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