half  1.11
IEEE 754-based half-precision floating point library
 All Classes Namespaces Files Functions Variables Typedefs Macros Pages
Half-precision floating point library

This is a C++ header-only library to provide an IEEE 754 conformant 16-bit half-precision floating point type along with corresponding arithmetic operators, type conversions and common mathematical functions. It aims for both efficiency and ease of use, trying to accurately mimic the behaviour of the builtin floating point types at the best performance possible. It is hosted on SourceForge.net.

News

November 16, 2013 - Release 1.11.0

Version 1.11.0 of the library has been released. It further increases the flexibility of the rounding, by allowing the round to nearest behaviour to be configured more precisely. By default half-way cases during round to nearest are rounded away from zero (and thus analogous to the round() function), but by redefining HALF_ROUND_TIES_TO_EVEN to 1 this can be changed to the slower but less biased round to even. In addition to that the support for C++11 mathematical functions in light of unsupported single-precision versions has been completed by providing erf(), erfc(), lgamma() and tgamma() even if their <cmath> counterparts are unavailable. Furthermore, it fixes a bug that made it impossible to disable support for C++11 mathematical functions in VC++ 2013.

November 9, 2013 - Release 1.10.0

Version 1.10.0 of the library has been released. It adds the possibility of changing the default rounding mode for halfs (used for any float-to-half conversion) at compile-time by redefining the HALF_ROUND_STYLE preprocessor symbol. In addition to that the behaviour of the half_cast() function was adjusted to respect the specified rounding mode for conversions to/from integer types, without a roundtrip over an additional conversion to/from float. The half_cast() now only supports built-in arithmetic types (apart from halfs, of course) and any further conversions from them to custom types have to be done explicitly. The usage and checks of C++11 features has been extended by support for type traits and TMP facilities from <type_traits>, controllable with the HALF_ENABLE_CPP11_TYPE_TRAITS symbol. Last but not least, the library now supports non-IEEE single-precision implementations, which are pretty unlikely nowadays, though.

more


Download and Installation

The library in its most recent version can be obtained from here, see the Release Notes for further information:

If you are interested in previous versions of the library, see the SourceForge download page.

Comfortably enough, the library consists of just a single header file containing all the functionality, which can be directly included by your projects, without the neccessity to build anything or link to anything.

Whereas this library is fully C++98-compatible, it can profit from certain C++11 features. Support for those features is checked and enabled automatically at compile (or rather preprocessing) time, but can be explicitly enabled or disabled by defining the corresponding preprocessor symbols to either 1 or 0 yourself. This is useful when the automatic detection fails (for more exotic implementations) or when a feature should be explicitly disabled:

C++11 feature Used for Enabled for (and newer) Override with
long long integer type functions returning long long VC++ 2003, gcc, clang HALF_ENABLE_CPP11_LONG_LONG
static assertions extended compile-time checks VC++ 2010, gcc 4.3, clang 2.9 HALF_ENABLE_CPP11_STATIC_ASSERT
generalized constant expressions constant operations gcc 4.6, clang 3.1 HALF_ENABLE_CPP11_CONSTEXPR
noexcept specifications proper noexcept functions gcc 4.6, clang 3.0 HALF_ENABLE_CPP11_NOEXCEPT
user-defined literals half-precision literals gcc 4.7, clang 3.1 HALF_ENABLE_CPP11_USER_LITERALS
type traits from <type_traits> TMP and extended checks VC++ 2010, libstdc++ 4.3, libc++ HALF_ENABLE_CPP11_TYPE_TRAITS
sized integer types from <cstdint> more flexible type sizes VC++ 2010, libstdc++ 4.3, libc++ HALF_ENABLE_CPP11_CSTDINT
certain new <cmath> functions corresponding half implementations VC++ 2013, libstdc++ 4.3, libc++ HALF_ENABLE_CPP11_CMATH
std::hash from <functional> hash function for halfs VC++ 2010, libstdc++ 4.3, libc++ HALF_ENABLE_CPP11_HASH

The library has been tested successfully with Visual C++ 2005 - 2013, gcc 4.4 - 4.8 and clang 3.1. Please contact me if you have any problems, suggestions or even just success testing it on other platforms.


Documentation

What follows are some general words about the usage of the library and its implementation. For a complete reference documentation of its iterface you should consult the API Documentation.

