// Copyright (c) 2007, Google Inc.
// All rights reserved.
//
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// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
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// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
// ---
// Author: Geoff Pike
//
// This file provides a minimal cache that can hold a <key, value> pair
// with little if any wasted space. The types of the key and value
// must be unsigned integral types or at least have unsigned semantics
// for >>, casting, and similar operations.
//
// Synchronization is not provided. However, the cache is implemented
// as an array of cache entries whose type is chosen at compile time.
// If a[i] is atomic on your hardware for the chosen array type then
// raciness will not necessarily lead to bugginess. The cache entries
// must be large enough to hold a partial key and a value packed
// together. The partial keys are bit strings of length
// kKeybits - kHashbits, and the values are bit strings of length kValuebits.
//
// In an effort to use minimal space, every cache entry represents
// some <key, value> pair; the class provides no way to mark a cache
// entry as empty or uninitialized. In practice, you may want to have
// reserved keys or values to get around this limitation. For example, in
// tcmalloc's PageID-to-sizeclass cache, a value of 0 is used as
// "unknown sizeclass."
//
// Usage Considerations
// --------------------
//
// kHashbits controls the size of the cache. The best value for
// kHashbits will of course depend on the application. Perhaps try
// tuning the value of kHashbits by measuring different values on your
// favorite benchmark. Also remember not to be a pig; other
// programs that need resources may suffer if you are.
//
// The main uses for this class will be when performance is
// critical and there's a convenient type to hold the cache's
// entries. As described above, the number of bits required
// for a cache entry is (kKeybits - kHashbits) + kValuebits. Suppose
// kKeybits + kValuebits is 43. Then it probably makes sense to
// chose kHashbits >= 11 so that cache entries fit in a uint32.
//
// On the other hand, suppose kKeybits = kValuebits = 64. Then
// using this class may be less worthwhile. You'll probably
// be using 128 bits for each entry anyway, so maybe just pick
// a hash function, H, and use an array indexed by H(key):
// void Put(K key, V value) { a_[H(key)] = pair<K, V>(key, value); }
// V GetOrDefault(K key, V default) { const pair<K, V> &p = a_[H(key)]; ... }
// etc.
//
// Further Details
// ---------------
//
// For caches used only by one thread, the following is true:
// 1. For a cache c,
// (c.Put(key, value), c.GetOrDefault(key, 0)) == value
// and
// (c.Put(key, value), <...>, c.GetOrDefault(key, 0)) == value
// if the elided code contains no c.Put calls.
//
// 2. Has(key) will return false if no <key, value> pair with that key
// has ever been Put. However, a newly initialized cache will have
// some <key, value> pairs already present. When you create a new
// cache, you must specify an "initial value." The initialization
// procedure is equivalent to Clear(initial_value), which is
// equivalent to Put(k, initial_value) for all keys k from 0 to
// 2^kHashbits - 1.
//
// 3. If key and key' differ then the only way Put(key, value) may
// cause Has(key') to change is that Has(key') may change from true to
// false. Furthermore, a Put() call that doesn't change Has(key')
// doesn't change GetOrDefault(key', ...) either.
//
// Implementation details:
//
// This is a direct-mapped cache with 2^kHashbits entries; the hash
// function simply takes the low bits of the key. We store whole keys
// if a whole key plus a whole value fits in an entry. Otherwise, an
// entry is the high bits of a key and a value, packed together.
// E.g., a 20 bit key and a 7 bit value only require a uint16 for each
// entry if kHashbits >= 11.
//
// Alternatives to this scheme will be added as needed.
#ifndef TCMALLOC_PACKED_CACHE_INL_H_
#define TCMALLOC_PACKED_CACHE_INL_H_
#include "config.h"
#include <stddef.h> // for size_t
#ifdef HAVE_STDINT_H
#include <stdint.h> // for uintptr_t
#endif
#include "base/basictypes.h"
#include "internal_logging.h"
// A safe way of doing "(1 << n) - 1" -- without worrying about overflow
// Note this will all be resolved to a constant expression at compile-time
#define N_ONES_(IntType, N) \
( (N) == 0 ? 0 : ((static_cast<IntType>(1) << ((N)-1))-1 + \
(static_cast<IntType>(1) << ((N)-1))) )
// The types K and V provide upper bounds on the number of valid keys
// and values, but we explicitly require the keys to be less than
// 2^kKeybits and the values to be less than 2^kValuebits. The size of
// the table is controlled by kHashbits, and the type of each entry in
// the cache is T. See also the big comment at the top of the file.
