1 // Copyright 2019 Ulf Adams
2 //
3 // The contents of this file may be used under the terms of the Apache License,
4 // Version 2.0.
5 //
6 // (See accompanying file LICENSE-Apache or copy at
7 // http://www.apache.org/licenses/LICENSE-2.0)
8 //
9 // Alternatively, the contents of this file may be used under the terms of
10 // the Boost Software License, Version 1.0.
11 // (See accompanying file LICENSE-Boost or copy at
12 // https://www.boost.org/LICENSE_1_0.txt)
13 //
14 // Unless required by applicable law or agreed to in writing, this software
15 // is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
16 // KIND, either express or implied.
17
18 #include "stdio_private.h"
19
20 #ifdef RYU_DEBUG
21 #include <inttypes.h>
22 #endif
23
24 #include "ryu/common.h"
25 #include "ryu/d2s_intrinsics.h"
26
27 #if __SIZEOF_DOUBLE__ == 8
28 #define FLOAT64 double
29 #elif __SIZEOF_LONG_DOUBLE__ == 8
30 #define FLOAT64 long double
31 #endif
32
33 #ifdef FLOAT64
34
35 #define DOUBLE_MANTISSA_BITS 52
36 #define DOUBLE_EXPONENT_BITS 11
37 #define DOUBLE_EXPONENT_BIAS 1023
38
39 #if defined(_MSC_VER)
40 #include <intrin.h>
41
floor_log2(const uint64_t value)42 static inline uint32_t floor_log2(const uint64_t value) {
43 long index;
44 return _BitScanReverse64(&index, value) ? index : 64;
45 }
46
47 #else
48
floor_log2(const uint64_t value)49 static inline uint32_t floor_log2(const uint64_t value) {
50 return 63 - __builtin_clzll(value);
51 }
52
53 #endif
54
55 // The max function is already defined on Windows.
max32(int32_t a,int32_t b)56 static inline int32_t max32(int32_t a, int32_t b) {
57 return a < b ? b : a;
58 }
59
int64Bits2Double(uint64_t bits)60 static inline FLOAT64 int64Bits2Double(uint64_t bits) {
61 FLOAT64 f;
62 memcpy(&f, &bits, sizeof(FLOAT64));
63 return f;
64 }
65
66 FLOAT64
__atod_engine(uint64_t m10,int e10)67 __atod_engine(uint64_t m10, int e10)
68 {
69 #ifdef RYU_DEBUG
70 printf("m10 = %ld\n", m10);
71 printf("e10 = %d\n", e10);
72 printf("m10 * 10^e10 = %" PRIu64 " * 10^%d\n", m10, e10);
73 #endif
74
75 // Convert to binary float m2 * 2^e2, while retaining information about whether the conversion
76 // was exact (trailingZeros).
77 int32_t e2;
78 uint64_t m2;
79 bool trailingZeros;
80 if (e10 >= 0) {
81 // The length of m * 10^e in bits is:
82 // log2(m10 * 10^e10) = log2(m10) + e10 log2(10) = log2(m10) + e10 + e10 * log2(5)
83 //
84 // We want to compute the DOUBLE_MANTISSA_BITS + 1 top-most bits (+1 for the implicit leading
85 // one in IEEE format). We therefore choose a binary output exponent of
86 // log2(m10 * 10^e10) - (DOUBLE_MANTISSA_BITS + 1).
87 //
88 // We use floor(log2(5^e10)) so that we get at least this many bits; better to
89 // have an additional bit than to not have enough bits.
90 e2 = floor_log2(m10) + e10 + log2pow5(e10) - (DOUBLE_MANTISSA_BITS + 1);
91
92 // We now compute [m10 * 10^e10 / 2^e2] = [m10 * 5^e10 / 2^(e2-e10)].
93 // To that end, we use the DOUBLE_POW5_SPLIT table.
94 int j = e2 - e10 - ceil_log2pow5(e10) + DOUBLE_POW5_BITCOUNT;
95 assert(j >= 0);
96 uint64_t pow5[2];
97 __double_computePow5(e10, pow5);
98 m2 = mulShift64(m10, pow5, j);
99
100 // We also compute if the result is exact, i.e.,
101 // [m10 * 10^e10 / 2^e2] == m10 * 10^e10 / 2^e2.
