V8 Project
utils-arm64.cc
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1 // Copyright 2013 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 #if V8_TARGET_ARCH_ARM64
6 
8 
9 
10 namespace v8 {
11 namespace internal {
12 
13 #define __ assm->
14 
15 
16 int CountLeadingZeros(uint64_t value, int width) {
17  // TODO(jbramley): Optimize this for ARM64 hosts.
18  DCHECK((width == 32) || (width == 64));
19  int count = 0;
20  uint64_t bit_test = 1UL << (width - 1);
21  while ((count < width) && ((bit_test & value) == 0)) {
22  count++;
23  bit_test >>= 1;
24  }
25  return count;
26 }
27 
28 
29 int CountLeadingSignBits(int64_t value, int width) {
30  // TODO(jbramley): Optimize this for ARM64 hosts.
31  DCHECK((width == 32) || (width == 64));
32  if (value >= 0) {
33  return CountLeadingZeros(value, width) - 1;
34  } else {
35  return CountLeadingZeros(~value, width) - 1;
36  }
37 }
38 
39 
40 int CountTrailingZeros(uint64_t value, int width) {
41  // TODO(jbramley): Optimize this for ARM64 hosts.
42  DCHECK((width == 32) || (width == 64));
43  int count = 0;
44  while ((count < width) && (((value >> count) & 1) == 0)) {
45  count++;
46  }
47  return count;
48 }
49 
50 
51 int CountSetBits(uint64_t value, int width) {
52  // TODO(jbramley): Would it be useful to allow other widths? The
53  // implementation already supports them.
54  DCHECK((width == 32) || (width == 64));
55 
56  // Mask out unused bits to ensure that they are not counted.
57  value &= (0xffffffffffffffffUL >> (64-width));
58 
59  // Add up the set bits.
60  // The algorithm works by adding pairs of bit fields together iteratively,
61  // where the size of each bit field doubles each time.
62  // An example for an 8-bit value:
63  // Bits: h g f e d c b a
64  // \ | \ | \ | \ |
65  // value = h+g f+e d+c b+a
66  // \ | \ |
67  // value = h+g+f+e d+c+b+a
68  // \ |
69  // value = h+g+f+e+d+c+b+a
70  value = ((value >> 1) & 0x5555555555555555) + (value & 0x5555555555555555);
71  value = ((value >> 2) & 0x3333333333333333) + (value & 0x3333333333333333);
72  value = ((value >> 4) & 0x0f0f0f0f0f0f0f0f) + (value & 0x0f0f0f0f0f0f0f0f);
73  value = ((value >> 8) & 0x00ff00ff00ff00ff) + (value & 0x00ff00ff00ff00ff);
74  value = ((value >> 16) & 0x0000ffff0000ffff) + (value & 0x0000ffff0000ffff);
75  value = ((value >> 32) & 0x00000000ffffffff) + (value & 0x00000000ffffffff);
76 
77  return value;
78 }
79 
80 
81 uint64_t LargestPowerOf2Divisor(uint64_t value) {
82  return value & -value;
83 }
84 
85 
86 int MaskToBit(uint64_t mask) {
87  DCHECK(CountSetBits(mask, 64) == 1);
88  return CountTrailingZeros(mask, 64);
89 }
90 
91 
92 } } // namespace v8::internal
93 
94 #endif // V8_TARGET_ARCH_ARM64
#define DCHECK(condition)
Definition: logging.h:205
int CountLeadingSignBits(int64_t value, int width)
int MaskToBit(uint64_t mask)
int CountLeadingZeros(uint64_t value, int width)
uint64_t LargestPowerOf2Divisor(uint64_t value)
int CountSetBits(uint64_t value, int width)
int CountTrailingZeros(uint64_t value, int width)
Debugger support for the V8 JavaScript engine.
Definition: accessors.cc:20