X-Git-Url: http://git.efficios.com/?a=blobdiff_plain;f=gdb%2Fbcache.h;h=929375642046047641443de196a10b46614fdd85;hb=389fe8647555af73fca362bb066786b8cfe52761;hp=61fbbe6c5914c406f1a186ebe24dd68c2ad6da87;hpb=af5f3db67c5dcbbc7b2038fe781d03301b94783d;p=deliverable%2Fbinutils-gdb.git diff --git a/gdb/bcache.h b/gdb/bcache.h index 61fbbe6c59..9293756420 100644 --- a/gdb/bcache.h +++ b/gdb/bcache.h @@ -2,13 +2,13 @@ Written by Fred Fish Rewritten by Jim Blandy - Copyright 1999, 2000, 2002 Free Software Foundation, Inc. + 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 2 of the License, or + 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, @@ -17,9 +17,7 @@ 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, write to the Free Software - Foundation, Inc., 59 Temple Place - Suite 330, - Boston, MA 02111-1307, USA. */ + along with this program. If not, see . */ #ifndef BCACHE_H #define BCACHE_H 1 @@ -48,43 +46,184 @@ 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. + 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. */ + 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. -struct bcache; + Size of bcache VS hashtab: -/* 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. */ -extern void *bcache (const void *addr, int length, struct bcache *bcache); + 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. -/* Free all the storage used by BCACHE. */ -extern void bcache_xfree (struct bcache *bcache); + 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 ... -/* Create a new bcache object. */ -extern struct bcache *bcache_xmalloc (void); + 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: -/* Print statistics on BCACHE's memory usage and efficacity at - eliminating duplication. TYPE should be a string describing the - kind of data BCACHE holds. Statistics are printed using - `printf_filtered' and its ilk. */ -extern void print_bcache_statistics (struct bcache *bcache, char *type); -extern int bcache_memory_used (struct bcache *bcache); + 4 (slot) / 4 (chain length) + 1 (length) + 4 (chain) = 6 bytes -/* The hash function */ -extern unsigned long hash(const void *addr, int length); + 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 +{ + /* Allocate a bcache. HASH_FN and COMPARE_FN can be used to pass in + custom hash, and compare functions to be used by this bcache. If + HASH_FUNCTION is NULL fast_hash() is used and if COMPARE_FUNCTION is + NULL memcmp() is used. */ + + explicit bcache (unsigned long (*hash_fn)(const void *, + int length) = nullptr, + int (*compare_fn)(const void *, const void *, + int length) = nullptr) + : m_hash_function (hash_fn == nullptr ? default_hash : hash_fn), + m_compare_function (compare_fn == nullptr ? compare : compare_fn) + { + } + + ~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 ment 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, int *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 (); + +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; + + /* Hash function to be used for this bcache object. */ + unsigned long (*m_hash_function)(const void *addr, int length); + + /* Compare function to be used for this bcache object. */ + int (*m_compare_function)(const void *, const void *, int length); + + /* Default compare function. */ + static int compare (const void *addr1, const void *addr2, int length); + + /* Default hash function. */ + static unsigned long default_hash (const void *ptr, int length) + { + return fast_hash (ptr, length, 0); + } + + /* Expand the hash table. */ + void expand_hash_table (); +}; + +} /* namespace gdb */ #endif /* BCACHE_H */