Basic usage

To make use of the library just include its only header file half.hpp, which defines all half-precision functionality inside the half_float namespace. The actual 16-bit half-precision data type is represented by the half type. This type behaves like the builtin floating point types as much as possible, supporting the usual arithmetic, comparison and streaming operators, which makes its use pretty straight-forward:

half a(3.4), b(5);
half c = a * b;
c += 3;
if(c > a)
std::cout << c << std::endl;

Additionally the half_float namespace also defines half-precision versions for all mathematical functions of the C++ standard library, which can be used directly through ADL:

half a(-3.14159);
half s = sin(abs(a));
long l = lround(s);

You may also specify explicit half-precision literals, since the library provides a user-defined literal inside the half_float::literal namespace, which you just need to import (assuming support for C++11 user-defined literals):

using namespace half_float::literal;
half x = 1.0_h;

Furthermore the library provides proper specializations for std::numeric_limits, defining various implementation properties, and std::hash for hashing half-precision numbers (assuming support for C++11 std::hash). Similar to the corresponding preprocessor symbols from <cmath> the library also defines the HUGE_VALH constant and maybe the FP_FAST_FMAH symbol.

Conversions and rounding

The half is explicitly constructible/convertible from a single-precision float argument. Thus it is also explicitly constructible/convertible from any type implicitly convertible to float, but constructing it from types like double or int will involve the usual warnings arising when implicitly converting those to float because of the lost precision. On the one hand those warnings are intentional, because converting those types to half neccessarily also reduces precision. But on the other hand they are raised for explicit conversions from those types, when the user knows what he is doing. So if those warnings keep bugging you, then you won't get around first explicitly converting to float before converting to half, or use the half_cast() described below. In addition you can also directly assign float values to halfs.

In contrast to the float-to-half conversion, which reduces precision, the conversion from half to float (and thus to any other type implicitly convertible to float) is implicit, because all values represetable with half-precision are also representable with single-precision. This way the half-to-float conversion behaves similar to the builtin float-to-double conversion and all arithmetic expressions involving both half-precision and single-precision arguments will be of single-precision type. This way you can also directly use the mathematical functions of the C++ standard library, though in this case you will invoke the single-precision versions which will also return single-precision values, which is (even if maybe performing the exact same computation, see below) not as conceptually clean when working in a half-precision environment.

The default rounding mode for conversions from float to half uses truncation (round toward zero, but mapping overflows to infinity) for rounding values not representable exactly in half-precision. This is the fastest rounding possible and is usually sufficient. But by redefining the HALF_ROUND_STYLE preprocessor symbol (before including half.hpp) this default can be overridden with one of the other standard rounding modes using their respective constants or the equivalent values of std::float_round_style (it can even be synchronized with the underlying single-precision implementation by defining it to std::numeric_limits<float>::round_style):

std::float_round_style value rounding
std::round_indeterminate -1 fastest (default)
std::round_toward_zero 0 toward zero
std::round_to_nearest 1 to nearest
std::round_toward_infinity 2 toward positive infinity
std::round_toward_neg_infinity 3 toward negative infinity

In addition to changing the overall default rounding mode one can also use the half_cast(). This converts between half and any built-in arithmetic type using a configurable rounding mode (or the default rounding mode if none is specified). In addition to a configurable rounding mode, half_cast() has two other differences to a mere static_cast: (1) Floating point types are explicitly cast to float before being converted to half-precision and thus any warnings for narrowing conversions are suppressed. (2) Conversions to/from integer types are performed directly using the given rounding mode, without any intermediate conversion to/from float.

half a = half_cast<half>(4.2);
half b = half_cast<half,std::numeric_limits<float>::round_style>(4.2f);
assert( half_cast<int, std::round_to_nearest>( 0.7_h ) == 1 );
assert( half_cast<half,std::round_toward_zero>( 4097 ) == 4096.0_h );
assert( half_cast<half,std::round_toward_infinity>( 4097 ) == 4100.0_h );

When using round to nearest (either as default or thorugh half_cast()) ties are by default resolved by rounding them away from zero (and thus equal to the behaviour of the round() function). But by redefining the HALF_ROUND_TIES_TO_EVEN preprocessor symbol to 1 (before including half.hpp) this default can be changed to the slightly slower but less biased and more IEEE-conformant behaviour of rounding half-way cases to the nearest even value.

