StringMap.cpp revision b5bb9f5b5cfe89f4b7626671f4923d50f8d4cd6a 
1//===--- StringMap.cpp - String Hash table map implementation -------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by Chris Lattner and is distributed under
6// the University of Illinois Open Source 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 <cassert>
16using namespace llvm;
17
18StringMapImpl::StringMapImpl(unsigned InitSize, unsigned itemSize) {
19  assert((InitSize & (InitSize-1)) == 0 &&
20         "Init Size must be a power of 2 or zero!");
21  NumBuckets = InitSize ? InitSize : 512;
22  ItemSize = itemSize;
23  NumItems = 0;
24
25  TheTable = new ItemBucket[NumBuckets+1]();
26  memset(TheTable, 0, NumBuckets*sizeof(ItemBucket));
27
28  // Allocate one extra bucket, set it to look filled so the iterators stop at
29  // end.
30  TheTable[NumBuckets].Item = (StringMapEntryBase*)2;
31}
32
33
34/// HashString - Compute a hash code for the specified string.
35///
36static unsigned HashString(const char *Start, const char *End) {
37  // Bernstein hash function.
38  unsigned int Result = 0;
39  // TODO: investigate whether a modified bernstein hash function performs
40  // better: http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx
41  //   X*33+c -> X*33^c
42  while (Start != End)
43    Result = Result * 33 + *Start++;
44  Result = Result + (Result >> 5);
45  return Result;
46}
47
48/// LookupBucketFor - Look up the bucket that the specified string should end
49/// up in.  If it already exists as a key in the map, the Item pointer for the
50/// specified bucket will be non-null.  Otherwise, it will be null.  In either
51/// case, the FullHashValue field of the bucket will be set to the hash value
52/// of the string.
53unsigned StringMapImpl::LookupBucketFor(const char *NameStart,
54                                         const char *NameEnd) {
55  unsigned HTSize = NumBuckets;
56  unsigned FullHashValue = HashString(NameStart, NameEnd);
57  unsigned BucketNo = FullHashValue & (HTSize-1);
58
59  unsigned ProbeAmt = 1;
60  while (1) {
61    ItemBucket &Bucket = TheTable[BucketNo];
62    StringMapEntryBase *BucketItem = Bucket.Item;
63    // If we found an empty bucket, this key isn't in the table yet, return it.
64    if (BucketItem == 0) {
65      Bucket.FullHashValue = FullHashValue;
66      return BucketNo;
67    }
68
69    // If the full hash value matches, check deeply for a match.  The common
70    // case here is that we are only looking at the buckets (for item info
71    // being non-null and for the full hash value) not at the items.  This
72    // is important for cache locality.
73    if (Bucket.FullHashValue == FullHashValue) {
74      // Do the comparison like this because NameStart isn't necessarily
75      // null-terminated!
76      char *ItemStr = (char*)BucketItem+ItemSize;
77      unsigned ItemStrLen = BucketItem->getKeyLength();
78      if (unsigned(NameEnd-NameStart) == ItemStrLen &&
79          memcmp(ItemStr, NameStart, ItemStrLen) == 0) {
80        // We found a match!
81        return BucketNo;
82      }
83    }
84
85    // Okay, we didn't find the item.  Probe to the next bucket.
86    BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
87
88    // Use quadratic probing, it has fewer clumping artifacts than linear
89    // probing and has good cache behavior in the common case.
90    ++ProbeAmt;
91  }
92}
93
94
95/// FindKey - Look up the bucket that contains the specified key. If it exists
96/// in the map, return the bucket number of the key.  Otherwise return -1.
97/// This does not modify the map.
98int StringMapImpl::FindKey(const char *KeyStart, const char *KeyEnd) const {
99  unsigned HTSize = NumBuckets;
100  unsigned FullHashValue = HashString(KeyStart, KeyEnd);
101  unsigned BucketNo = FullHashValue & (HTSize-1);
102
103  unsigned ProbeAmt = 1;
104  while (1) {
105    ItemBucket &Bucket = TheTable[BucketNo];
106    StringMapEntryBase *BucketItem = Bucket.Item;
107    // If we found an empty bucket, this key isn't in the table yet, return.
108    if (BucketItem == 0)
109      return -1;
110
111    // If the full hash value matches, check deeply for a match.  The common
112    // case here is that we are only looking at the buckets (for item info
113    // being non-null and for the full hash value) not at the items.  This
114    // is important for cache locality.
115    if (Bucket.FullHashValue == FullHashValue) {
116      // Do the comparison like this because NameStart isn't necessarily
117      // null-terminated!
118      char *ItemStr = (char*)BucketItem+ItemSize;
119      unsigned ItemStrLen = BucketItem->getKeyLength();
120      if (unsigned(KeyEnd-KeyStart) == ItemStrLen &&
121          memcmp(ItemStr, KeyStart, ItemStrLen) == 0) {
122        // We found a match!
123        return BucketNo;
124      }
125    }
126
127    // Okay, we didn't find the item.  Probe to the next bucket.
128    BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
129
130    // Use quadratic probing, it has fewer clumping artifacts than linear
131    // probing and has good cache behavior in the common case.
132    ++ProbeAmt;
133  }
134}
135
136
137/// RehashTable - Grow the table, redistributing values into the buckets with
138/// the appropriate mod-of-hashtable-size.
139void StringMapImpl::RehashTable() {
140  unsigned NewSize = NumBuckets*2;
141  // Allocate one extra bucket which will always be non-empty.  This allows the
142  // iterators to stop at end.
143  ItemBucket *NewTableArray = new ItemBucket[NewSize+1]();
144  memset(NewTableArray, 0, NewSize*sizeof(ItemBucket));
145  NewTableArray[NewSize].Item = (StringMapEntryBase*)2;
146
147  // Rehash all the items into their new buckets.  Luckily :) we already have
148  // the hash values available, so we don't have to rehash any strings.
149  for (ItemBucket *IB = TheTable, *E = TheTable+NumBuckets; IB != E; ++IB) {
150    if (IB->Item) {
151      // Fast case, bucket available.
152      unsigned FullHash = IB->FullHashValue;
153      unsigned NewBucket = FullHash & (NewSize-1);
154      if (NewTableArray[NewBucket].Item == 0) {
155        NewTableArray[FullHash & (NewSize-1)].Item = IB->Item;
156        NewTableArray[FullHash & (NewSize-1)].FullHashValue = FullHash;
157        continue;
158      }
159
160      unsigned ProbeSize = 1;
161      do {
162        NewBucket = (NewBucket + ProbeSize++) & (NewSize-1);
163      } while (NewTableArray[NewBucket].Item);
164
165      // Finally found a slot.  Fill it in.
166      NewTableArray[NewBucket].Item = IB->Item;
167      NewTableArray[NewBucket].FullHashValue = FullHash;
168    }
169  }
170
171  delete[] TheTable;
172
173  TheTable = NewTableArray;
174  NumBuckets = NewSize;
175}
176