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      1 /*
      2  * Copyright (C) 2008 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 /*
     18  * Mutex-free cache.  Each entry has two 32-bit keys, one 32-bit value,
     19  * and a 32-bit version.
     20  */
     21 #include "Dalvik.h"
     22 
     23 #include <stdlib.h>
     24 
     25 /*
     26  * I think modern C mandates that the results of a boolean expression are
     27  * 0 or 1.  If not, or we suddenly turn into C++ and bool != int, use this.
     28  */
     29 #define BOOL_TO_INT(x)  (x)
     30 //#define BOOL_TO_INT(x)  ((x) ? 1 : 0)
     31 
     32 #define CPU_CACHE_WIDTH         32
     33 #define CPU_CACHE_WIDTH_1       (CPU_CACHE_WIDTH-1)
     34 
     35 #define ATOMIC_LOCK_FLAG        (1 << 31)
     36 
     37 /*
     38  * Allocate cache.
     39  */
     40 AtomicCache* dvmAllocAtomicCache(int numEntries)
     41 {
     42     AtomicCache* newCache;
     43 
     44     newCache = (AtomicCache*) calloc(1, sizeof(AtomicCache));
     45     if (newCache == NULL)
     46         return NULL;
     47 
     48     newCache->numEntries = numEntries;
     49 
     50     newCache->entryAlloc = calloc(1,
     51         sizeof(AtomicCacheEntry) * numEntries + CPU_CACHE_WIDTH);
     52     if (newCache->entryAlloc == NULL)
     53         return NULL;
     54 
     55     /*
     56      * Adjust storage to align on a 32-byte boundary.  Each entry is 16 bytes
     57      * wide.  This ensures that each cache entry sits on a single CPU cache
     58      * line.
     59      */
     60     assert(sizeof(AtomicCacheEntry) == 16);
     61     newCache->entries = (AtomicCacheEntry*)
     62         (((int) newCache->entryAlloc + CPU_CACHE_WIDTH_1) & ~CPU_CACHE_WIDTH_1);
     63 
     64     return newCache;
     65 }
     66 
     67 /*
     68  * Free cache.
     69  */
     70 void dvmFreeAtomicCache(AtomicCache* cache)
     71 {
     72     if (cache != NULL) {
     73         free(cache->entryAlloc);
     74         free(cache);
     75     }
     76 }
     77 
     78 
     79 /*
     80  * Update a cache entry.
     81  *
     82  * In the event of a collision with another thread, the update may be skipped.
     83  *
     84  * We only need "pCache" for stats.
     85  */
     86 void dvmUpdateAtomicCache(u4 key1, u4 key2, u4 value, AtomicCacheEntry* pEntry,
     87     u4 firstVersion
     88 #if CALC_CACHE_STATS > 0
     89     , AtomicCache* pCache
     90 #endif
     91     )
     92 {
     93     /*
     94      * The fields don't match, so we want to update them.  There is a risk
     95      * that another thread is also trying to update them, so we grab an
     96      * ownership flag to lock out other threads.
     97      *
     98      * If the lock flag was already set in "firstVersion", somebody else
     99      * was in mid-update, and we don't want to continue here.  (This means
    100      * that using "firstVersion" as the "before" argument to the CAS would
    101      * succeed when it shouldn't and vice-versa -- we could also just pass
    102      * in (firstVersion & ~ATOMIC_LOCK_FLAG) as the first argument.)
    103      *
    104      * NOTE: we don't deal with the situation where we overflow the version
    105      * counter and trample the ATOMIC_LOCK_FLAG (at 2^31).  Probably not
    106      * a real concern.
    107      */
    108     if ((firstVersion & ATOMIC_LOCK_FLAG) != 0 ||
    109         android_atomic_release_cas(
    110                 firstVersion, firstVersion | ATOMIC_LOCK_FLAG,
    111                 (volatile s4*) &pEntry->version) != 0)
    112     {
    113         /*
    114          * We couldn't get the write lock.  Return without updating the table.
    115          */
    116 #if CALC_CACHE_STATS > 0
    117         pCache->fail++;
    118 #endif
    119         return;
    120     }
    121 
    122     /* must be even-valued on entry */
    123     assert((firstVersion & 0x01) == 0);
    124 
    125 #if CALC_CACHE_STATS > 0
    126     /* for stats, assume a key value of zero indicates an empty entry */
    127     if (pEntry->key1 == 0)
    128         pCache->fills++;
    129     else
    130         pCache->misses++;
    131 #endif
    132 
    133     /*
    134      * We have the write lock, but somebody could be reading this entry
    135      * while we work.  We use memory barriers to ensure that the state
    136      * is always consistent when the version number is even.
    137      */
    138     u4 newVersion = (firstVersion | ATOMIC_LOCK_FLAG) + 1;
    139     assert((newVersion & 0x01) == 1);
    140 
    141     pEntry->version = newVersion;
    142 
    143     android_atomic_release_store(key1, (int32_t*) &pEntry->key1);
    144     pEntry->key2 = key2;
    145     pEntry->value = value;
    146 
    147     newVersion++;
    148     android_atomic_release_store(newVersion, (int32_t*) &pEntry->version);
    149 
    150     /*
    151      * Clear the lock flag.  Nobody else should have been able to modify
    152      * pEntry->version, so if this fails the world is broken.
    153      */
    154     assert(newVersion == ((firstVersion + 2) | ATOMIC_LOCK_FLAG));
    155     if (android_atomic_release_cas(
    156             newVersion, newVersion & ~ATOMIC_LOCK_FLAG,
    157             (volatile s4*) &pEntry->version) != 0)
    158     {
    159         //LOGE("unable to reset the instanceof cache ownership");
    160         dvmAbort();
    161     }
    162 }
    163 
    164 
    165 /*
    166  * Dump the "instanceof" cache stats.
    167  */
    168 void dvmDumpAtomicCacheStats(const AtomicCache* pCache)
    169 {
    170     if (pCache == NULL)
    171         return;
    172     dvmFprintf(stdout,
    173         "Cache stats: trv=%d fai=%d hit=%d mis=%d fil=%d %d%% (size=%d)\n",
    174         pCache->trivial, pCache->fail, pCache->hits,
    175         pCache->misses, pCache->fills,
    176         (pCache->hits == 0) ? 0 :
    177             pCache->hits * 100 /
    178                 (pCache->fail + pCache->hits + pCache->misses + pCache->fills),
    179         pCache->numEntries);
    180 }
    181