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      1 //===--- StringMap.cpp - String Hash table map implementation -------------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file implements the StringMap class.
     11 //
     12 //===----------------------------------------------------------------------===//
     13 
     14 #include "llvm/ADT/StringMap.h"
     15 #include "llvm/ADT/StringExtras.h"
     16 #include "llvm/Support/Compiler.h"
     17 #include <cassert>
     18 using namespace llvm;
     19 
     20 StringMapImpl::StringMapImpl(unsigned InitSize, unsigned itemSize) {
     21   ItemSize = itemSize;
     22 
     23   // If a size is specified, initialize the table with that many buckets.
     24   if (InitSize) {
     25     init(InitSize);
     26     return;
     27   }
     28 
     29   // Otherwise, initialize it with zero buckets to avoid the allocation.
     30   TheTable = nullptr;
     31   NumBuckets = 0;
     32   NumItems = 0;
     33   NumTombstones = 0;
     34 }
     35 
     36 void StringMapImpl::init(unsigned InitSize) {
     37   assert((InitSize & (InitSize-1)) == 0 &&
     38          "Init Size must be a power of 2 or zero!");
     39   NumBuckets = InitSize ? InitSize : 16;
     40   NumItems = 0;
     41   NumTombstones = 0;
     42 
     43   TheTable = (StringMapEntryBase **)calloc(NumBuckets+1,
     44                                            sizeof(StringMapEntryBase **) +
     45                                            sizeof(unsigned));
     46 
     47   // Allocate one extra bucket, set it to look filled so the iterators stop at
     48   // end.
     49   TheTable[NumBuckets] = (StringMapEntryBase*)2;
     50 }
     51 
     52 
     53 /// LookupBucketFor - Look up the bucket that the specified string should end
     54 /// up in.  If it already exists as a key in the map, the Item pointer for the
     55 /// specified bucket will be non-null.  Otherwise, it will be null.  In either
     56 /// case, the FullHashValue field of the bucket will be set to the hash value
     57 /// of the string.
     58 unsigned StringMapImpl::LookupBucketFor(StringRef Name) {
     59   unsigned HTSize = NumBuckets;
     60   if (HTSize == 0) {  // Hash table unallocated so far?
     61     init(16);
     62     HTSize = NumBuckets;
     63   }
     64   unsigned FullHashValue = HashString(Name);
     65   unsigned BucketNo = FullHashValue & (HTSize-1);
     66   unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
     67 
     68   unsigned ProbeAmt = 1;
     69   int FirstTombstone = -1;
     70   while (1) {
     71     StringMapEntryBase *BucketItem = TheTable[BucketNo];
     72     // If we found an empty bucket, this key isn't in the table yet, return it.
     73     if (LLVM_LIKELY(!BucketItem)) {
     74       // If we found a tombstone, we want to reuse the tombstone instead of an
     75       // empty bucket.  This reduces probing.
     76       if (FirstTombstone != -1) {
     77         HashTable[FirstTombstone] = FullHashValue;
     78         return FirstTombstone;
     79       }
     80 
     81       HashTable[BucketNo] = FullHashValue;
     82       return BucketNo;
     83     }
     84 
     85     if (BucketItem == getTombstoneVal()) {
     86       // Skip over tombstones.  However, remember the first one we see.
     87       if (FirstTombstone == -1) FirstTombstone = BucketNo;
     88     } else if (LLVM_LIKELY(HashTable[BucketNo] == FullHashValue)) {
     89       // If the full hash value matches, check deeply for a match.  The common
     90       // case here is that we are only looking at the buckets (for item info
     91       // being non-null and for the full hash value) not at the items.  This
     92       // is important for cache locality.
     93 
     94       // Do the comparison like this because Name isn't necessarily
     95       // null-terminated!
     96       char *ItemStr = (char*)BucketItem+ItemSize;
     97       if (Name == StringRef(ItemStr, BucketItem->getKeyLength())) {
     98         // We found a match!
     99         return BucketNo;
    100       }
    101     }
    102 
    103     // Okay, we didn't find the item.  Probe to the next bucket.
    104     BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
    105 
    106     // Use quadratic probing, it has fewer clumping artifacts than linear
    107     // probing and has good cache behavior in the common case.
    108     ++ProbeAmt;
    109   }
    110 }
    111 
    112 
    113 /// FindKey - Look up the bucket that contains the specified key. If it exists
    114 /// in the map, return the bucket number of the key.  Otherwise return -1.
    115 /// This does not modify the map.
    116 int StringMapImpl::FindKey(StringRef Key) const {
    117   unsigned HTSize = NumBuckets;
    118   if (HTSize == 0) return -1;  // Really empty table?
    119   unsigned FullHashValue = HashString(Key);
    120   unsigned BucketNo = FullHashValue & (HTSize-1);
    121   unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
    122 
    123   unsigned ProbeAmt = 1;
    124   while (1) {
    125     StringMapEntryBase *BucketItem = TheTable[BucketNo];
    126     // If we found an empty bucket, this key isn't in the table yet, return.
    127     if (LLVM_LIKELY(!BucketItem))
    128       return -1;
    129 
    130     if (BucketItem == getTombstoneVal()) {
    131       // Ignore tombstones.
