指定浮点恒定值按位在C / C ++ [英] specify floating point constant value bitwise in c/c++

问题描述

这就是我要做的：

  //让Bin2Float是一个神奇的宏程序包中指定的位模式为浮动作为一个常量常量浮MyInf = Bin2Float（01111111,10000000,00000000,00000000）;
 

我们都知道如何打包的位模式为整数（二进制常数黑客）和输入到这个神奇的原型宏是一样的将是相应的32位整数二进制常量宏。打包进位整型常量是没有问题的。但是，随着指针和双关工会上场后，我意​​识到型夯实整成浮动，但是，导致许多问题（有些在MSVC这边，一些海合会侧）。因此，这里是需求列表：

1. 在海湾合作委员会（C模式）必须编译，G ++，MSVC（即使我不得不使用条件编译做两个不同的版本）


2. 必须编译为C和C ++


3. 在生成的程序集code，必须编译成硬codeD不变，
   不能动态计算


4. 不能使用的memcpy


5. 不得使用静态或全局变量


6. 不能使用基于指针型双关，以避免问题
   严格别名

解决方案

首先，很少有必要以这种方式指定浮点常量。对于无穷大，使用 INFINITY 。对于NaN时，请使用 NAN 或 nanf（串）。这些在中所定义;文件math.h＆GT; 。编译器可能会编译 INFINITY 和 NAN 某种汇编语言常量（可能是在读只有数据部分，可以以指令立即字段，等等）而形成。然而，这不能保证除了由编译器实现者，由于C标准并不能保证它。 nanf 很可能导致在函数调用，虽​​然编译器就可以优化它一个常数，如果字符串是一个常数。对于有限的数字，用十六进制浮点常量（例如，0x3.4p5）。唯一的IEEE 754浮点对象，你不能完全指定这样下来的最后一点，是NaN的。在男和 nanf 功能未完全由C标准的规定，所以你没有尾数的完全控制位，除非实现提供它。

我不熟悉你暗示二进制常数黑客。假设你有一个宏 Bin2Unsigned ，提供了一个 unsigned int类型，那么你可以使用这样的：

 常量浮动MyInf =（工会{无符号U;浮动F;}）{Bin2Unsigned（...）} .F;
 

也就是说，无论你相信与否，标准的C语法和语义到那里的位reinter PTED为float $ P $点。显然，位的帧间pretation取决于实现。然而，通过工会复合文字和reinter preting是由C标准的规定。

我测试用gcc 4.2.1版（苹果公司建立5666），针对x86_64的，用-O3和默认选项，否则，产生的装配code使用的常量，。长2139095040 。

This is what I am trying to do:

//Let Bin2Float be a magic macro that packages specified bit pattern into float as a constant

const float MyInf = Bin2Float(01111111,10000000,00000000,00000000);

We all know how to package the bit patterns into integers ("binary constant" hacks) and the input to this magic prototype macro is the same as would be for corresponding 32-bit integer binary constant macro. Packaging the bits into integer constant is not a problem. But, after playing with pointer and union punning, I realized that type-punning the integer into float, however, leads to many issues (some on MSVC side, some on gcc side). So here is the list of requirements:

1. Must compile under gcc (C mode), g++, MSVC (even if I have to use conditional compiling to do two separate versions)
2. Must compile for both C and C++
3. In resulting assembly code, must compile into hardcoded constant, not be dynamically computed
4. Must not use memcpy
5. Must not use static or global variables
6. Must not use the pointer-based type punning to avoid issues with strict aliasing

解决方案

First, there is rarely a need to specify floating-point constants in this way. For infinity, use INFINITY. For a NaN, use either NAN or nanf(string). These are defined in <math.h>. The compiler is likely to compile INFINITY and NAN to some sort of assembly-language constant (could be in the read-only data section, could be formed in immediate fields of instructions, et cetera). However, this cannot be guaranteed except by the compiler implementors, since the C standard does not guarantee it. nanf is likely to result in a function call, although the compiler is free to optimize it to a constant, if the string is a constant. For finite numbers, use hexadecimal floating-point constants (e.g., "0x3.4p5"). The only IEEE 754 floating-point object you cannot completely specify this way, down the last bit, is NaNs. The nan and nanf functions are not fully specified by the C standard, so you do not have full control of the significand bits unless the implementation provides it.

I am unfamiliar with the binary constant hacks you allude to. Supposing you have a macro Bin2Unsigned that provides an unsigned int, then you can use this:

const float MyInf = (union { unsigned u; float f; }) { Bin2Unsigned(…) } .f;

That is, believe it or not, standard C syntax and semantics up to the point where the bits are reinterpreted as a float. Obviously, the interpretation of the bits depends on the implementation. However, the compound literal and reinterpreting through a union is specified by the C standard.

I tested with gcc version 4.2.1 (Apple Inc. build 5666), targeting x86_64, with -O3 and default options otherwise, and the resulting assembly code used a constant, .long 2139095040.

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