1 HALF-PRECISION FLOATING-POINT LIBRARY (Version 2.1.0)
2 -----------------------------------------------------
4 This is a C++ header-only library to provide an IEEE 754 conformant 16-bit
5 half-precision floating-point type along with corresponding arithmetic
6 operators, type conversions and common mathematical functions. It aims for both
7 efficiency and ease of use, trying to accurately mimic the behaviour of the
8 built-in floating-point types at the best performance possible.
11 INSTALLATION AND REQUIREMENTS
12 -----------------------------
14 Conveniently, the library consists of just a single header file containing all
15 the functionality, which can be directly included by your projects, without the
16 neccessity to build anything or link to anything.
18 Whereas this library is fully C++98-compatible, it can profit from certain
19 C++11 features. Support for those features is checked automatically at compile
20 (or rather preprocessing) time, but can be explicitly enabled or disabled by
21 predefining the corresponding preprocessor symbols to either 1 or 0 yourself
22 before including half.hpp. This is useful when the automatic detection fails
23 (for more exotic implementations) or when a feature should be explicitly
26 - 'long long' integer type for mathematical functions returning 'long long'
27 results (enabled for VC++ 2003 and icc 11.1 and newer, gcc and clang,
28 overridable with 'HALF_ENABLE_CPP11_LONG_LONG').
30 - Static assertions for extended compile-time checks (enabled for VC++ 2010,
31 gcc 4.3, clang 2.9, icc 11.1 and newer, overridable with
32 'HALF_ENABLE_CPP11_STATIC_ASSERT').
34 - Generalized constant expressions (enabled for VC++ 2015, gcc 4.6, clang 3.1,
35 icc 14.0 and newer, overridable with 'HALF_ENABLE_CPP11_CONSTEXPR').
37 - noexcept exception specifications (enabled for VC++ 2015, gcc 4.6,
38 clang 3.0, icc 14.0 and newer, overridable with 'HALF_ENABLE_CPP11_NOEXCEPT').
40 - User-defined literals for half-precision literals to work (enabled for
41 VC++ 2015, gcc 4.7, clang 3.1, icc 15.0 and newer, overridable with
42 'HALF_ENABLE_CPP11_USER_LITERALS').
44 - Thread-local storage for per-thread floating-point exception flags (enabled
45 for VC++ 2015, gcc 4.8, clang 3.3, icc 15.0 and newer, overridable with
46 'HALF_ENABLE_CPP11_THREAD_LOCAL').
48 - Type traits and template meta-programming features from <type_traits>
49 (enabled for VC++ 2010, libstdc++ 4.3, libc++ and newer, overridable with
50 'HALF_ENABLE_CPP11_TYPE_TRAITS').
52 - Special integer types from <cstdint> (enabled for VC++ 2010, libstdc++ 4.3,
53 libc++ and newer, overridable with 'HALF_ENABLE_CPP11_CSTDINT').
55 - Certain C++11 single-precision mathematical functions from <cmath> for
56 floating-point classification during conversions from higher precision types
57 (enabled for VC++ 2013, libstdc++ 4.3, libc++ and newer, overridable with
58 'HALF_ENABLE_CPP11_CMATH').
60 - Floating-point environment control from <cfenv> for possible exception
61 propagation to the built-in floating-point platform (enabled for VC++ 2013,
62 libstdc++ 4.3, libc++ and newer, overridable with 'HALF_ENABLE_CPP11_CFENV').
64 - Hash functor 'std::hash' from <functional> (enabled for VC++ 2010,
65 libstdc++ 4.3, libc++ and newer, overridable with 'HALF_ENABLE_CPP11_HASH').
67 The library has been tested successfully with Visual C++ 2005-2015, gcc 4-8
68 and clang 3-8 on 32- and 64-bit x86 systems. Please contact me if you have any
69 problems, suggestions or even just success testing it on other platforms.
75 What follows are some general words about the usage of the library and its
76 implementation. For a complete documentation of its interface consult the
77 corresponding website http://half.sourceforge.net. You may also generate the
78 complete developer documentation from the library's only include file's doxygen
79 comments, but this is more relevant to developers rather than mere users.
