lib/Support/StringMap.cpp

   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 "llvm/Support/DJB.h"
  18 #include "llvm/Support/MathExtras.h"
  19 #include <cassert>
  20
  21 using namespace llvm;
  22
  23 /// Returns the number of buckets to allocate to ensure that the DenseMap can
  24 /// accommodate \p NumEntries without need to grow().
  25 static unsigned getMinBucketToReserveForEntries(unsigned NumEntries) {
  26   // Ensure that "NumEntries * 4 < NumBuckets * 3"
  27   if (NumEntries == 0)
  28     return 0;
  29   // +1 is required because of the strict equality.
  30   // For example if NumEntries is 48, we need to return 401.
  31   return NextPowerOf2(NumEntries * 4 / 3 + 1);
  32 }
  33
  34 StringMapImpl::StringMapImpl(unsigned InitSize, unsigned itemSize) {
  35   ItemSize = itemSize;
  36
  37   // If a size is specified, initialize the table with that many buckets.
  38   if (InitSize) {
  39     // The table will grow when the number of entries reach 3/4 of the number of
  40     // buckets. To guarantee that "InitSize" number of entries can be inserted
  41     // in the table without growing, we allocate just what is needed here.
  42     init(getMinBucketToReserveForEntries(InitSize));
  43     return;
  44   }
  45
  46   // Otherwise, initialize it with zero buckets to avoid the allocation.
  47   TheTable = nullptr;
  48   NumBuckets = 0;
  49   NumItems = 0;
  50   NumTombstones = 0;
  51 }
  52
  53 void StringMapImpl::init(unsigned InitSize) {
  54   assert((InitSize & (InitSize-1)) == 0 &&
  55          "Init Size must be a power of 2 or zero!");
  56
  57   unsigned NewNumBuckets = InitSize ? InitSize : 16;
  58   NumItems = 0;
  59   NumTombstones = 0;
  60
  61   TheTable = static_cast<StringMapEntryBase **>(
  62       safe_calloc(NewNumBuckets+1,
  63                   sizeof(StringMapEntryBase **) + sizeof(unsigned)));
  64
  65   // Set the member only if TheTable was successfully allocated
  66   NumBuckets = NewNumBuckets;
  67
  68   // Allocate one extra bucket, set it to look filled so the iterators stop at
  69   // end.
  70   TheTable[NumBuckets] = (StringMapEntryBase*)2;
  71 }
  72
  73 /// LookupBucketFor - Look up the bucket that the specified string should end
  74 /// up in.  If it already exists as a key in the map, the Item pointer for the
  75 /// specified bucket will be non-null.  Otherwise, it will be null.  In either
  76 /// case, the FullHashValue field of the bucket will be set to the hash value
  77 /// of the string.
  78 unsigned StringMapImpl::LookupBucketFor(StringRef Name) {
  79   unsigned HTSize = NumBuckets;
  80   if (HTSize == 0) {  // Hash table unallocated so far?
  81     init(16);
  82     HTSize = NumBuckets;
  83   }
  84   unsigned FullHashValue = djbHash(Name, 0);
  85   unsigned BucketNo = FullHashValue & (HTSize-1);
  86   unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
  87
  88   unsigned ProbeAmt = 1;
  89   int FirstTombstone = -1;
  90   while (true) {
  91     StringMapEntryBase *BucketItem = TheTable[BucketNo];
  92     // If we found an empty bucket, this key isn't in the table yet, return it.
  93     if (LLVM_LIKELY(!BucketItem)) {
  94       // If we found a tombstone, we want to reuse the tombstone instead of an
  95       // empty bucket.  This reduces probing.
  96       if (FirstTombstone != -1) {
  97         HashTable[FirstTombstone] = FullHashValue;
  98         return FirstTombstone;
  99       }
 100
 101       HashTable[BucketNo] = FullHashValue;
 102       return BucketNo;
 103     }
 104
 105     if (BucketItem == getTombstoneVal()) {
 106       // Skip over tombstones.  However, remember the first one we see.
 107       if (FirstTombstone == -1) FirstTombstone = BucketNo;
 108     } else if (LLVM_LIKELY(HashTable[BucketNo] == FullHashValue)) {
 109       // If the full hash value matches, check deeply for a match.  The common
 110       // case here is that we are only looking at the buckets (for item info
 111       // being non-null and for the full hash value) not at the items.  This
 112       // is important for cache locality.
 113
 114       // Do the comparison like this because Name isn't necessarily
 115       // null-terminated!
 116       char *ItemStr = (char*)BucketItem+ItemSize;
 117       if (Name == StringRef(ItemStr, BucketItem->getKeyLength())) {
 118         // We found a match!
 119         return BucketNo;
 120       }
 121     }
 122
 123     // Okay, we didn't find the item.  Probe to the next bucket.
 124     BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
 125
 126     // Use quadratic probing, it has fewer clumping artifacts than linear
 127     // probing and has good cache behavior in the common case.
 128     ++ProbeAmt;
 129   }
 130 }
 131
 132 /// FindKey - Look up the bucket that contains the specified key. If it exists
 133 /// in the map, return the bucket number of the key.  Otherwise return -1.
 134 /// This does not modify the map.
 135 int StringMapImpl::FindKey(StringRef Key) const {
 136   unsigned HTSize = NumBuckets;
 137   if (HTSize == 0) return -1;  // Really empty table?
