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Assuming I am really pressed for memory and want a smaller range (similar to short vs int). Shader languages already support half for a floating-point type with half the precision (not just convert back and forth for the value to be between -1 and 1, that is, return a float like this: shortComingIn / maxRangeOfShort). Is there an implementation that already exists for a 2-byte float?

I am also interested to know any (historical?) reasons as to why there is no 2-byte float.

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  • It's called half-precision floating point in IEEE lingo, and implementations exist, just not in the C standard primitives (which C++ uses by extension). The C standard only dictates single-precision, double-precision, and long double floating point (which could be 80-bit or 128-bit). Commented Apr 23, 2011 at 21:01
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    A question should be exactly that: A question. If you want references to implementations of half for C++, that's a question. If you're interested in historical reasons that float is a four-byte entity, that's a different question. Commented Apr 23, 2011 at 21:01
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    You can use half C++ library half.sourceforge.net Commented Feb 21, 2018 at 8:12
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    Half-precision floating point has now been in the IEEE spec for ten years. Does anyone know why it's still not a built-in type in C++? Commented Nov 12, 2018 at 17:16
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    No need to be insolent, bro. The world’s fastest processors have hardware support for half precision. It’s used all the time in machine learning, graphics, and video games. The film industry uses it extensively for rendering. But if it’s people who don’t understand the use cases who are defining the languages I guess that would answer my question. Commented Mar 12, 2020 at 3:44

10 Answers 10

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TL;DR: 16-bit floats do exist and there are various software as well as hardware implementations

There are currently 2 common standard 16-bit float formats: IEEE-754 binary16 and Google's bfloat16. Since they're standardized, obviously anyone who knows the spec can write an implementation. Some examples:

Or if you don't want to use them, you can also design a different 16-bit float format and implement it

2-byte floats are generally not used, because even float's precision is not enough for normal operations and double should always be used by default unless you're limited by bandwidth or cache size. Floating-point literals are also double when using without a suffix in C and C-like languages. See

However less-than-32-bit floats do exist. They're mainly used for storage purposes, like in graphics when 96 bits per pixel (32 bits per channel * 3 channels) are far too wasted, and will be converted to a normal 32-bit float for calculations (except on some special hardware). Various 10, 11, 14-bit float types exist in OpenGL. Many HDR formats use a 16-bit float for each channel, and Direct3D 9.0 as well as some GPUs like the Radeon R300 and R420 have a 24-bit float format. A 24-bit float is also supported by compilers in some 8-bit microcontrollers like PIC where 32-bit float support is too costly. 8-bit or narrower float types are less useful but due to their simplicity, they're often taught in computer science curriculum. Besides, a small float is also used in ARM's instruction encoding for small floating-point immediates.

The IEEE 754-2008 revision officially added a 16-bit float format, A.K.A binary16 or half-precision, with a 5-bit exponent and an 11-bit mantissa

Some compilers had support for IEEE-754 binary16, but mainly for conversion or vectorized operations and not for computation (because they're not precise enough). For example ARM's toolchain has __fp16 which can be chosen between 2 variants: IEEE and alternative depending on whether you want more range or NaN/inf representations. GCC and Clang also support __fp16 along with the standardized name _Float16. See How to enable __fp16 type on gcc for x86_64

Recently due to the rise of AI, another format called bfloat16 (brain floating-point format) which is a simple truncation of the top 16 bits of IEEE-754 binary32 became common

The motivation behind the reduced mantissa is derived from Google's experiments that showed that it is fine to reduce the mantissa so long it's still possible to represent tiny values closer to zero as part of the summation of small differences during training. Smaller mantissa brings a number of other advantages such as reducing the multiplier power and physical silicon area.

  • float32: 242=576 (100%)
  • float16: 112=121 (21%)
  • bfloat16: 82=64 (11%)

Many compilers like GCC and ICC now also gained the ability to support bfloat16

More information about bfloat16:

In cases where bfloat16 is not enough there's also the rise of a new 19-bit type called TensorFloat

