如何在x86和x86_64上使用Gcc 4.6.0 libquadmath和__float128 [英] How to use Gcc 4.6.0 libquadmath and __float128 on x86 and x86_64

问题描述

我有中等大小的C99程序，它使用 long double type（80bit）来进行浮点运算。我想用新的GCC 4.6扩展 __ float128 来提高精度。就像我得到的那样，这是一个软件模拟的128位精度数学。

我应该如何将我的程序从经典的80位双精度浮点数转换为128浮点数与完全精确的软件仿真？
我需要改变什么？编译器的标志，源？

我的程序读取完整的精度值与 strtod ，做了很多不同的操作（例如+ - * / sin，cos，exp和其他来自< math.h> ）和 printf

PPS：有是C语言gcc模式的文档： http://gcc.gnu.org/onlinedocs/gcc /Floating-Types.html
$ b

GNU C编译器支持128位（TFmode）浮动类型。其他类型包括算术运算符：加，减，乘，除;一元算术运算符;关系运算符;相等运算符...在i386，x86_64上支持__float128类型$ b $我应该如何将我的程序从80位的经典长双倍数据转换为四倍数据浮点数128位软件仿真的全精度？我需要改变什么？编译器标志，源代码？

您需要最新的软件，支持 __ float128 类型（4.6和更新）和libquadmath（仅在 x86和x86_64目标上受支持; IA64和HPPA与更新的GCC）。你应该添加链接器标记 -lquadmath （找不到-lquadmath'会显示你没有安装libquadmath） / p>

• 添加 #include< quadmath.h> 标题以具有宏和函数定义。


• 您应该将所有 long double 变量定义修改为 __ float128 。
  
• 复数变量可以变为 __ complex128 类型（ quadmath.h ）或直接使用 typedef _Complex float __attribute __（（mode（TC）））_Complex128;


• 所有简单的算术运算都由GCC自动处理（转换为像 __ * tf3（））这样的辅助函数的调用。 如果使用 LDBL _ * 等任何宏，请将它们替换为 FLT128 _ * （完整列表 http://gcc.gnu.org/onlinedocs/ libquadmath / Typedef-and-constants.html＃Typedef-and-constants ）
  
• 如果你需要一些特定的常量像pi（ M_PI ）或e（ M_E ），使用预定义的常量（ q suffix（<$ c （code> M_qq ），如 M_PIq 和 M_Eq （完整列表 http://gcc.gnu.org/onlinedocs/libquadmath/Typedef-and- constants.html＃Typedef-and-constants ）


• 用户定义的常量可以用 Q 所有的数学函数调用应该被替换为 * q 版本 1.3000011111111Q
，像 sqrtq（）， sinq（）（完整列表 http： //gcc.gnu.org/onlinedocs/libquadmath/Math-Library-Routines.html#Math-Library-Routing ）

• 从字符串中读取四元组与 __ float128 strtoflt128（const char * s，char ** sp） - http://gcc.gnu.org/onlinedocs/libquadmath/strtoflt128.html#strtoflt128 （警告，在较旧的libquadmags中，strtoflt128可能会有一些错误，请仔细检查）


• 打印 __ float128 是在 quadmath_snprintf 函数的帮助下完成的。在最近使用glibc的linux发行版中，函数将被libquadmath自动注册以处理 Q （也可以是 q ）长度 a，a，e，E，f，f，g， sprintf s，就像 L 长时间加倍一样。示例： printf（％Qe，1.2Q）， http://gcc.gnu.org/onlinedocs/libquadmath/quadmath_005fsnprintf.html#quadmath_005fsnprintf



您应该也知道，既然4.6 Gfortran将使用DOUBLE PRECISION的 __ float128 类型，如果 -fdefault-real-8 被给出了，没有选项 -fdefault-double-8 。这可能是个问题，因为在软件计算中，128位双精度比许多平台上的标准精度双精度要慢得多。 （感谢glennglockwood http://glennklockwood.blogspot.com/2014/ 02 / linux-perf-libquadmath-and-gfortrans.html ）

I have medium size C99 program which uses long double type (80bit) for floating-point computation. I want to improve precision with new GCC 4.6 extension __float128. As I get, it is a software-emulated 128-bit precision math.