 138   unsigned FullHashValue = djbHash(Key, 0);
 139   unsigned BucketNo = FullHashValue & (HTSize-1);
 140   unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
 141
 142   unsigned ProbeAmt = 1;
 143   while (true) {
 144     StringMapEntryBase *BucketItem = TheTable[BucketNo];
 145     // If we found an empty bucket, this key isn't in the table yet, return.
 146     if (LLVM_LIKELY(!BucketItem))
 147       return -1;
 148
 149     if (BucketItem == getTombstoneVal()) {
 150       // Ignore tombstones.
 151     } else if (LLVM_LIKELY(HashTable[BucketNo] == FullHashValue)) {
 152       // If the full hash value matches, check deeply for a match.  The common
 153       // case here is that we are only looking at the buckets (for item info
 154       // being non-null and for the full hash value) not at the items.  This
 155       // is important for cache locality.
 156
 157       // Do the comparison like this because NameStart isn't necessarily
 158       // null-terminated!
 159       char *ItemStr = (char*)BucketItem+ItemSize;
 160       if (Key == StringRef(ItemStr, BucketItem->getKeyLength())) {
 161         // We found a match!
 162         return BucketNo;
 163       }
 164     }
 165
 166     // Okay, we didn't find the item.  Probe to the next bucket.
 167     BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
 168
 169     // Use quadratic probing, it has fewer clumping artifacts than linear
 170     // probing and has good cache behavior in the common case.
 171     ++ProbeAmt;
 172   }
 173 }
 174
 175 /// RemoveKey - Remove the specified StringMapEntry from the table, but do not
 176 /// delete it.  This aborts if the value isn't in the table.
 177 void StringMapImpl::RemoveKey(StringMapEntryBase *V) {
 178   const char *VStr = (char*)V + ItemSize;
 179   StringMapEntryBase *V2 = RemoveKey(StringRef(VStr, V->getKeyLength()));
 180   (void)V2;
 181   assert(V == V2 && "Didn't find key?");
 182 }
 183
 184 /// RemoveKey - Remove the StringMapEntry for the specified key from the
 185 /// table, returning it.  If the key is not in the table, this returns null.
 186 StringMapEntryBase *StringMapImpl::RemoveKey(StringRef Key) {
 187   int Bucket = FindKey(Key);
 188   if (Bucket == -1) return nullptr;
 189
 190   StringMapEntryBase *Result = TheTable[Bucket];
 191   TheTable[Bucket] = getTombstoneVal();
 192   --NumItems;
 193   ++NumTombstones;
 194   assert(NumItems + NumTombstones <= NumBuckets);
 195
 196   return Result;
 197 }
 198
 199 /// RehashTable - Grow the table, redistributing values into the buckets with
 200 /// the appropriate mod-of-hashtable-size.
 201 unsigned StringMapImpl::RehashTable(unsigned BucketNo) {
 202   unsigned NewSize;
 203   unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
 204
 205   // If the hash table is now more than 3/4 full, or if fewer than 1/8 of
 206   // the buckets are empty (meaning that many are filled with tombstones),
 207   // grow/rehash the table.
 208   if (LLVM_UNLIKELY(NumItems * 4 > NumBuckets * 3)) {
 209     NewSize = NumBuckets*2;
 210   } else if (LLVM_UNLIKELY(NumBuckets - (NumItems + NumTombstones) <=
 211                            NumBuckets / 8)) {
 212     NewSize = NumBuckets;
 213   } else {
 214     return BucketNo;
 215   }
 216
 217   unsigned NewBucketNo = BucketNo;
 218   // Allocate one extra bucket which will always be non-empty.  This allows the
 219   // iterators to stop at end.
 220   auto NewTableArray = static_cast<StringMapEntryBase **>(
 221       safe_calloc(NewSize+1, sizeof(StringMapEntryBase *) + sizeof(unsigned)));
 222
 223   unsigned *NewHashArray = (unsigned *)(NewTableArray + NewSize + 1);
 224   NewTableArray[NewSize] = (StringMapEntryBase*)2;
 225
 226   // Rehash all the items into their new buckets.  Luckily :) we already have
 227   // the hash values available, so we don't have to rehash any strings.
 228   for (unsigned I = 0, E = NumBuckets; I != E; ++I) {
 229     StringMapEntryBase *Bucket = TheTable[I];
 230     if (Bucket && Bucket != getTombstoneVal()) {
 231       // Fast case, bucket available.
 232       unsigned FullHash = HashTable[I];
 233       unsigned NewBucket = FullHash & (NewSize-1);
 234       if (!NewTableArray[NewBucket]) {
 235         NewTableArray[FullHash & (NewSize-1)] = Bucket;
 236         NewHashArray[FullHash & (NewSize-1)] = FullHash;
 237         if (I == BucketNo)
 238           NewBucketNo = NewBucket;
 239         continue;
 240       }
 241
 242       // Otherwise probe for a spot.
 243       unsigned ProbeSize = 1;
 244       do {
 245         NewBucket = (NewBucket + ProbeSize++) & (NewSize-1);
 246       } while (NewTableArray[NewBucket]);
 247
 248       // Finally found a slot.  Fill it in.
 249       NewTableArray[NewBucket] = Bucket;
 250       NewHashArray[NewBucket] = FullHash;
 251       if (I == BucketNo)
 252         NewBucketNo = NewBucket;
 253     }
 254   }
 255
 256   free(TheTable);
 257
 258   TheTable = NewTableArray;
 259   NumBuckets = NewSize;
 260   NumTombstones = 0;
 261   return NewBucketNo;
 262 }