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11 Comments

"GCC and Clang also support __fp16 along with the standardized name _Float16" - _Float16 doesn't seem to be supported in GCC. GCC half page doesn't mention this name, and the only answer in linked question claims they didn't find the way to enable it.
Many other compilers like armcc or Keil also support that keyword. But _Float16 isn't available on all targets: The _Float16 type is supported on AArch64 systems by default, and on ARM systems when the IEEE format for 16-bit floating-point types is selected with -mfp16-format=ieee
MSVC supports HALF via DirectX: learn.microsoft.com/en-us/windows/win32/dxmath/half-data-type
As I said doubles are not a disadvantage on a 64b cpu other than vector unit throughput. And you need to consider the algorithm being used first, if you don't you're just fumbling around blind with any type. Correct assessment of significant digits is covered in grade 10 chemistry. The float 7 digits are for a conservative convertion from decimal to float and back to decimal, which you only do once, for the input and final output.(The majority of numbers preserve 8 or 9) Internally a float has slightly more than precision than this. 7 what remains after cutting off typical rounding errors.
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Re: Implementations: Someone has apparently written half for C, which would (of course) work in C++: https://storage.googleapis.com/google-code-archive-downloads/v2/code.google.com/cellperformance-snippets/half.c

Re: Why is float four bytes: Probably because below that, their precision is so limited. In IEEE-754, a "half" only has 11 bits of significand precision, yielding about 3.311 decimal digits of precision (vs. 24 bits in a single yielding between 6 and 9 decimal digits of precision, or 53 bits in a double yielding between 15 and 17 decimal digits of precision).

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4 Comments

Right. 10 bits = 3.01 decimal digits, which is inadequate for most number-crunching tasks.
@dan04 It's 11, including the implicit one bit.
OK, 3.31 decimal digits. Not that it makes much of a difference.
@dan04 It's a 10 bits representable difference.
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If you're low on memory, did you consider dropping the float concept? Floats use up a lot of bits just for saving where the decimal point is. You can work around this if you know where you need the decimal point, let's say you want to save a Dollar value, you could just save it in Cents:

uint16_t cash = 50000;
std::cout << "Cash: $" << (cash / 100) << "." << ((cash % 100) < 10 ? "0" : "") << (cash % 100) << std::endl;

That is of course only an option if it's possible for you to predetermine the position of the decimal point. But if you can, always prefer it, because this also speeds up all calculations!

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that is not correct what if cash = 402 you will print 42
@Et7f3XIV You are right, it's amazing how careless I answered on this page 8 years ago :(
Or if you include <iomanip> header. You will able to code that way: std::cout << "Cash: $" << (cash / 100) << "." << std::setfill('0') << std::setw(2) << (cash % 100) << std::endl;
it's called fixed-point arithmetic when you know where the radix point is
Fixed point is essentially integer math with a superficial dot added. float16 has larger range than int16. There is tradeoff. An IEEE float16 reliably has about 3 significant decimal digits over the whole range, very small to huge, while an int16 is an exact index of count of 65536 units regardless of where you fix the point. The precision at the low end of int16 is one digit but it is known to be exactly accurate, and 5 digits at the high end. Where you need accuracy as a percent of the whole and a wide range use float, for an exact count like tracking inventory use int or fixed point.
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There is an IEEE 754 standard for 16-bit floats.

It's a new format, having been standardized in 2008 based on a GPU released in 2002.

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1 Comment

Yes. He did mention half in his question.
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To go a bit further than Kiralein on switching to integers, we could define a range and permit the integer values of a short to represent equal divisions over the range, with some symmetry if straddling zero:

short mappedval = (short)(val/range);

Differences between these integer versions and using half precision floats:

  1. Integers are equally spaced over the range, whereas floats are more densely packed near zero
  2. Using integers will use integer math in the CPU rather than floating-point. That is often faster because integer operations are simpler. Having said that, mapping the values onto an asymmetric range would require extra additions etc to retrieve the value at the end.
  3. The absolute precision loss is more predictable; you know the error in each value so the total loss can be calculated in advance, given the range. Conversely, the relative error is more predictable using floating point.
  4. There may be a small selection of operations which you can do using pairs of values, particularly bitwise operations, by packing two shorts into an int. This can halve the number of cycles needed (or more, if short operations involve a cast to int) and maintains 32-bit width. This is just a diluted version of bit-slicing where 32 bits are acted on in parallel, which is used in crypto.
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3

The C++23 standard supports optional extended precision floating point types which includes
std::float16_t (IEEE 16 bit floating point type) with 5 bits of exponent and 11 bits of precision and
std::bfloat16_t (Google's brain floating point type) with 8 bits of exponent and 8 bits of precision (truncated IEEE 32 bit floating point type) and two additional floating point types with 64 and 128 bits of storage.

However at the time of this answer only Clang and GCC seem to support these floating point types.

