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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_DIY_FP_H_
6 #define V8_DIY_FP_H_
7 
8 #include <stdint.h>
9 
10 #include "src/base/logging.h"
11 
12 namespace v8 {
13 namespace internal {
14 
15 // This "Do It Yourself Floating Point" class implements a floating-point number
16 // with a uint64 significand and an int exponent. Normalized DiyFp numbers will
17 // have the most significant bit of the significand set.
18 // Multiplication and Subtraction do not normalize their results.
19 // DiyFp are not designed to contain special doubles (NaN and Infinity).
20 class DiyFp {
21  public:
22   static const int kSignificandSize = 64;
23 
DiyFp()24   DiyFp() : f_(0), e_(0) {}
DiyFp(uint64_t f,int e)25   DiyFp(uint64_t f, int e) : f_(f), e_(e) {}
26 
27   // this = this - other.
28   // The exponents of both numbers must be the same and the significand of this
29   // must be bigger than the significand of other.
30   // The result will not be normalized.
Subtract(const DiyFp & other)31   void Subtract(const DiyFp& other) {
32     DCHECK(e_ == other.e_);
33     DCHECK(f_ >= other.f_);
34     f_ -= other.f_;
35   }
36 
37   // Returns a - b.
38   // The exponents of both numbers must be the same and this must be bigger
39   // than other. The result will not be normalized.
Minus(const DiyFp & a,const DiyFp & b)40   static DiyFp Minus(const DiyFp& a, const DiyFp& b) {
41     DiyFp result = a;
42     result.Subtract(b);
43     return result;
44   }
45 
46 
47   // this = this * other.
48   void Multiply(const DiyFp& other);
49 
50   // returns a * b;
Times(const DiyFp & a,const DiyFp & b)51   static DiyFp Times(const DiyFp& a, const DiyFp& b) {
52     DiyFp result = a;
53     result.Multiply(b);
54     return result;
55   }
56 
Normalize()57   void Normalize() {
58     DCHECK_NE(f_, 0);
59     uint64_t f = f_;
60     int e = e_;
61 
62     // This method is mainly called for normalizing boundaries. In general
63     // boundaries need to be shifted by 10 bits. We thus optimize for this case.
64     const uint64_t k10MSBits = static_cast<uint64_t>(0x3FF) << 54;
65     while ((f & k10MSBits) == 0) {
66       f <<= 10;
67       e -= 10;
68     }
69     while ((f & kUint64MSB) == 0) {
70       f <<= 1;
71       e--;
72     }
73     f_ = f;
74     e_ = e;
75   }
76 
Normalize(const DiyFp & a)77   static DiyFp Normalize(const DiyFp& a) {
78     DiyFp result = a;
79     result.Normalize();
80     return result;
81   }
82 
f()83   uint64_t f() const { return f_; }
e()84   int e() const { return e_; }
85 
set_f(uint64_t new_value)86   void set_f(uint64_t new_value) { f_ = new_value; }
set_e(int new_value)87   void set_e(int new_value) { e_ = new_value; }
88 
89  private:
90   static const uint64_t kUint64MSB = static_cast<uint64_t>(1) << 63;
91 
92   uint64_t f_;
93   int e_;
94 };
95 
96 }  // namespace internal
97 }  // namespace v8
98 
99 #endif  // V8_DIY_FP_H_
100