MSVC++ 限制关键字和局部变量 [英] MSVC++ restrict keyword and local variables

问题描述

我已经阅读了许多关于 restrict 关键字的帖子.但几乎我能找到的每个例子似乎都只涉及一个函数的输入参数，也许是一个单一的值.我需要澄清一下我的理解.

I've read a number of posts on the restrict keyword. But virtually every example I can find seem to refer to input parameters only to a function and, perhaps a single value. I need to clarify my understanding.

我发现了一个函数，它的输入参数和局部变量看起来都完全违反了关键字规则.

I've found a function that looks like it totally violates the rules of the keyword with both an input parameter and a local variable.

使用指向缓冲区的 void* 调用此函数，并将指针声明为 __restrict(这是 Microsoft Visual C++).然而在函数的后面，声明了一个 UCHAR* 类型的局部变量指针，并使其指向相同的受限输入参数缓冲区.

This function is called with a void* to a buffer and the pointer is declared as __restrict (this is Microsoft Visual C++). Yet later in the function, a local variable pointer of type UCHAR* is declared and made to point to that same restricted input parameter buffer.

这是我正在谈论的功能的一个严重削减版本:

Here is a seriously chopped down version of the function I'm talking about:

void Foo(int nVersion, int nX, int nY, int nWidth, void * __restrict pBuffer) const
{
    // ... blah blah blah
    UCHAR * __restrict pBufferPtr = ((UCHAR *) pBuffer) + 10;  // Isn't this aliasing?
    const void * __restrict pData =  (blah blah blah);     //... Get from some function call;
    memcpy(pBufferPtr, pData, nWidth);
}

上面的例子是否违反了restrict的规则?

Does the above example violate the rules of restrict?

推荐答案

restrict 关键字仅表示指针应指向内存的唯一部分.在上面的代码中，pBuffer指向的东西，我们称之为A，pBufferPtr指向A+10，PData指向的是完全不同的东西，B，所以没有违规.

The restrict keyword only means that the pointers should point to unique portions of memory. In the above code, pBuffer points to something, let's call it A, pBufferPtr points to A+10, PData points to something completely different, B, so there's no violations.

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