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If your CPU supports F16C, then you can get something up and running fairly quickly with something such as:

// needs to be compiled with -mf16c enabled
#include <immintrin.h>
#include <cstdint>

struct float16
{
private:
  uint16_t _value;
public:

  inline float16() : _value(0) {}
  inline float16(const float16&) = default;
  inline float16(float16&&) = default;
  inline float16(const float f) : _value(_cvtss_sh(f, _MM_FROUND_CUR_DIRECTION)) {}

  inline float16& operator = (const float16&) = default;
  inline float16& operator = (float16&&) = default;
  inline float16& operator = (const float f) { _value = _cvtss_sh(f, _MM_FROUND_CUR_DIRECTION); return *this; }

  inline operator float () const 
    { return _cvtsh_ss(_value); }

  inline friend std::istream& operator >> (std::istream& input, float16& h) 
  { 
    float f = 0;
    input >> f;
    h._value = _cvtss_sh(f, _MM_FROUND_CUR_DIRECTION);
    return input;
  }
};

Maths is still performed using 32-bit floats (the F16C extensions only provides conversions between 16/32-bit floats - no instructions exist to compute arithmetic with 16-bit floats).

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2 Comments

This can be done without immintrin.h. See this answer: stackoverflow.com/a/64493446/1413259
@wolfram77 Fairly sure what you linked is about bfloat16, whereas this answer here is about half floats.
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There are probably a variety of types in different implementations. A float equivalent of stdint.h seems like a good idea. Call (alias?) the types by their sizes. (float16_t?) A float being 4 bytes is only right now, but it probably won't get smaller. Terms like half and long mostly become meaningless with time. With 128 or 256-bit computers they could come to mean anything.

I'm working with images (1+1+1 byte/pixel) and I want to express each pixel's value relative to the average. So floating point or carefully fixed point, but not 4 times as big as the raw data please. A 16-bit float sounds about right.

This GCC 7.3 doesn't know "half", maybe in a C++ context.

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4 Comments

128 and 256b processing is a specialty domain that is unlikely to see much of a market in general computing, with a possible exception of a single long number unit within an otherwise 64bit CPU. Anyway "long double" and "long long int" are already reserved in C++ [presumably for 128bit] though most compilers currently set them as duplicate 64bit types or x87 80bit float on x86_64 machines. long double is not to be confused with "double double math" which is two 64b floats mashed together (Slightly faster processing than using software implemented arbitrary precision math.).
Mainframe CPUs have been between 32 and 64bit since the vacuum tube days. 8 and 16 were only used for low cost or low power consumption. Very few use cases need more than 7 significant digits of precision(32bit). 64b floats ~15 sig digits (x87 unit takes 64bit input, uses 80bit internally and returns 64bit for 19 sig digits ) 128-256b computations are very niche. 64bit address space is unlikely to be exceeded in a single machine for operational reasons and 128bit for elementary physics limitations. 8*(2^128) silicon atoms [number of bits in 128bit address space] weighs 130 tons
@MaxPower are you sure? The first 64-bit computer was released in 1961, far later than the vacuum tube era. And "long long int" are already reserved in C++ [presumably for 128bit] is absolutely wrong. long long is already there since C++11 and has at least 64 bits
@phuclv You need to work on comprehending what you reply to before posting. Yes, 128bits is at least 64bits, ask anyone the math really works. if(128>=64)std::cout<<"True\n"; else std::cout<<"False\n"; ENIAC was decimal in hardware and could calculate 10 or 20 decimal digit numbers. (This is a little better than 40bit and 80bit binary); EDVAC used 44bit words; SWAC used 37bit words with both single or double precision(74bit) ; EDSAC 34 bit using two 17bit words ; Manchester Mark 1 used 40bit numbers 20 bit instructions; MEG/Mercury floating-point unit used 40bit, 30mantissa 10exponent
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2 byte float is available in clang C compiler , The data type is represented as __fp16.

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Various compilers now support three different half precision formats:

  • __fp16 is mostly used as a storage format. It is promoted to float as soon as you do calculations on it. Calculations on __fp16 will give a float result. __fp16 has 5 bits exponent and 10 bits mantissa.
  • _Float16 is the same as __fp16, but used as an interchange and arithmetic format. Calculations on _Float16 will give a _Float16 result.
  • __bf16 is a storage format with less precision. It has 8 bits exponent and 7 bits mantissa.

All three types are supported by compilers for the ARM architecture and now also by compilers for x86 processors. The AVX512_FP16 instruction set extension will be supported by Intel's forthcoming Golden Cove processors and it is supported by the latest Clang, Gnu, and Intel compilers. Vectors of _Float16 are defined as __m128h, __m256h, and __m512h on compilers that support AVX512_FP16 .

References:

https://developer.arm.com/documentation/100067/0612/Other-Compiler-specific-Features/Half-precision-floating-point-data-types

https://clang.llvm.org/docs/LanguageExtensions.html#half-precision-floating-point

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