Line data Source code
1 : // Copyright 2016 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 : #include "src/string-case.h"
6 :
7 : #include "src/assert-scope.h"
8 : #include "src/base/logging.h"
9 : #include "src/globals.h"
10 : #include "src/utils.h"
11 :
12 : namespace v8 {
13 : namespace internal {
14 :
15 : // FastAsciiConvert tries to do character processing on a word_t basis if
16 : // source and destination strings are properly aligned. Natural alignment of
17 : // string data depends on kTaggedSize so we define word_t via Tagged_t.
18 : using word_t = std::make_unsigned<Tagged_t>::type;
19 :
20 : const word_t kWordTAllBitsSet = std::numeric_limits<word_t>::max();
21 : const word_t kOneInEveryByte = kWordTAllBitsSet / 0xFF;
22 : const word_t kAsciiMask = kOneInEveryByte << 7;
23 :
24 : #ifdef DEBUG
25 : bool CheckFastAsciiConvert(char* dst, const char* src, int length, bool changed,
26 : bool is_to_lower) {
27 : bool expected_changed = false;
28 : for (int i = 0; i < length; i++) {
29 : if (dst[i] == src[i]) continue;
30 : expected_changed = true;
31 : if (is_to_lower) {
32 : DCHECK('A' <= src[i] && src[i] <= 'Z');
33 : DCHECK(dst[i] == src[i] + ('a' - 'A'));
34 : } else {
35 : DCHECK('a' <= src[i] && src[i] <= 'z');
36 : DCHECK(dst[i] == src[i] - ('a' - 'A'));
37 : }
38 : }
39 : return (expected_changed == changed);
40 : }
41 : #endif
42 :
43 : // Given a word and two range boundaries returns a word with high bit
44 : // set in every byte iff the corresponding input byte was strictly in
45 : // the range (m, n). All the other bits in the result are cleared.
46 : // This function is only useful when it can be inlined and the
47 : // boundaries are statically known.
48 : // Requires: all bytes in the input word and the boundaries must be
49 : // ASCII (less than 0x7F).
50 : static inline word_t AsciiRangeMask(word_t w, char m, char n) {
51 : // Use strict inequalities since in edge cases the function could be
52 : // further simplified.
53 : DCHECK(0 < m && m < n);
54 : // Has high bit set in every w byte less than n.
55 184737 : word_t tmp1 = kOneInEveryByte * (0x7F + n) - w;
56 : // Has high bit set in every w byte greater than m.
57 184737 : word_t tmp2 = w + kOneInEveryByte * (0x7F - m);
58 184737 : return (tmp1 & tmp2 & (kOneInEveryByte * 0x80));
59 : }
60 :
61 : template <bool is_lower>
62 6870 : int FastAsciiConvert(char* dst, const char* src, int length,
63 : bool* changed_out) {
64 : #ifdef DEBUG
65 : char* saved_dst = dst;
66 : #endif
67 : const char* saved_src = src;
68 : DisallowHeapAllocation no_gc;
69 : // We rely on the distance between upper and lower case letters
70 : // being a known power of 2.
71 : DCHECK_EQ('a' - 'A', 1 << 5);
72 : // Boundaries for the range of input characters than require conversion.
73 : static const char lo = is_lower ? 'A' - 1 : 'a' - 1;
74 : static const char hi = is_lower ? 'Z' + 1 : 'z' + 1;
75 : bool changed = false;
76 6870 : const char* const limit = src + length;
77 :
78 : // dst is newly allocated and always aligned.
79 : DCHECK(IsAligned(reinterpret_cast<Address>(dst), sizeof(word_t)));
80 : // Only attempt processing one word at a time if src is also aligned.
81 13740 : if (IsAligned(reinterpret_cast<Address>(src), sizeof(word_t))) {
82 : // Process the prefix of the input that requires no conversion one aligned
83 : // (machine) word at a time.
84 15420 : while (src <= limit - sizeof(word_t)) {
85 8065 : const word_t w = *reinterpret_cast<const word_t*>(src);
86 8065 : if ((w & kAsciiMask) != 0) return static_cast<int>(src - saved_src);
87 7984 : if (AsciiRangeMask(w, lo, hi) != 0) {
88 : changed = true;
89 : break;
90 : }
91 4491 : *reinterpret_cast<word_t*>(dst) = w;
92 4491 : src += sizeof(word_t);
93 4491 : dst += sizeof(word_t);
94 : }
95 : // Process the remainder of the input performing conversion when
96 : // required one word at a time.
97 359863 : while (src <= limit - sizeof(word_t)) {
98 176780 : const word_t w = *reinterpret_cast<const word_t*>(src);
99 176780 : if ((w & kAsciiMask) != 0) return static_cast<int>(src - saved_src);
100 : word_t m = AsciiRangeMask(w, lo, hi);
101 : // The mask has high (7th) bit set in every byte that needs
102 : // conversion and we know that the distance between cases is
103 : // 1 << 5.
104 176753 : *reinterpret_cast<word_t*>(dst) = w ^ (m >> 2);
105 176753 : src += sizeof(word_t);
106 176753 : dst += sizeof(word_t);
107 : }
108 : }
109 : // Process the last few bytes of the input (or the whole input if
110 : // unaligned access is not supported).
111 243308 : while (src < limit) {
112 118395 : char c = *src;
113 118395 : if ((c & kAsciiMask) != 0) return static_cast<int>(src - saved_src);
114 118273 : if (lo < c && c < hi) {
115 25178 : c ^= (1 << 5);
116 : changed = true;
117 : }
118 118273 : *dst = c;
119 118273 : ++src;
120 118273 : ++dst;
121 : }
122 :
123 : DCHECK(
124 : CheckFastAsciiConvert(saved_dst, saved_src, length, changed, is_lower));
125 :
126 6640 : *changed_out = changed;
127 6640 : return length;
128 : }
129 :
130 : template int FastAsciiConvert<false>(char* dst, const char* src, int length,
131 : bool* changed_out);
132 : template int FastAsciiConvert<true>(char* dst, const char* src, int length,
133 : bool* changed_out);
134 :
135 : } // namespace internal
136 120216 : } // namespace v8
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