浮点型NaN有效载荷有什么用途？ [英] What uses do floating point NaN payloads have?

问题描述

我知道IEEE 754定义NaN具有以下按位表示：

• 符号位可以是 0 或 1


• 指数字段包含所有 1
• 尾数的某些位用于指定它是否是一个安静的NaN或信号NaN

• 尾数不能全部是 0 位，因为该位模式被保留用于表示无穷大


• 尾数的其余位形成有效载荷
  

当计算的输入是NaN时，有效载荷被传播（如同整个NaN一样）到浮点计算的结果，尽管我没有了解这种传播的细节或标准是否规定了如何完成。谁设置了原始有效载荷？如果我添加两个具有不同有效载荷的NaN，会发生什么？

但是最重要的是：我以前从来没有见过NaN的有效载荷。这个有效载荷字段有什么用途？

解决方案

当IEEE754和NaN被开发时，这被认为是一个好主意。实际上我已经看到它用来存储NaN创建的原因。

今天，我不会在可移植代码中使用它，原因有几个。你有多确定这个有效载荷能够存活下来，例如一个任务？如果你指定x = y，那你有多确定x和Na有相同的有效载荷？你有多确定它能算算吗？如果a或b是NaN，那么op b应该是一个NaN，或者两个NaN中的一个，如果它们都是NaN的话。当然是这样吗？我不会愿意赌它。

I know that IEEE 754 defines NaNs to have the following bitwise representation:

• The sign bit can be either 0 or 1
• The exponent field contains all 1 bits
• Some bits of the mantissa are used to specify whether it's a quiet NaN or signalling NaN
• The mantissa cannot be all 0 bits because that bit pattern is reserved for representing infinity
• The remaining bits of the mantissa form a payload

The payload is propagated (as is the NaN as a whole) to the result of a floating point calculation when the input of the calculation is NaN, though I have no knowledge of the details of this propagation or whether the standard specifies how this is done. Who sets the original payload? What happens if I add two NaNs with different payloads?

But most importantly: I've never seen NaN payloads used before. What uses does this payload field have?

解决方案

It was thought to be a good idea whenIEEE754 and NaN's were developed. I have actually seen it used to store the reason why a NaN was created.

Today, I wouldn't use it in portable code for several reasons. How sure are you that this payload will survive for example an assignment? If you assign x = y, how sure are you that x has the same NaN payload as y? And how sure are you that it will survive arithmetic? If a or b is an NaN, then a op b is supposed to be the one NaN, or one of the two NaNs if they are both NaN. Sure that this is the case? I wouldn't be willing to bet on it.

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