何时才能完全优化掉易失性变量? [英] When can a volatile variable be optimized away completely?

问题描述

考虑以下代码示例:

int main(void)
{
  volatile int a;
  static volatile int b;

  volatile int c;
  c = 20;
  static volatile int d;
  d = 30;

  volatile int e = 40;
  static volatile int f = 50;

  return 0;
}

在没有volatile的情况下，编译器可以优化所有变量，因为它们从未被读取.

Without volatile a compiler could optimize away all the variables since they are never read from.

我认为a和b可以被完全利用，因此可以对其进行优化，请参见未使用的volatile变量.

I think a and b can be optimized away since they are completely unused, see unused volatile variable.

我认为c和d不能删除，因为它们已被写入，并且实际上必须发生对volatile变量的写入. e应等效于c.

I think c and d can not be removed since they are written to, and writes to volatile variables must actually happen. e should be equivalent to c.

GCC不会优化f，但也不会发出任何写入指令.在数据部分中设置50. LLVM(clang)完全删除了f.

GCC does not optimize away f, but it also does not emit any instruction to write to it. 50 is set in a data section. LLVM (clang) removes f completely.

这些说法正确吗?

1. 如果从不访问volatile变量，则可以对其进行优化.
2. 静态或全局变量的初始化不算作访问.

推荐答案

写入易失性变量(甚至是自动变量)也视为可观察的行为.

Writes to volatile variables (even automatic ones) count as observable behaviour.

C11(N1570)5.1.2.3/6:

C11 (N1570) 5.1.2.3/6:

对符合标准的实现的最低要求是:

The least requirements on a conforming implementation are:

-严格根据摘要规则评估对易失对象的访问 机器.

— Accesses to volatile objects are evaluated strictly according to the rules of the abstract machine.

-在程序终止时，写入文件的所有数据应与以下结果相同: 按照抽象语义执行程序.

— At program termination, all data written into files shall be identical to the result that execution of the program according to the abstract semantics would have produced.

-交互式设备的输入和输出动态应按照 7.21.3.这些要求的目的是无缓冲或行缓冲输出 尽快出现，以确保提示消息实际上早于 一个等待输入的程序.

— The input and output dynamics of interactive devices shall take place as specified in 7.21.3. The intent of these requirements is that unbuffered or line-buffered output appear as soon as possible, to ensure that prompting messages actually appear prior to a program waiting for input.

这是程序的可观察到的行为.

问题是:初始化(e，f)是否算作访问"?正如桑德·戴克(Sander de Dycker)指出的那样，6.7.3说:

The question is: does initialization (e, f) count as an "access"? As pointed out by Sander de Dycker, 6.7.3 says:

对具有volatile限定类型的对象的访问构成由实现定义.

What constitutes an access to an object that has volatile-qualified type is implementation-defined.

这意味着是否可以优化e和f取决于编译器-但这必须记录在案！

which means it's up to the compiler whether or not e and f can be optimized away - but this must be documented!

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