1 // Copyright 2017 the V8 project authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style license that can be
3 // found in the LICENSE file.
4 
5 #ifndef V8_STRINGS_STRING_HASHER_INL_H_
6 #define V8_STRINGS_STRING_HASHER_INL_H_
7 
8 #include "src/strings/string-hasher.h"
9 
10 // Comment inserted to prevent header reordering.
11 #include <type_traits>
12 
13 #include "src/objects/name-inl.h"
14 #include "src/objects/objects.h"
15 #include "src/objects/string.h"
16 #include "src/strings/char-predicates-inl.h"
17 #include "src/utils/utils-inl.h"
18 
19 namespace v8 {
20 namespace internal {
21 
AddCharacterCore(uint32_t running_hash,uint16_t c)22 uint32_t StringHasher::AddCharacterCore(uint32_t running_hash, uint16_t c) {
23   running_hash += c;
24   running_hash += (running_hash << 10);
25   running_hash ^= (running_hash >> 6);
26   return running_hash;
27 }
28 
GetHashCore(uint32_t running_hash)29 uint32_t StringHasher::GetHashCore(uint32_t running_hash) {
30   running_hash += (running_hash << 3);
31   running_hash ^= (running_hash >> 11);
32   running_hash += (running_hash << 15);
33   int32_t hash = static_cast<int32_t>(running_hash & String::kHashBitMask);
34   int32_t mask = (hash - 1) >> 31;
35   return running_hash | (kZeroHash & mask);
36 }
37 
GetTrivialHash(int length)38 uint32_t StringHasher::GetTrivialHash(int length) {
39   DCHECK_GT(length, String::kMaxHashCalcLength);
40   // The hash of a large string is simply computed from the length.
41   // Ensure that the max length is small enough to be shifted without losing
42   // information.
43   STATIC_ASSERT(base::bits::CountLeadingZeros32(String::kMaxLength) >=
44                 String::kHashShift);
45   uint32_t hash = static_cast<uint32_t>(length);
46   return (hash << String::kHashShift) | String::kIsNotIntegerIndexMask;
47 }
48 
49 template <typename char_t>
HashSequentialString(const char_t * chars_raw,int length,uint64_t seed)50 uint32_t StringHasher::HashSequentialString(const char_t* chars_raw, int length,
51                                             uint64_t seed) {
52   STATIC_ASSERT(std::is_integral<char_t>::value);
53   STATIC_ASSERT(sizeof(char_t) <= 2);
54   using uchar = typename std::make_unsigned<char_t>::type;
55   const uchar* chars = reinterpret_cast<const uchar*>(chars_raw);
56   DCHECK_LE(0, length);
57   DCHECK_IMPLIES(0 < length, chars != nullptr);
58   if (length >= 1) {
59     if (IsDecimalDigit(chars[0]) && (length == 1 || chars[0] != '0')) {
60       if (length <= String::kMaxArrayIndexSize) {
61         // Possible array index; try to compute the array index hash.
62         uint32_t index = chars[0] - '0';
63         int i = 1;
64         do {
65           if (i == length) {
66             return MakeArrayIndexHash(index, length);
67           }
68         } while (TryAddArrayIndexChar(&index, chars[i++]));
69       }
70       // The following block wouldn't do anything on 32-bit platforms,
71       // because kMaxArrayIndexSize == kMaxIntegerIndexSize there, and
72       // if we wanted to compile it everywhere, then {index_big} would
73       // have to be a {size_t}, which the Mac compiler doesn't like to
74       // implicitly cast to uint64_t for the {TryAddIndexChar} call.
75 #if V8_HOST_ARCH_64_BIT
76       // No "else" here: if the block above was entered and fell through,
77       // we'll have to take this branch.
78       if (length <= String::kMaxIntegerIndexSize) {
79         // Not an array index, but it could still be an integer index.
80         // Perform a regular hash computation, and additionally check
81         // if there are non-digit characters.
82         uint32_t is_integer_index = 0;
83         uint32_t running_hash = static_cast<uint32_t>(seed);
84         uint64_t index_big = 0;
85         const uchar* end = &chars[length];
86         while (chars != end) {
87           if (is_integer_index == 0 &&
88               !TryAddIntegerIndexChar(&index_big, *chars)) {
89             is_integer_index = String::kIsNotIntegerIndexMask;
90           }
91           running_hash = AddCharacterCore(running_hash, *chars++);
92         }
93         uint32_t hash = (GetHashCore(running_hash) << String::kHashShift) |
94                         is_integer_index;
95         if (Name::ContainsCachedArrayIndex(hash)) {
96           // The hash accidentally looks like a cached index. Fix that by
97           // setting a bit that looks like a longer-than-cacheable string
98           // length.
99           hash |= (String::kMaxCachedArrayIndexLength + 1)
100                   << String::ArrayIndexLengthBits::kShift;
101         }
102         DCHECK(!Name::ContainsCachedArrayIndex(hash));
103         return hash;
104       }
105 #endif
106     }
107     // No "else" here: if the first character was a decimal digit, we might
108     // still have to take this branch.
109     if (length > String::kMaxHashCalcLength) {
110       return GetTrivialHash(length);
111     }
112   }
113 
114   // Non-index hash.
115   uint32_t running_hash = static_cast<uint32_t>(seed);
116   const uchar* end = &chars[length];
117   while (chars != end) {
118     running_hash = AddCharacterCore(running_hash, *chars++);
119   }
120 
121   return (GetHashCore(running_hash) << String::kHashShift) |
122          String::kIsNotIntegerIndexMask;
123 }
124 
operator()125 std::size_t SeededStringHasher::operator()(const char* name) const {
126   return StringHasher::HashSequentialString(
127       name, static_cast<int>(strlen(name)), hashseed_);
128 }
129 
130 }  // namespace internal
131 }  // namespace v8
132 
133 #endif  // V8_STRINGS_STRING_HASHER_INL_H_
134