是否允许编译器删除无限循环像Intel C ++编译器-O2？ [英] Are compilers allowed to remove infinite loops like Intel C++ Compiler with -O2 does?

问题描述

以下测试代码在VS中正确地使用调试或发布以及GCC。它对于具有调试的ICC也是正确的，但是在启用优化（ -O2 ）时不会正确。

The following testing code does correctly in VS either with debug or release, and also in GCC. It also does correctly for ICC with debug, but not when optimization enabled (-O2).

#include <cstdio>

class tClassA{
public:
  int m_first, m_last;

  tClassA() : m_first(0), m_last(0) {}
  ~tClassA() {}

  bool isEmpty() const {return (m_first == m_last);}
  void updateFirst() {m_first = m_first + 1;}
  void updateLast() {m_last = m_last + 1;}
  void doSomething() {printf("should not reach here\r\n");}
};

int main() {
  tClassA q;
  while(true) {
    while(q.isEmpty()) ;
    q.doSomething();
  }
  return 1;
}

isEmpty（））。然而，当在ICC（发布）下启用 -O2 时，它会无限地开始doSomething。

It is supposed to stop at while(q.isEmpty()). When -O2 enabled under ICC (release), however, it starts to "doSomething" infinitely.

是单线程程序， isEmpty（）应该被计算为 true 发现没有理由ICC应该这样行事？因为 while（q.isEmpty（））;

Since this is single-threaded program and isEmpty() should be evaluated as true, I can find no reason the ICC should behave in this way? Do I miss anything?

推荐答案

c $ c> loop不包含可能导致外部可见的副作用的语句，整个循环被优化为不存在。原因也一样：

Because the while (q.isEmpty()) ; loop contains no statements that can ever cause an externally-visible side-effect, the entire loop is being optimised out of existence. It's the same reason that:

for (int i = 0; i < 10; i++)
    ;

可以优化出来，只要 i 不是 volatile （存储到 volatile 对象是程序的外部可见效果的一部分） 。

could be optimised out of existence, as long as i was not volatile (stores to volatile objects are part of the "externally visible" effects of a program).

在C语言中，它实际上是一个突出的骨头，关于是否允许以这种方式优化无限循环（我不知道什么情况是用C ++）。据我所知，在这个问题上从未达成共识 - 聪明和知识渊博的人已经采取了双方。

In the C language, it is actually an outstanding bone of contention as to whether an infinite loop is allowed to be optimised away in this manner (I do not know what the situation is with C++). As far as I know, a consensus has never been reached on this issue - smart and knowledgeable people have taken both sides.

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