1 // Copyright 2020 The Tint Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 #include "src/writer/float_to_string.h"
16
17 #include <cmath>
18 #include <cstring>
19 #include <functional>
20 #include <iomanip>
21 #include <limits>
22 #include <sstream>
23
24 #include "src/debug.h"
25
26 namespace tint {
27 namespace writer {
28
FloatToString(float f)29 std::string FloatToString(float f) {
30 // Try printing the float in fixed point, with a smallish limit on the
31 // precision
32 std::stringstream fixed;
33 fixed.flags(fixed.flags() | std::ios_base::showpoint | std::ios_base::fixed);
34 fixed.precision(9);
35 fixed << f;
36
37 // If this string can be parsed without loss of information, use it
38 auto float_equal_no_warning = std::equal_to<float>();
39 if (float_equal_no_warning(std::stof(fixed.str()), f)) {
40 auto str = fixed.str();
41 while (str.length() >= 2 && str[str.size() - 1] == '0' &&
42 str[str.size() - 2] != '.') {
43 str.pop_back();
44 }
45
46 return str;
47 }
48
49 // Resort to scientific, with the minimum precision needed to preserve the
50 // whole float
51 std::stringstream sci;
52 sci.precision(std::numeric_limits<float>::max_digits10);
53 sci << f;
54 return sci.str();
55 }
56
FloatToBitPreservingString(float f)57 std::string FloatToBitPreservingString(float f) {
58 // For the NaN case, avoid handling the number as a floating point value.
59 // Some machines will modify the top bit in the mantissa of a NaN.
60
61 std::stringstream ss;
62
63 uint32_t float_bits = 0u;
64 std::memcpy(&float_bits, &f, sizeof(float_bits));
65
66 // Handle the sign.
67 const uint32_t kSignMask = 1u << 31;
68 if (float_bits & kSignMask) {
69 // If `f` is -0.0 print -0.0.
70 ss << '-';
71 // Strip sign bit.
72 float_bits = float_bits & (~kSignMask);
73 }
74
75 switch (std::fpclassify(f)) {
76 case FP_ZERO:
77 case FP_NORMAL:
78 std::memcpy(&f, &float_bits, sizeof(float_bits));
79 ss << FloatToString(f);
80 break;
81
82 default: {
83 // Infinity, NaN, and Subnormal
84 // TODO(dneto): It's unclear how Infinity and NaN should be handled.
85 // See https://github.com/gpuweb/gpuweb/issues/1769
86
87 // std::hexfloat prints 'nan' and 'inf' instead of an
88 // explicit representation like we want. Split it out
89 // manually.
90 const int kExponentBias = 127;
91 const int kExponentMask = 0x7f800000;
92 const int kMantissaMask = 0x007fffff;
93 const int kMantissaBits = 23;
94
95 int mantissaNibbles = (kMantissaBits + 3) / 4;
96
97 const int biased_exponent =
98 static_cast<int>((float_bits & kExponentMask) >> kMantissaBits);
99 int exponent = biased_exponent - kExponentBias;
100 uint32_t mantissa = float_bits & kMantissaMask;
101
102 ss << "0x";
103
104 if (exponent == 128) {
105 if (mantissa == 0) {
106 // Infinity case.
107 ss << "1p+128";
108 } else {
109 // NaN case.
110 // Emit the mantissa bits as if they are left-justified after the
111 // binary point. This is what SPIRV-Tools hex float emitter does,
112 // and it's a justifiable choice independent of the bit width
113 // of the mantissa.
114 mantissa <<= (4 - (kMantissaBits % 4));
115 // Remove trailing zeroes, for tidyness.
116 while (0 == (0xf & mantissa)) {
117 mantissa >>= 4;
118 mantissaNibbles--;
119 }
120 ss << "1." << std::hex << std::setfill('0')
121 << std::setw(mantissaNibbles) << mantissa << "p+128";
122 }
123 } else {
124 // Subnormal, and not zero.
125 TINT_ASSERT(Writer, mantissa != 0);
126 const int kTopBit = (1 << kMantissaBits);
127
128 // Shift left until we get 1.x
129 while (0 == (kTopBit & mantissa)) {
130 mantissa <<= 1;
131 exponent--;
132 }
133 // Emit the leading 1, and remove it from the mantissa.
134 ss << "1";
135 mantissa = mantissa ^ kTopBit;
136 mantissa <<= 1;
137 exponent++;
138
139 // Emit the fractional part.
140 if (mantissa) {
141 // Remove trailing zeroes, for tidyness
142 while (0 == (0xf & mantissa)) {
143 mantissa >>= 4;
144 mantissaNibbles--;
145 }
146 ss << "." << std::hex << std::setfill('0')
147 << std::setw(mantissaNibbles) << mantissa;
148 }
149 // Emit the exponent
150 ss << "p" << std::showpos << std::dec << exponent;
151 }
152 }
153 }
154 return ss.str();
155 }
156
157 } // namespace writer
158 } // namespace tint
159