83 To make use of the library just include its only header file half.hpp, which
84 defines all half-precision functionality inside the 'half_float' namespace. The
85 actual 16-bit half-precision data type is represented by the 'half' type, which
86 uses the standard IEEE representation with 1 sign bit, 5 exponent bits and 11
87 mantissa bits (including the hidden bit) and supports all types of special
88 values, like subnormal values, infinity and NaNs. This type behaves like the
89 built-in floating-point types as much as possible, supporting the usual
90 arithmetic, comparison and streaming operators, which makes its use pretty
93 using half_float::half;
98 std::cout << c << std::endl;
100 Additionally the 'half_float' namespace also defines half-precision versions
101 for all mathematical functions of the C++ standard library, which can be used
102 directly through ADL:
105 half s = sin(abs(a));
108 You may also specify explicit half-precision literals, since the library
109 provides a user-defined literal inside the 'half_float::literal' namespace,
110 which you just need to import (assuming support for C++11 user-defined literals):
112 using namespace half_float::literal;
115 Furthermore the library provides proper specializations for
116 'std::numeric_limits', defining various implementation properties, and
117 'std::hash' for hashing half-precision numbers (assuming support for C++11
118 'std::hash'). Similar to the corresponding preprocessor symbols from <cmath>
119 the library also defines the 'HUGE_VALH' constant and maybe the 'FP_FAST_FMAH'
122 CONVERSIONS AND ROUNDING
124 The half is explicitly constructible/convertible from a single-precision float
125 argument. Thus it is also explicitly constructible/convertible from any type
126 implicitly convertible to float, but constructing it from types like double or
127 int will involve the usual warnings arising when implicitly converting those to
128 float because of the lost precision. On the one hand those warnings are
129 intentional, because converting those types to half neccessarily also reduces
130 precision. But on the other hand they are raised for explicit conversions from
131 those types, when the user knows what he is doing. So if those warnings keep
132 bugging you, then you won't get around first explicitly converting to float
133 before converting to half, or use the 'half_cast' described below. In addition
134 you can also directly assign float values to halfs.
136 In contrast to the float-to-half conversion, which reduces precision, the
137 conversion from half to float (and thus to any other type implicitly
138 convertible from float) is implicit, because all values represetable with
139 half-precision are also representable with single-precision. This way the
140 half-to-float conversion behaves similar to the builtin float-to-double
141 conversion and all arithmetic expressions involving both half-precision and
142 single-precision arguments will be of single-precision type. This way you can
143 also directly use the mathematical functions of the C++ standard library,
144 though in this case you will invoke the single-precision versions which will
145 also return single-precision values, which is (even if maybe performing the
146 exact same computation, see below) not as conceptually clean when working in a
147 half-precision environment.
149 The default rounding mode for conversions between half and more precise types
150 as well as for rounding results of arithmetic operations and mathematical
151 functions rounds to the nearest representable value. But by predefining the
152 'HALF_ROUND_STYLE' preprocessor symbol this default can be overridden with one
153 of the other standard rounding modes using their respective constants or the
154 equivalent values of 'std::float_round_style' (it can even be synchronized with
155 the built-in single-precision implementation by defining it to
156 'std::numeric_limits<float>::round_style'):
158 - 'std::round_indeterminate' (-1) for the fastest rounding.
160 - 'std::round_toward_zero' (0) for rounding toward zero.
162 - 'std::round_to_nearest' (1) for rounding to the nearest value (default).
164 - 'std::round_toward_infinity' (2) for rounding toward positive infinity.
166 - 'std::round_toward_neg_infinity' (3) for rounding toward negative infinity.