template <int kKeybits, typename T>
class PackedCache {
public:
typedef uintptr_t K;
typedef size_t V;
#ifdef TCMALLOC_SMALL_BUT_SLOW
// Decrease the size map cache if running in the small memory mode.
static const int kHashbits = 12;
#else
// We don't want the hash map to occupy 512K memory at Chromium, so
// kHashbits is decreased from 16 to 12.
static const int kHashbits = 12;
#endif
static const int kValuebits = 7;
static const bool kUseWholeKeys = kKeybits + kValuebits <= 8 * sizeof(T);
explicit PackedCache(V initial_value) {
COMPILE_ASSERT(kKeybits <= sizeof(K) * 8, key_size);
COMPILE_ASSERT(kValuebits <= sizeof(V) * 8, value_size);
COMPILE_ASSERT(kHashbits <= kKeybits, hash_function);
COMPILE_ASSERT(kKeybits - kHashbits + kValuebits <= kTbits,
entry_size_must_be_big_enough);
Clear(initial_value);
}
void Put(K key, V value) {
ASSERT(key == (key & kKeyMask));
ASSERT(value == (value & kValueMask));
array_[Hash(key)] = KeyToUpper(key) | value;
}
bool Has(K key) const {
ASSERT(key == (key & kKeyMask));
return KeyMatch(array_[Hash(key)], key);
}
V GetOrDefault(K key, V default_value) const {
// As with other code in this class, we touch array_ as few times
// as we can. Assuming entries are read atomically (e.g., their
// type is uintptr_t on most hardware) then certain races are
// harmless.
ASSERT(key == (key & kKeyMask));
T entry = array_[Hash(key)];
return KeyMatch(entry, key) ? EntryToValue(entry) : default_value;
}
void Clear(V value) {
ASSERT(value == (value & kValueMask));
for (int i = 0; i < 1 << kHashbits; i++) {
ASSERT(kUseWholeKeys || KeyToUpper(i) == 0);
array_[i] = kUseWholeKeys ? (value | KeyToUpper(i)) : value;
}
}
private:
// We are going to pack a value and the upper part of a key (or a
// whole key) into an entry of type T. The UPPER type is for the
// upper part of a key, after the key has been masked and shifted
// for inclusion in an entry.
typedef T UPPER;
static V EntryToValue(T t) { return t & kValueMask; }
// If we have space for a whole key, we just shift it left.
// Otherwise kHashbits determines where in a K to find the upper
// part of the key, and kValuebits determines where in the entry to
// put it.
static UPPER KeyToUpper(K k) {
if (kUseWholeKeys) {
return static_cast<T>(k) << kValuebits;
} else {
const int shift = kHashbits - kValuebits;
// Assume kHashbits >= kValuebits. It'd be easy to lift this assumption.
return static_cast<T>(k >> shift) & kUpperMask;
}
}
static size_t Hash(K key) {
return static_cast<size_t>(key) & N_ONES_(size_t, kHashbits);
}
// Does the entry match the relevant part of the given key?
static bool KeyMatch(T entry, K key) {
return kUseWholeKeys ?
(entry >> kValuebits == key) :
((KeyToUpper(key) ^ entry) & kUpperMask) == 0;
}
static const int kTbits = 8 * sizeof(T);
static const int kUpperbits = kUseWholeKeys ? kKeybits : kKeybits - kHashbits;
// For masking a K.
static const K kKeyMask = N_ONES_(K, kKeybits);
// For masking a T.
static const T kUpperMask = N_ONES_(T, kUpperbits) << kValuebits;
// For masking a V or a T.
static const V kValueMask = N_ONES_(V, kValuebits);
// array_ is the cache. Its elements are volatile because any
// thread can write any array element at any time.
volatile T array_[1 << kHashbits];
};
#undef N_ONES_
#endif // TCMALLOC_PACKED_CACHE_INL_H_