负NaN不是NaN？ [英] Negative NaN is not a NaN?

问题描述

在编写一些测试用例时，一些测试检查了NaN的结果。

While writing some test cases, and some of the tests check for the result of a NaN.

我尝试使用 std :: isnan 但是assert失败：

I tried using std::isnan but the assert failes:

Assertion `std::isnan(x)' failed.

打印 x 的值结果是它是负的NaN（ -nan ）在我的情况下是完全可以接受的。

After printing the value of x, it turned out it's negative NaN (-nan) which is totally acceptable in my case.

尝试使用事实上， NaN！= NaN 并使用 assert（x == x）

After trying to use the fact that NaN != NaN and using assert(x == x), the compiler does me a 'favor' and optimises the assert away.

使自己的 isNaN 函数也被优化。

Making my own isNaN function is being optimised away as well.

如何检查NaN 和 -NaN是否相等？

How can I check for both equality of NaN and -NaN?

推荐答案

这是令人尴尬的。

编译器（在这种情况下是GCC）优化掉比较的原因和 isnan 返回 false 是因为我团队中的某个人已打开 -ffast-math 。

The reason the compiler (GCC in this case) was optimising away the comparison and isnan returned false was because someone in my team had turned on -ffast-math.

从文档：

-ffast-math
    Sets -fno-math-errno, -funsafe-math-optimizations,
    -fno-trapping-math, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and fcx-limited-range.

    This option causes the preprocessor macro __FAST_MATH__ to be defined.

    This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions. 

请注意结束句子 - -ffast-math 是不安全的。

Notice the ending sentence - -ffast-math is unsafe.

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