How should I convert my program from classic long double of 80-bit to quad floats of 128 bit with software emulation of full precision? What need I change? Compiler flags, sources?

My program have reading of full precision values with strtod, doing a lot of different operations on them (like +-*/ sin, cos, exp and other from <math.h>) and printf-ing of them.

PS: despite that float128 is declared only for Fortran (REAL*16), the libquadmath is written in C and it uses float128. I'm unsure will GCC convert operations on float128 to runtime library or not and I'm unsure how to migrate from long double to __float128 in my sources.

PPS: There is a documentation on "C" language gcc mode: http://gcc.gnu.org/onlinedocs/gcc/Floating-Types.html

"GNU C compiler supports ... 128 bit (TFmode) floating types. Support for additional types includes the arithmetic operators: add, subtract, multiply, divide; unary arithmetic operators; relational operators; equality operators ... __float128 types are supported on i386, x86_64"

解决方案

How should I convert my program from classic long double of 80-bit to quad floats of 128 bit with software emulation of full precision? What need I change? Compiler flags, sources?

You need recent software, GCC version with support of __float128 type (4.6 and newer) and libquadmath (supported only on x86 and x86_64 targets; in IA64 and HPPA with newer GCC). You should add linker flag -lquadmath (the cannot find -lquadmath' will show that you have no libquadmath installed)

• Add #include <quadmath.h> header to have macro and function definitions.
• You should modify all long double variable definitions to __float128.
• Complex variables may be changed to __complex128 type (quadmath.h) or directly with typedef _Complex float __attribute__((mode(TC))) _Complex128;
• All simple arithmetic operations are automatically handled by GCC (converted to calls of helper functions like __*tf3()).
• If you use any macro like LDBL_*, replace them with FLT128_* (full list http://gcc.gnu.org/onlinedocs/libquadmath/Typedef-and-constants.html#Typedef-and-constants)
• If you need some specific constants like pi (M_PI) or e (M_E) with quadruple precision, use predefined constants with q suffix (M_*q), like M_PIq and M_Eq (full list http://gcc.gnu.org/onlinedocs/libquadmath/Typedef-and-constants.html#Typedef-and-constants)
• User-defined constants may be written with Q suffix, like 1.3000011111111Q
• All math function calls should be replaced with *q versions, like sqrtq(), sinq() (full list http://gcc.gnu.org/onlinedocs/libquadmath/Math-Library-Routines.html#Math-Library-Routines)
• Reading quad-float from string should be done with __float128 strtoflt128 (const char *s, char **sp) - http://gcc.gnu.org/onlinedocs/libquadmath/strtoflt128.html#strtoflt128 (Warning, in older libquadmaths there may be some bugs in strtoflt128, do a double check)
• Printing the __float128 is done with help of quadmath_snprintf function. On linux distributions with recent glibc the function will be automagically registered by libquadmath to handle Q (may be also q) length modifier of a, A, e, E, f, F, g, G conversion specifiers in all printfs/sprintfs, like it did L for long doubles. Example: printf ("%Qe", 1.2Q), http://gcc.gnu.org/onlinedocs/libquadmath/quadmath_005fsnprintf.html#quadmath_005fsnprintf

You should also know, that since 4.6 Gfortran will use __float128 type for DOUBLE PRECISION, if the option -fdefault-real-8 was given and there were no option -fdefault-double-8. This may be problem, since 128 long double is much slower than standard long double on many platforms due to software computation. (Thanks to post by glennglockwood http://glennklockwood.blogspot.com/2014/02/linux-perf-libquadmath-and-gfortrans.html)

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