V8 Project
misc-intrinsics.h
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1 // Copyright 2011 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_MISC_INTRINSICS_H_
6 #define V8_MISC_INTRINSICS_H_
7 
8 #include "include/v8.h"
9 #include "src/globals.h"
10 
11 namespace v8 {
12 namespace internal {
13 
14 // Returns the index of the leading 1 bit, counting the least significant bit at
15 // index 0. (1 << IntegerLog2(x)) is a mask for the most significant bit of x.
16 // Result is undefined if input is zero.
17 int IntegerLog2(uint32_t value);
18 
19 #if defined(__GNUC__)
20 
21 inline int IntegerLog2(uint32_t value) {
22  return 31 - __builtin_clz(value);
23 }
24 
25 #elif defined(_MSC_VER)
26 
27 #pragma intrinsic(_BitScanReverse)
28 
29 inline int IntegerLog2(uint32_t value) {
30  unsigned long result; // NOLINT: MSVC intrinsic demands this type.
31  _BitScanReverse(&result, value);
32  return result;
33 }
34 
35 #else
36 
37 // Default version using regular operations. Code taken from:
38 // http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
39 inline int IntegerLog2(uint32_t value) {
40  int result, shift;
41 
42  shift = (value > 0xFFFF) << 4;
43  value >>= shift;
44  result = shift;
45 
46  shift = (value > 0xFF) << 3;
47  value >>= shift;
48  result |= shift;
49 
50  shift = (value > 0xF) << 2;
51  value >>= shift;
52  result |= shift;
53 
54  shift = (value > 0x3) << 1;
55  value >>= shift;
56  result |= shift;
57 
58  result |= (value >> 1);
59 
60  return result;
61 }
62 #endif
63 
64 } } // namespace v8::internal
65 
66 #endif // V8_MISC_INTRINSICS_H_
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int IntegerLog2(uint32_t value)
Debugger support for the V8 JavaScript engine.
Definition: accessors.cc:20