负零的平方根 [英] Square root of negative zero

问题描述

在g ++版本4.8.0（32位mingw）下，-0.0（二进制表示0x8000000000000000）的平方根是NAN。但是我可以发誓早期的版本（我不知道是哪一个，但是最后一次运行我的完整测试套件）只返回了0.0，这对我来说似乎更加明智。

这是正确的吗？有没有在C ++标准的变化，或者这是一个g + +的变化？

解决方案

这是一个非标准的行为MinGW环境（我也间接地通过一个客户来观察我的工作软件，这个客户意外地失败了）。 code> sqrt（-0。），即 -0。，在编译时正确计算值的优化级别

我最终定义了自己的fixed sqrt（） as：

$ p $ double mysqrt（double x）{
if x == 0.）return x;否则返回sqrt（x）;




$ b $（b）（*）IEEE 754将 sqrt（-0。）定义为 -0。。 C或C ++编译器不必为浮点运算实现IEEE 754，但是GCC试图（在这个特定的情况下失败）。


Under g++ version 4.8.0 (32-bit mingw), the square root of -0.0 (binary representation 0x8000000000000000) is NAN. But I could have sworn that earlier versions (I'm not sure which, but the last time I ran my full test suite) returned simply 0.0, which seems more sensible to me.

Is this right? Has something in the C++ standard changed, or is this a g++ change?

解决方案

This is a non-standard* behavior of the MinGW environment (that I have also observed, indirectly, through a customer for whom the software I work on was failing unexpectedly).

You may have seen the correct result for sqrt(-0.), i.e. -0., with a higher optimization level where the value was computed correctly at compile-time, or with a previous version of the run-time in which the bug was not present.

I ended up defining my own "fixed" sqrt() as:

double mysqrt(double x) {
  if (x == 0.) return x; else return sqrt(x);
}

---

(*) IEEE 754 defines sqrt(-0.) as -0.. A C or C++ compiler does not have to implement IEEE 754 for floating-point arithmetic, but GCC tries to (and in this particular instance, fails).

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