168 In addition to changing the overall default rounding mode one can also use the
169 'half_cast'. This converts between half and any built-in arithmetic type using
170 a configurable rounding mode (or the default rounding mode if none is
171 specified). In addition to a configurable rounding mode, 'half_cast' has
172 another big difference to a mere 'static_cast': Any conversions are performed
173 directly using the given rounding mode, without any intermediate conversion
174 to/from 'float'. This is especially relevant for conversions to integer types,
175 which don't necessarily truncate anymore. But also for conversions from
176 'double' or 'long double' this may produce more precise results than a
177 pre-conversion to 'float' using the single-precision implementation's current
180 half a = half_cast<half>(4.2);
181 half b = half_cast<half,std::numeric_limits<float>::round_style>(4.2f);
182 assert( half_cast<int, std::round_to_nearest>( 0.7_h ) == 1 );
183 assert( half_cast<half,std::round_toward_zero>( 4097 ) == 4096.0_h );
184 assert( half_cast<half,std::round_toward_infinity>( 4097 ) == 4100.0_h );
185 assert( half_cast<half,std::round_toward_infinity>( std::numeric_limits<double>::min() ) > 0.0_h );
187 ACCURACY AND PERFORMANCE
189 From version 2.0 onward the library is implemented without employing the
190 underlying floating-point implementation of the system (except for conversions,
191 of course), providing an entirely self-contained half-precision implementation
192 with results independent from the system's existing single- or double-precision
193 implementation and its rounding behaviour.
195 As to accuracy, many of the operators and functions provided by this library
196 are exact to rounding for all rounding modes, i.e. the error to the exact
197 result is at most 0.5 ULP (unit in the last place) for rounding to nearest and
198 less than 1 ULP for all other rounding modes. This holds for all the operations
199 required by the IEEE 754 standard and many more. Specifically the following
200 functions might exhibit a deviation from the correctly rounded exact result by
201 1 ULP for a select few input values: 'expm1', 'log1p', 'pow', 'atan2', 'erf',
202 'erfc', 'lgamma', 'tgamma' (for more details see the documentation of the
203 individual functions). All other functions and operators are always exact to
204 rounding or independent of the rounding mode altogether.
206 The increased IEEE-conformance and cleanliness of this implementation comes
207 with a certain performance cost compared to doing computations and mathematical
208 functions in hardware-accelerated single-precision. On average and depending on
209 the platform, the arithemtic operators are about 75% as fast and the
210 mathematical functions about 33-50% as fast as performing the corresponding
211 operations in single-precision and converting between the inputs and outputs.
212 However, directly computing with half-precision values is a rather rare
213 use-case and usually using actual 'float' values for all computations and
214 temproraries and using 'half's only for storage is the recommended way. But
215 nevertheless the goal of this library was to provide a complete and
216 conceptually clean IEEE-confromant half-precision implementation and in the few
217 cases when you do need to compute directly in half-precision you do so for a
218 reason and want accurate results.
220 If necessary, this internal implementation can be overridden by predefining the
221 'HALF_ARITHMETIC_TYPE' preprocessor symbol to one of the built-in
222 floating-point types ('float', 'double' or 'long double'), which will cause the
223 library to use this type for computing arithmetic operations and mathematical
224 functions (if available). However, due to using the platform's floating-point
225 implementation (and its rounding behaviour) internally, this might cause
226 results to deviate from the specified half-precision rounding mode. It will of
227 course also inhibit the automatic exception detection described below.
229 The conversion operations between half-precision and single-precision types can
230 also make use of the F16C extension for x86 processors by using the
231 corresponding compiler intrinsics from <immintrin.h>. Support for this is
232 checked at compile-time by looking for the '__F16C__' macro which at least gcc
233 and clang define based on the target platform. It can also be enabled manually
234 by predefining the 'HALF_ENABLE_F16C_INTRINSICS' preprocessor symbol to 1, or 0
235 for explicitly disabling it. However, this will directly use the corresponding
236 intrinsics for conversion without checking if they are available at runtime
237 (possibly crashing if they are not), so make sure they are supported on the
238 target platform before enabling this.
242 The half-precision implementation supports all 5 required floating-point
243 exceptions from the IEEE standard to indicate erroneous inputs or inexact
244 results during operations. These are represented by exception flags which
245 actually use the same values as the corresponding 'FE_...' flags defined in
246 C++11's <cfenv> header if supported, specifically:
248 - 'FE_INVALID' for invalid inputs to an operation.
