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:
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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位浮点格式不是十六进制和十进制浮点类型 ，格式和 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，则可以使用其对十进制浮点数的内置软件支持. IBM还使用特殊的double-double格式，到目前为止，仍是PowerPC上long double的默认格式
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 单和双精度分别
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 80位扩展精度，因此在这些平台上， long double通常映射为与double 相同的类型.但是，最近long double正在缓慢地迁移到 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，该格式是通过双精度双精度算法，其范围与IEEE-754双精度精度相同.它的精度略低于四倍精度，但速度更快，因为它可以利用硬件双重运算.如上所述，您可以在两种格式之间选择 -mabi=ibmlongdouble/ieeelongdouble选项.在某些仅支持32位浮点数的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
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
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