Mingw32 std::isnan -ffast-math

I am compiling the following code with the -ffast-math option:

我正在用-ffast-math选项编译下面的代码:

#include <limits>
#include <cmath>
#include <iostream>

int main() {
    std::cout << std::isnan(std::numeric_limits<double>::quiet_NaN() ) << std::endl;
}

I am getting 0 as output. How can my code tell whether a floating point number is NaN when it is compiled with -ffast-math?

我得到0作为输出。我的代码如何判断一个浮点数是NaN，它是用- ffastmath编译的?

Note: On linux, std::isnan works even with -ffast-math.

注:在linux上，std::isnan甚至与- ffastmath一起工作。

2 个解决方案

#1

Since -ffast-math instructs GCC not to handle NaNs, it is expected that isnan() has an undefined behaviour. Returning 0 is therefore valid.

由于-ffast-math指示GCC不处理NaNs，因此期望isnan()有一个未定义的行为。因此返回0是有效的。

You can use the following fast replacement for isnan():

您可以使用以下快速替换isnan():

#if defined __FAST_MATH__
#   undef isnan
#endif
#if !defined isnan
#   define isnan isnan
#   include <stdint.h>
static inline int isnan(float f)
{
    union { float f; uint32_t x; } u = { f };
    return (u.x << 1) > 0xff000000u;
}
#endif

#2

On linux, the gcc flag -ffast-math breaks isnan(), isinf() and isfinite() - there may be other related functions that are also broken that I have not tested.

在linux上，gcc标志-ffast-math断开isnan()、isinf()和is限性()——可能还有其他与我没有测试过的相关函数。

The trick of wrapping the function/macro in parentheses also did not work (ie. (isnan)(x))

用括号括起函数/宏的技巧也不起作用。(isnan)(x))

Removing -ffast-math works ;-)

删除-ffast-math作品;-)

#1