    132     } else if (LLVM_LIKELY(HashTable[BucketNo] == FullHashValue)) {
    133       // If the full hash value matches, check deeply for a match.  The common
    134       // case here is that we are only looking at the buckets (for item info
    135       // being non-null and for the full hash value) not at the items.  This
    136       // is important for cache locality.
    137 
    138       // Do the comparison like this because NameStart isn't necessarily
    139       // null-terminated!
    140       char *ItemStr = (char*)BucketItem+ItemSize;
    141       if (Key == StringRef(ItemStr, BucketItem->getKeyLength())) {
    142         // We found a match!
    143         return BucketNo;
    144       }
    145     }
    146 
    147     // Okay, we didn't find the item.  Probe to the next bucket.
    148     BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
    149 
    150     // Use quadratic probing, it has fewer clumping artifacts than linear
    151     // probing and has good cache behavior in the common case.
    152     ++ProbeAmt;
    153   }
    154 }
    155 
    156 /// RemoveKey - Remove the specified StringMapEntry from the table, but do not
    157 /// delete it.  This aborts if the value isn't in the table.
    158 void StringMapImpl::RemoveKey(StringMapEntryBase *V) {
    159   const char *VStr = (char*)V + ItemSize;
    160   StringMapEntryBase *V2 = RemoveKey(StringRef(VStr, V->getKeyLength()));
    161   (void)V2;
    162   assert(V == V2 && "Didn't find key?");
    163 }
    164 
    165 /// RemoveKey - Remove the StringMapEntry for the specified key from the
    166 /// table, returning it.  If the key is not in the table, this returns null.
    167 StringMapEntryBase *StringMapImpl::RemoveKey(StringRef Key) {
    168   int Bucket = FindKey(Key);
    169   if (Bucket == -1) return nullptr;
    170 
    171   StringMapEntryBase *Result = TheTable[Bucket];
    172   TheTable[Bucket] = getTombstoneVal();
    173   --NumItems;
    174   ++NumTombstones;
    175   assert(NumItems + NumTombstones <= NumBuckets);
    176 
    177   return Result;
    178 }
    179 
    180 
    181 
    182 /// RehashTable - Grow the table, redistributing values into the buckets with
    183 /// the appropriate mod-of-hashtable-size.
    184 unsigned StringMapImpl::RehashTable(unsigned BucketNo) {
    185   unsigned NewSize;
    186   unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
    187 
    188   // If the hash table is now more than 3/4 full, or if fewer than 1/8 of
    189   // the buckets are empty (meaning that many are filled with tombstones),
    190   // grow/rehash the table.
    191   if (LLVM_UNLIKELY(NumItems * 4 > NumBuckets * 3)) {
    192     NewSize = NumBuckets*2;
    193   } else if (LLVM_UNLIKELY(NumBuckets - (NumItems + NumTombstones) <=
    194                            NumBuckets / 8)) {
    195     NewSize = NumBuckets;
    196   } else {
    197     return BucketNo;
    198   }
    199 
    200   unsigned NewBucketNo = BucketNo;
    201   // Allocate one extra bucket which will always be non-empty.  This allows the
    202   // iterators to stop at end.
    203   StringMapEntryBase **NewTableArray =
    204     (StringMapEntryBase **)calloc(NewSize+1, sizeof(StringMapEntryBase *) +
    205                                              sizeof(unsigned));
    206   unsigned *NewHashArray = (unsigned *)(NewTableArray + NewSize + 1);
    207   NewTableArray[NewSize] = (StringMapEntryBase*)2;
    208 
    209   // Rehash all the items into their new buckets.  Luckily :) we already have
    210   // the hash values available, so we don't have to rehash any strings.
    211   for (unsigned I = 0, E = NumBuckets; I != E; ++I) {
    212     StringMapEntryBase *Bucket = TheTable[I];
    213     if (Bucket && Bucket != getTombstoneVal()) {
    214       // Fast case, bucket available.
    215       unsigned FullHash = HashTable[I];
    216       unsigned NewBucket = FullHash & (NewSize-1);
    217       if (!NewTableArray[NewBucket]) {
    218         NewTableArray[FullHash & (NewSize-1)] = Bucket;
    219         NewHashArray[FullHash & (NewSize-1)] = FullHash;
    220         if (I == BucketNo)
    221           NewBucketNo = NewBucket;
    222         continue;
    223       }
    224 
    225       // Otherwise probe for a spot.
    226       unsigned ProbeSize = 1;
    227       do {
    228         NewBucket = (NewBucket + ProbeSize++) & (NewSize-1);
    229       } while (NewTableArray[NewBucket]);
    230 
    231       // Finally found a slot.  Fill it in.
    232       NewTableArray[NewBucket] = Bucket;
    233       NewHashArray[NewBucket] = FullHash;
    234       if (I == BucketNo)
    235         NewBucketNo = NewBucket;
    236     }
    237   }
    238 
    239   free(TheTable);
    240 
    241   TheTable = NewTableArray;
    242   NumBuckets = NewSize;
    243   NumTombstones = 0;
    244   return NewBucketNo;
    245 }
    246