C ++编译器中存在哪些不常见的浮点大小? [英] What uncommon floating-point sizes exist in C++ compilers?

问题描述

C ++ 14草案标准对于float，double和long double的特定要求似乎比较安静，尽管这些大小似乎很常见:

The C++14 draft standard seems rather quiet about the specific requirements for float, double and long double, although these sizes seem to be common:

• float:IEEE 32位浮点表示形式(大约7位精度，指数范围为1e-38..1e + 38)

• float: IEEE 32-bit floating-point representation (roughly 7 digits of precision, exponent range of 1e-38..1e+38)

double:IEEE 64位浮点表示形式(大约16位精度，指数范围为1e-308..1e + 308)

double: IEEE 64-bit floating-point representation (roughly 16 digits of precision, exponent range of 1e-308..1e+308)

long double:80位浮点表示形式(大约19位精度，指数范围为1e-4951..1e + 4932)

long double: 80-bit floating-point representation (roughly 19 digits of precision, exponent range of 1e-4951..1e+4932)

除这些以外，当前有哪些C ++编译器和系统使用浮点大小?

What C++ compilers and systems currently use floating-point sizes other than these?

我对使用标准类型而不是库的更长，更短和非二进制表示感兴趣，因为我的主要兴趣是C ++程序的可移植性.

I'm interested in longer, shorter, and non-binary representations using the standard types, not libraries, as my primary interest is portability of C++ programs.

推荐答案

如果仅询问以位为单位的大小，那么奇数大小的类型仅存在于一些不使用的旧平台中8位(或2的幂)字节，如 Unisys ClearPath Dorado服务器，具有36位浮点数和72位双倍的.直到现在，那只野兽仍处于活跃的发展之中. 最新版本是在2018年.大型机和服务器的寿命很长，因此您仍然可以看到一些PDP-10和其他架构在现代中正在使用，并具有现代编译器支持

If you're only asking about size in bits then odd-sized types only exist in some older platforms that don't use 8-bit (or another power of 2) bytes like the Unisys ClearPath Dorado Servers with 36-bit float and 72-bit double. That beast is still even in active development until now. The last version was in 2018. Mainframes and servers live a very long life so you can still see some PDP-10 and other architectures in use in modern times, with modern compiler support

如果您在意 formats ，那么有很多标准兼容的32位，64位和128位浮点格式不是 VAX格式.实际上，IBM z是使用十进制浮点硬件的非常罕见的现代平台之一，尽管如果使用GCC和其他一些编译器，则可以使用其

If you care about the formats then there are lots of standard compliant 32, 64 and 128-bit floating-point formats that aren't IEEE-754 like the hex and decimal floating point types in IBM z, Cray formats and VAX formats. In fact IBM z is one of the very rare modern platforms with decimal float hardware, although if you use GCC and some other compilers you can use their built-in software support for decimal float. IBM also uses the special double-double format which is still the default for long double on PowerPC until now

一些用于微控制器的现代C/C ++编译器中还有其他一些非标准的24位浮点数

There are also some other non-standard 24-bit floats in a few modern C/C++ compilers for microcontrollers

以下是大多数可用的浮点格式的摘要.另请参见是否有任何现实中的CPU不使用IEEE 754?.有关更多信息，请继续下一节

Here's the summary of most of the available floating-point formats. See also Do any real-world CPUs not use IEEE 754?. For more information continue to the next section

类型映射到硬件类型.因此，如果有FPU，则浮点类型将是CPU上可用的任何值.在现代计算机中，IEEE-754是硬件中的主要格式，并且由于C ++标准float和double的要求，必须至少将其映射到IEEE-754

Types in C++ are generally mapped to hardware types for performance reasons. Therefore floating-point types will be whatever available on the CPU if it ever has an FPU. In modern computers IEEE-754 is the dominant format in hardware, and due to the requirements in C++ standard float and double must be mapped to at least IEEE-754 single and double precision respectively

除了x86和其他一些具有 IEEE-754四倍精度许多编译器，例如GCC或Clang.由于该程序是使用内置的软件库实现的，因此性能会大大降低.根据您是希望更快的执行速度还是更高的精度，仍然可以自由选择long double映射到的类型.例如，在x86上，GCC具有 -mlong-double-64/80/128和-m96/128bit-long-double选项设置long double的填充和格式.该选项在许多其他体系结构中也可用，例如 S/390和zSeries