249 - 'FE_DIVBYZERO' for finite inputs producing infinite results.
250 - 'FE_OVERFLOW' if a result is too large to represent finitely.
251 - 'FE_UNDERFLOW' for a subnormal or zero result after rounding.
252 - 'FE_INEXACT' if a result needed rounding to be representable.
253 - 'FE_ALL_EXCEPT' as a convenient OR of all possible exception flags.
255 The internal exception flag state will start with all flags cleared and is
256 maintained per thread if C++11 thread-local storage is supported, otherwise it
257 will be maintained globally and will theoretically NOT be thread-safe (while
258 practically being as thread-safe as a simple integer variable can be). These
259 flags can be managed explicitly using the library's error handling functions,
260 which again try to mimic the built-in functions for handling floating-point
261 exceptions from <cfenv>. You can clear them with 'feclearexcept' (which is the
262 only way a flag can be cleared), test them with 'fetestexcept', explicitly
263 raise errors with 'feraiseexcept' and save and restore their state using
264 'fegetexceptflag' and 'fesetexceptflag'. You can also throw corresponding C++
265 exceptions based on the current flag state using 'fethrowexcept'.
267 However, any automatic exception detection and handling during half-precision
268 operations and functions is DISABLED by default, since it comes with a minor
269 performance overhead due to runtime checks, and reacting to IEEE floating-point
270 exceptions is rarely ever needed in application code. But the library fully
271 supports IEEE-conformant detection of floating-point exceptions and various
272 ways for handling them, which can be enabled by pre-defining the corresponding
273 preprocessor symbols to 1. They can be enabled individually or all at once and
274 they will be processed in the order they are listed here:
276 - 'HALF_ERRHANDLING_FLAGS' sets the internal exception flags described above
277 whenever the corresponding exception occurs.
278 - 'HALF_ERRHANDLING_ERRNO' sets the value of 'errno' from <cerrno> similar to
279 the behaviour of the built-in floating-point types when 'MATH_ERRNO' is used.
280 - 'HALF_ERRHANDLING_FENV' will propagate exceptions to the built-in
281 floating-point implementation using 'std::feraiseexcept' if support for
282 C++11 floating-point control is enabled. However, this does not synchronize
283 exceptions: neither will clearing propagate nor will it work in reverse.
284 - 'HALF_ERRHANDLING_THROW_...' can be defined to a string literal which will
285 be used as description message for a C++ exception that is thrown whenever
286 a 'FE_...' exception occurs, similar to the behaviour of 'fethrowexcept'.
288 If any of the above error handling is activated, non-quiet operations on
289 half-precision values will also raise a 'FE_INVALID' exception whenever
290 they encounter a signaling NaN value, in addition to transforming the value
291 into a quiet NaN. If error handling is disabled, signaling NaNs will be
292 treated like quiet NaNs (while still getting explicitly quieted if propagated
293 to the result). There can also be additional treatment of overflow and
294 underflow errors after they have been processed as above, which is ENABLED by
295 default (but of course only takes effect if any other exception handling is
296 activated) unless overridden by pre-defining the corresponding preprocessor
299 - 'HALF_ERRHANDLING_OVERFLOW_TO_INEXACT' will cause overflow errors to also
300 raise a 'FE_INEXACT' exception.
301 - 'HALF_ERRHANDLING_UNDERFLOW_TO_INEXACT' will cause underflow errors to also
302 raise a 'FE_INEXACT' exception. This will also slightly change the
303 behaviour of the underflow exception, which will ONLY be raised if the
304 result is actually inexact due to underflow. If this is disabled, underflow
305 exceptions will be raised for ANY (possibly exact) subnormal result.
311 This library is developed by CHRISTIAN RAU and released under the MIT License
312 (see LICENSE.txt). If you have any questions or problems with it, feel free to
313 contact me at rauy@users.sourceforge.net.
315 Additional credit goes to JEROEN VAN DER ZIJP for his paper on "Fast Half Float
316 Conversions", whose algorithms have been used in the library for converting
317 between half-precision and single-precision values.