cached obstack implementation.
Written by Fred Fish <fnf@cygnus.com>
Rewritten by Jim Blandy <jimb@cygnus.com>
Copyright (C) 1999-2020 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/>. */
#ifndef BCACHE_H
#define BCACHE_H 1
/* A bcache is a data structure for factoring out duplication in
read-only structures. You give the bcache some string of bytes S.
If the bcache already contains a copy of S, it hands you back a
pointer to its copy. Otherwise, it makes a fresh copy of S, and
hands you back a pointer to that. In either case, you can throw
away your copy of S, and use the bcache's.
The "strings" in question are arbitrary strings of bytes --- they
can contain zero bytes. You pass in the length explicitly when you
call the bcache function.
This means that you can put ordinary C objects in a bcache.
However, if you do this, remember that structs can contain `holes'
between members, added for alignment. These bytes usually contain
garbage. If you try to bcache two objects which are identical from
your code's point of view, but have different garbage values in the
structure's holes, then the bcache will treat them as separate
strings, and you won't get the nice elimination of duplicates you
were hoping for. So, remember to memset your structures full of
zeros before bcaching them!
You shouldn't modify the strings you get from a bcache, because:
- You don't necessarily know who you're sharing space with. If I
stick eight bytes of text in a bcache, and then stick an eight-byte
structure in the same bcache, there's no guarantee those two
objects don't actually comprise the same sequence of bytes. If
they happen to, the bcache will use a single byte string for both
of them. Then, modifying the structure will change the string. In
bizarre ways.
- Even if you know for some other reason that all that's okay,
there's another problem. A bcache stores all its strings in a hash
table. If you modify a string's contents, you will probably change
its hash value. This means that the modified string is now in the
wrong place in the hash table, and future bcache probes will never
find it. So by mutating a string, you give up any chance of
sharing its space with future duplicates.
Size of bcache VS hashtab:
For bcache, the most critical cost is size (or more exactly the
overhead added by the bcache). It turns out that the bcache is
remarkably efficient.
Assuming a 32-bit system (the hash table slots are 4 bytes),
ignoring alignment, and limit strings to 255 bytes (1 byte length)
we get ...
bcache: This uses a separate linked list to track the hash chain.
The numbers show roughly 100% occupancy of the hash table and an
average chain length of 4. Spreading the slot cost over the 4
chain elements:
4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes
hashtab: This uses a more traditional re-hash algorithm where the
chain is maintained within the hash table. The table occupancy is
kept below 75% but we'll assume its perfect:
4 (slot) x 4/3 (occupancy) + 1 (length) = 6 1/3 bytes
So a perfect hashtab has just slightly larger than an average
bcache.
It turns out that an average hashtab is far worse. Two things
hurt:
- Hashtab's occupancy is more like 50% (it ranges between 38% and
75%) giving a per slot cost of 4x2 vs 4x4/3.
- the string structure needs to be aligned to 8 bytes which for
hashtab wastes 7 bytes, while for bcache wastes only 3.
This gives:
hashtab: 4 x 2 + 1 + 7 = 16 bytes
bcache 4 / 4 + 1 + 4 + 3 = 9 bytes
The numbers of GDB debugging GDB support this. ~40% vs ~70% overhead.
Speed of bcache VS hashtab (the half hash hack):
While hashtab has a typical chain length of 1, bcache has a chain
length of round 4. This means that the bcache will require
something like double the number of compares after that initial
hash. In both cases the comparison takes the form:
a.length == b.length && memcmp (a.data, b.data, a.length) == 0
That is lengths are checked before doing the memcmp.
For GDB debugging GDB, it turned out that all lengths were 24 bytes
(no C++ so only psymbols were cached) and hence, all compares
required a call to memcmp. As a hack, two bytes of padding
(mentioned above) are used to store the upper 16 bits of the
string's hash value and then that is used in the comparison vis:
a.half_hash == b.half_hash && a.length == b.length && memcmp
(a.data, b.data, a.length)
The numbers from GDB debugging GDB show this to be a remarkable
100% effective (only necessary length and memcmp tests being
performed).
Mind you, looking at the wall clock, the same GDB debugging GDB
showed only marginal speed up (0.780 vs 0.773s). Seems GDB is too
busy doing something else :-(
*/
namespace gdb {
struct bstring;
struct bcache
{
virtual ~bcache ();
/* 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.
Since the cached value is meant to be read-only, return a const
buffer. If ADDED is not NULL, set *ADDED to true if the bytes
were newly added to the cache, or to false if the bytes were
found in the cache. */
const void *insert (const void *addr, int length, bool *added = nullptr);
/* Print statistics on this bcache's memory usage and efficacity at
eliminating duplication. TYPE should be a string describing the
kind of data this bcache holds. Statistics are printed using
`printf_filtered' and its ilk. */
void print_statistics (const char *type);
int memory_used ();
protected:
/* Hash function to be used for this bcache object. Defaults to
fast_hash. */
virtual unsigned long hash (const void *addr, int length);
/* Compare function to be used for this bcache object. Defaults to
memcmp. */
virtual int compare (const void *left, const void *right, int length);
private:
/* All the bstrings are allocated here. */
struct obstack m_cache {};
/* How many hash buckets we're using. */
unsigned int m_num_buckets = 0;
/* Hash buckets. This table is allocated using malloc, so when we
grow the table we can return the old table to the system. */
struct bstring **m_bucket = nullptr;
/* Statistics. */
unsigned long m_unique_count = 0; /* number of unique strings */
long m_total_count = 0; /* total number of strings cached, including dups */
long m_unique_size = 0; /* size of unique strings, in bytes */
long m_total_size = 0; /* total number of bytes cached, including dups */
long m_structure_size = 0; /* total size of bcache, including infrastructure */
/* Number of times that the hash table is expanded and hence
re-built, and the corresponding number of times that a string is
[re]hashed as part of entering it into the expanded table. The
total number of hashes can be computed by adding TOTAL_COUNT to
expand_hash_count. */
unsigned long m_expand_count = 0;
unsigned long m_expand_hash_count = 0;
/* Number of times that the half-hash compare hit (compare the upper
16 bits of hash values) hit, but the corresponding combined
length/data compare missed. */
unsigned long m_half_hash_miss_count = 0;
/* Expand the hash table. */
void expand_hash_table ();
};
} /* namespace gdb */
#endif /* BCACHE_H */
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