对局部变量允许C＃编译器优化和从内存中重新获取价值 [英] Allowed C# Compiler optimization on local variables and refetching value from memory

问题描述

修改：我问发生了什么，当两个线程的同时访问同一个数据，而不适当的同步（此编辑之前，这一点是不是前pressed明确）。

EDIT: I am asking what happens when two threads concurrently access the same data without proper synchronization (before this edit, that point was not expressed clearly).

我有一个关于是由C＃编译器和JIT编译器执行的优化问题。

I have a question about the optimizations that are performed by the C# compiler and by the JIT compiler.

考虑以下简化的例子：

class Example {
    private Action _action;

    private void InvokeAction() {
        var local = this._action;
        if (local != null) {
            local();
        }
    }
}

请在本例中忽略读 _action 可能生成缓存和过时的价值，因为没有挥发性的说明，也没有任何其他的同步控制器。这是不是问题的关键：）

Please ignore in the example that reading _action might yield a cached and outdated value as there is no volatile specifier nor any other sychronization. That is not the point :)

允许优化分配局部变量出来，而不是阅读的编译器（或实际的抖动在运行时） _action 一倍的内存：

Is the compiler (or actually the jitter at runtime) allowed to optimize the assignment to the local variable out and instead reading _action from memory twice:

class Example {
    private Action _action;

    private void InvokeAction() {
        if (this._action != null) {
            this._action(); // might be set to null by an other thread.
        }
    }
}

这可能会抛出一个的NullReferenceException 当现场 _action 设置为空由并发任务。

which might throw a NullReferenceException when the field _action is set to null by a concurrent assignment.

当然在本实施例中这样的优化，将没有任何意义，因为它会更快存储该值在寄存器，因此使用局部变量。但在更复杂的情况，有没有保证，这种按预期工作无需从内存中重新阅读的价值？

Of course in this example such an "optimization" would not make any sense because it would be faster to store the value in a register and thus using the local variable. But in more complex cases, is there a guarantee that this works as expected without re-reading the value from memory?

推荐答案

这是根据在ECMA规范中定义的内存模型法律的优化。如果_action是易失性内存模式将保证值只读一次，所以这种优化可能不会发生。

It is legal optimization according to the memory model defined in the ECMA specification. If the _action were volatile, memory model would guarantee that the value is read only once and so this optimization could not happen.

不过，我认为，目前微软的CLR实现不优化局部变量了。

However, I think that current Microsoft's CLR implementations do not optimize local variables away.

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