#define HALF_ROUND_TIES_TO_EVEN 1
#include <half.hpp>
...
assert( half_cast<int,std::round_to_nearest>(3.5_h)
== half_cast<int,std::round_to_nearest>(4.5_h) );

Implementation

For performance reasons (and ease of implementation) many of the mathematical functions provided by the library as well as all arithmetic operations are actually carried out in single-precision under the hood, calling to the C++ standard library implementations of those functions whenever appropriate, meaning the arguments are converted to floats and the result back to half. But to reduce the conversion overhead as much as possible any temporary values inside of lengthy expressions are kept in single-precision as long as possible, while still maintaining a strong half-precision type to the outside world. Only when finally assigning the value to a half or calling a function that works directly on halfs is the actual conversion done (or never, when further converting the result to float).

This approach has two implications. First of all you have to treat the documentation on this site as a simplified version, describing the behaviour of the library as if implemented this way. The actual argument and return types of functions and operators may involve other internal types (feel free to generate the exact developer documentation from the Doxygen comments in the library's header file if you really need to). But nevertheless the behaviour is exactly like specified in the documentation. The other implication is, that in the presence of rounding errors or over-/underflows arithmetic expressions may produce different results when compared to converting to half-precision after each individual operation:

half a = (std::numeric_limits<half>::max() * 2.0_h) / 2.0_h; // a = MAX
half b = std::numeric_limits<half>::max() * 2.0_h; // b = INF
b /= 2.0_h; // b stays INF

But this should only be a problem in very few cases. One last word has to be said when talking about performance. Even with its efforts in reducing conversion overhead as much as possible, the software half-precision implementation can most probably not beat the direct use of single-precision computations. Usually using actual float values for all computations and temproraries and using halfs only for storage is the recommended way. On the one hand this somehow makes the provided mathematical functions obsolete (especially in light of the implicit conversion from half to float), but nevertheless the goal of this library was to provide a complete and conceptually clean half-precision implementation, to which the standard mathematical functions belong, even if usually not needed.

IEEE conformance

The half type uses the standard IEEE representation with 1 sign bit, 5 exponent bits and 10 mantissa bits (11 when counting the hidden bit). It supports all types of special values, like subnormal values, infinity and NaNs. But there are some limitations to the complete conformance to the IEEE 754 standard:

  • The implementation does not differentiate between signalling and quiet NaNs, this means operations on halfs are not specified to trap on signalling NaNs (though they may, see last point).
  • Though arithmetic operations are internally rounded to single-precision using the underlying single-precision implementation's current rounding mode, those values are then converted to half-precision using the default half-precision rounding mode (changed by defining HALF_ROUND_STYLE and HALF_ROUND_TIES_TO_EVEN accordingly). This mixture of rounding modes is also the reason why std::numeric_limits<half>::round_style may actually return std::round_indeterminate when half- and single-precision rounding modes don't match.
  • Because of internal truncation it may also be that certain single-precision NaNs will be wrongly converted to half-precision infinity, though this is very unlikely to happen, since most single-precision implementations don't tend to only set the lowest bits of a NaN mantissa.
  • The implementation does not provide any floating point exceptions, thus arithmetic operations or mathematical functions are not specified to invoke proper floating point exceptions. But due to many functions implemented in single-precision, those may still invoke floating point exceptions of the underlying single-precision implementation.

Some of those points could have been circumvented by controlling the floating point environment using <cfenv> or implementing a similar exception mechanism. But this would have required excessive runtime checks giving two high an impact on performance for something that is rarely ever needed. If you really need to rely on proper floating point exceptions, it is recommended to explicitly perform computations using the built-in floating point types to be on the safe side. In the same way, if you really need to rely on a particular rounding behaviour, it is recommended to either use single-precision computations and explicitly convert the result to half-precision using half_cast() and specifying the desired rounding mode, or synchronize the default half-precision rounding mode to the rounding mode of the single-precision implementation (most likely HALF_ROUND_STYLE=1, HALF_ROUND_TIES_TO_EVEN=1). But this is really considered an expert-scenario that should be used only when necessary, since actually working with half-precision usually comes with a certain tolerance/ignorance of exactness considerations and proper rounding comes with a certain performance cost.


Credits and Contact

This library is developed by Christian Rau and released under the MIT License. If you have any questions or problems with it, feel free to contact me at rauy AT users.sourceforge.net or use any of the other means for support.

Additional credit goes to Jeroen van der Zijp for his paper on Fast Half Float Conversions, whose algorithms have been used in the library for converting between half-precision and single-precision values.