102 // This can only be the case if 2^e2 divides m10 * 10^e10, which in turn requires that the
103 // largest power of 2 that divides m10 + e10 is greater than e2. If e2 is less than e10, then
104 // the result must be exact. Otherwise we use the existing multipleOfPowerOf2 function.
105 trailingZeros = e2 < e10 || (e2 - e10 < 64 && multipleOfPowerOf2(m10, e2 - e10));
106 } else {
107 e2 = floor_log2(m10) + e10 - ceil_log2pow5(-e10) - (DOUBLE_MANTISSA_BITS + 1);
108 int j = e2 - e10 + ceil_log2pow5(-e10) - 1 + DOUBLE_POW5_INV_BITCOUNT;
109 uint64_t pow5[2];
110 __double_computeInvPow5(-e10, pow5);
111 m2 = mulShift64(m10, pow5, j);
112 trailingZeros = multipleOfPowerOf5(m10, -e10);
113 #ifdef RYU_DEBUG
114 printf("pow5 %016lx_%016lx j %d trailingZeros %d\n", pow5[0], pow5[1], j, trailingZeros);
115 #endif
116 }
117
118 #ifdef RYU_DEBUG
119 printf("m2 * 2^e2 = %" PRIu64 " * 2^%d\n", m2, e2);
120 #endif
121
122 // Compute the final IEEE exponent.
123 uint32_t ieee_e2 = (uint32_t) max32(0, e2 + DOUBLE_EXPONENT_BIAS + floor_log2(m2));
124
125 if (ieee_e2 > 0x7fe) {
126 // Final IEEE exponent is larger than the maximum representable; return +/-Infinity.
127 uint64_t ieee = (0x7ffull << DOUBLE_MANTISSA_BITS);
128 return int64Bits2Double(ieee);
129 }
130
131 // We need to figure out how much we need to shift m2. The tricky part is that we need to take
132 // the final IEEE exponent into account, so we need to reverse the bias and also special-case
133 // the value 0.
134 int32_t shift = (ieee_e2 == 0 ? 1 : ieee_e2) - e2 - DOUBLE_EXPONENT_BIAS - DOUBLE_MANTISSA_BITS;
135 assert(shift >= 0);
136 #ifdef RYU_DEBUG
137 printf("ieee_e2 = %d\n", ieee_e2);
138 printf("shift = %d\n", shift);
139 #endif
140
141 // We need to round up if the exact value is more than 0.5 above the value we computed. That's
142 // equivalent to checking if the last removed bit was 1 and either the value was not just
143 // trailing zeros or the result would otherwise be odd.
144 //
145 // We need to update trailingZeros given that we have the exact output exponent ieee_e2 now.
146 trailingZeros &= (m2 & ((1ull << (shift - 1)) - 1)) == 0;
147 uint64_t lastRemovedBit = (m2 >> (shift - 1)) & 1;
148 bool roundUp = (lastRemovedBit != 0) && (!trailingZeros || (((m2 >> shift) & 1) != 0));
149
150 #ifdef RYU_DEBUG
151 printf("roundUp = %d\n", roundUp);
152 printf("ieee_m2 = %" PRIu64 "\n", (m2 >> shift) + roundUp);
153 #endif
154 uint64_t ieee_m2 = (m2 >> shift) + roundUp;
155 assert(ieee_m2 <= (1ull << (DOUBLE_MANTISSA_BITS + 1)));
156 ieee_m2 &= (1ull << DOUBLE_MANTISSA_BITS) - 1;
157 if (ieee_m2 == 0 && roundUp) {
158 // Due to how the IEEE represents +/-Infinity, we don't need to check for overflow here.
159 ieee_e2++;
160 }
161 uint64_t ieee = (((uint64_t)ieee_e2) << DOUBLE_MANTISSA_BITS) | ieee_m2;
162 return int64Bits2Double(ieee);
163 }
164
165 #endif /* FLOAT64 */
166
