挥发性功能 [英] Volatile function

问题描述

摘要：这是什么关键字挥发性当C和C ++中应用到函数声明呢？

Summary: What does the keyword volatile do when applied to a function declaration in C and in C++?

详细信息

我看到它是可以通过编译标记为函数挥发性。但是，我不知道是什么编译器优化（如果有的话），这个prevents。比如我创建了以下测试案例：

I see that it's possible to compile a function that is marked as volatile. However, I'm not sure what compiler optimization (if any) this prevents. For instance I created the following test case:

volatile int foo() {
  return 1;
}

int main() {
  int total = 0;
  int i = 0;
  for(i = 0; i < 100; i++) {
    total += foo();
  }

  return total;
}

当我编译铛-emit-LLVM -S -O3 test.c以（GCC也将工作，但LLVM IR更具可读性在我看来），我得到

When I compile with clang -emit-llvm -S -O3 test.c (gcc would also work but the llvm IR is more readable in my opinion) I get:

target triple = "x86_64-unknown-linux-gnu"

define i32 @foo() #0 {
  ret i32 1
}

define i32 @main() #0 {
  ret i32 100
}

所以，很显然，编译器能够优化掉的调用函数富（）让的main（）返回一个常量，即使富（）标记为挥发性。所以我的问题是，是否挥发性做任何事情时，在限制编译器优化方面应用到函数声明。

So obviously the compiler was able to optimize away the calls to function foo() so that main() returns a constant, even though foo() is marked as volatile. So my question is whether volatile does anything at all when applied to a function declaration in terms of limiting compiler optimizations.

（注意：我对这个问题的兴趣主要是好奇明白什么挥发性确实，而不是解决任何特定的问题。）

(Note my interest in this question is mostly curiosity to understand what volatile does rather than to solve any specific problem.)

（我也标志着这个问题，因为C和C ++并不是因为我认为他们是同一种语言，而是因为我想知道是否有什么挥发性确实在这种情况下，这两种语言）。

(Also I have marked this question as both C and C++ not because I think they are the same language, but because I am interested to know if there are differences in what volatile does in this case in these two languages).

推荐答案

在您的code时，挥发性关键字不适用的功能，但对返回类型，它是等价的：

In your code, the volatile keyword does not apply to the function, but to the return type, it is the equivalent of:

typedef volatile int Type;
Type foo();

现在，在C ++中，你可以做一个的成员的函数挥发性，以同样的方式，在常量预选赛，并且行为是一样的：

Now, in C++ you can make a member function volatile, in the same way that the const qualifier, and the behavior is the same:

struct test {
   void vfunction() volatile;
};

基本上你不能调用类型的挥发性（分别为常量）实例非易失性（alterantively非const）功能：

Basically you cannot call a non-volatile (alterantively non-const) function on a volatile (const respectively) instance of the type:

struct test {
   void vfunction() volatile;
   void function();
};
volatile test t;
t.vfunction();      // ok
t.function();       // error

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