导致时间旅行的未定义行为 [英] Undefined behavior causing time travel

问题描述

来自msdn博客的这篇文章的一个例子使我ticker：

它说这个函数：

  void unwitting bool door_is_open）
 {
 if（door_is_open）{
 walk_on_in（）; 
} else {
 ring_bell（）; 
 
 //使用后备值等待门打开
 fallback = value_or_fallback（nullptr）; 
 wait_for_door_to_open（fallback）; 
} 
} 
  

可以优化到这一个：

  void unwitting（bool door_is_open）
 {
 walk_on_in 
} 
  

因为调用 value_or_fallback（nullptr）是未定义的行为（这在本文前面已经证明）。

现在我不明白的是：运行时进入未定义的行为，到达那条线。在第一段的所有可观察到的效果在运行时间进入UB之前已经解决的意义上，不应该发生在之前/发生之后的概念？

解决方案

在推理中有一个流程。

当编译器编写器说： ，有两种不同的解释：

• 大多数人听到：我们识别未定义的行为，


ul>



因此，在您的情况下：






• 解除引用 nullptr

• 因此执行 value_or_fallback（nullptr）是未定义行为


• 因此执行 else 分支是未定义的行为


• 因此 door_is_open false 是未定义的行为






程序员发现她将遵循使用条款）， door_is_open 必须 true ，编译器可以清除 else 分支。





（*）我很轻slightly，Raymond Chen实际上是以这种方式制定的...




One example of this article from a msdn blog made me ticker:

It says that this function:

void unwitting(bool door_is_open)
{
 if (door_is_open) {
  walk_on_in();
 } else {
  ring_bell();

  // wait for the door to open using the fallback value
  fallback = value_or_fallback(nullptr);
  wait_for_door_to_open(fallback);
 }
}

Can be optimized into this one:

void unwitting(bool door_is_open)
{
    walk_on_in();
}

Because calling value_or_fallback(nullptr) is undefined behavior (this is proven earlier in the article).

Now what I don’t understand is this: the run time enters undefined behavior only when it reaches that line. Shouldn’t the happen-before / happen-after concept applies here, in the sense that all observable effects of the first paragraph have be resolved before the run time enters UB?

解决方案

There is a flow in the reasoning.

When a compiler writer says: we use Undefined Behavior to optimize a program, there are two different interpretations:

• most people hear: we identify Undefined Behavior and decide we can do whatever we want (*)
• the compiler writer meant: we assume Undefined Behavior does not occur

Thus, in your case:

• dereferencing a nullptr is Undefined Behavior
• thus executing value_or_fallback(nullptr) is Undefined Behavior
• thus executing the else branch is Undefined Behavior
• thus door_is_open being false is Undefined Behavior

And since Undefined Behavior does not occur (the programmer swears she will follow the terms of use), door_is_open is necessarily true and the compiler can elide the else branch.

(*) I am slightly annoyed that Raymond Chen actually formulated it this way...

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