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misc-intrinsics.h
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27 
28 #ifndef V8_MISC_INTRINSICS_H_
29 #define V8_MISC_INTRINSICS_H_
30 
31 #include "../include/v8.h"
32 #include "globals.h"
33 
34 namespace v8 {
35 namespace internal {
36 
37 // Returns the index of the leading 1 bit, counting the least significant bit at
38 // index 0. (1 << IntegerLog2(x)) is a mask for the most significant bit of x.
39 // Result is undefined if input is zero.
40 int IntegerLog2(uint32_t value);
41 
42 #if defined(__GNUC__)
43 
44 inline int IntegerLog2(uint32_t value) {
45  return 31 - __builtin_clz(value);
46 }
47 
48 #elif defined(_MSC_VER)
49 
50 #pragma intrinsic(_BitScanReverse)
51 
52 inline int IntegerLog2(uint32_t value) {
53  unsigned long result; // NOLINT: MSVC intrinsic demands this type.
54  _BitScanReverse(&result, value);
55  return result;
56 }
57 
58 #else
59 
60 // Default version using regular operations. Code taken from:
61 // http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
62 inline int IntegerLog2(uint32_t value) {
63  int result, shift;
64 
65  shift = (value > 0xFFFF) << 4;
66  value >>= shift;
67  result = shift;
68 
69  shift = (value > 0xFF) << 3;
70  value >>= shift;
71  result |= shift;
72 
73  shift = (value > 0xF) << 2;
74  value >>= shift;
75  result |= shift;
76 
77  shift = (value > 0x3) << 1;
78  value >>= shift;
79  result |= shift;
80 
81  result |= (value >> 1);
82 
83  return result;
84 }
85 #endif
86 
87 } } // namespace v8::internal
88 
89 #endif // V8_MISC_INTRINSICS_H_
int IntegerLog2(uint32_t value)