Hardware support for types with higher precision is not common except on x86 and a few other rare platforms with 80-bit extended precision, therefore long double is usually mapped to the same type as double on those platforms. However recently long double is being slowly migrated to IEEE-754 quadruple precision in many compilers like GCC or Clang. Since that one is implemented with the built-in software library, performance is a lot worse. Depending on whether you favor faster execution or higher precision you're still free to choose whatever type long double maps to though. For example on x86 GCC has -mlong-double-64/80/128 and -m96/128bit-long-double options to set the padding and format of long double. The option is also available in many other architectures like the S/390 and zSeries

默认情况下，PowerPC OTOH使用完全不同的128位长双精度格式，该格式是通过平台上也使用了这种技巧，以获得接近两倍的精度

PowerPC OTOH by default uses a completely different 128-bit long double format implemented using double-double arithmetic and has the same range as IEEE-754 double precision. Its precision is slightly lower than quadruple precision but it's a lot faster because it can utilize the hardware double arithmetic. As above, you can choose between the 2 formats with the -mabi=ibmlongdouble/ieeelongdouble options. That trick is also used in some platforms where only 32-bit float is supported to get near-double precision

IBM z大型机通常使用 IBM十六进制浮点格式，如今它们仍在使用.但是它们也支持IEEE -754以外的-754二进制和十进制浮点类型

IBM z mainframes traditionally use IBM hex float formats and they still use it nowadays. But they do also support IEEE-754 binary and decimal floating-point types in addition to that

浮点数的格式可以是基数为16 S/390®十六进制格式，基数为2 IEEE-754二进制格式或基数为10 IEEE-754十进制格式.格式基于十六进制和二进制的三种操作数长度:短(32位)，长(64位)和扩展(128位).格式还基于三种十进制操作数长度:_Decimal32(32位)，_ Decimal64(64位)和_Decimal128(128位).

The format of floating-point numbers can be either base 16 S/390® hexadecimal format, base 2 IEEE-754 binary format, or base 10 IEEE-754 decimal format. The formats are based on three operand lengths for hexadecimal and binary: short (32 bits), long (64 bits), and extended (128 bits). The formats are also based on three operand lengths for decimal: _Decimal32 (32 bits), _Decimal64 (64 bits), and _Decimal128 (128 bits).

浮点数字

其他体系结构可能具有其他浮点格式，例如VAX或Cray.但是，由于这些大型机仍在使用，它们的较新的硬件版本也包括对IEEE-754的支持，就像IBM对其大型机所做的那样.

Other architectures may have other floating-point formats, like VAX or Cray. However since those mainframes are still being used, their newer hardware version also include support for IEEE-754 just like how IBM did with their mainframes

在没有FPU的现代平台上，浮点类型通常为IEEE-754单精度和双精度，以实现更好的互操作性和库支持.但是，在8位微控制器上，即使单精度也太昂贵了，因此某些编译器支持非标准模式，其中float是24位类型.例如， XC8编译器使用 24位浮点格式是32位格式的截断形式，并且 NXP的MRK使用不同的24位浮点格式

On modern platforms without FPU the floating-point types are usually IEEE-754 single and double precision for better interoperability and library support. However on 8-bit microcontrollers even single precision is too costly, therefore some compilers support a non-standard mode where float is a 24-bit type. For example the XC8 compiler uses a 24-bit floating-point format that is a truncated form of the 32-bit format, and NXP's MRK uses a different 24-bit float format

由于需要更窄浮点类型的图形和AI应用程序的兴起，许多平台还引入了诸如IEEE-754 binary16和Google的bfloat16之类的16位浮点格式，并且编译器也对其提供了有限的支持，例如GCC中的 __fp16

Due to the rise of graphics and AI applications that require a narrower floating-point type, 16-bit float formats like IEEE-754 binary16 and Google's bfloat16 are also introduced to in many platforms and compilers also have some limited support for them, like __fp16 in GCC

这篇关于C ++编译器中存在哪些不常见的浮点大小?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！