用"const constant"声明变量的目的是什么? [英] what the purpose of declaring a variable with "const constant"?

问题描述

在Metal shader中，声明像const constant Vertex *vertexArray [[buffer(0)]]这样的变量的目的是什么(我指的是const constant)?为什么仅靠常数还不够?另外，constant和const有什么区别?

In Metal shader, What is the purpose of declaring a variable like const constant Vertex *vertexArray [[buffer(0)]] (with const constant I mean)? why constant alone is not enough? also, what is the difference between constant and const ?

以同样的方式const device和constant有什么区别?

Also in the same way what is the difference between const device and constant ?

推荐答案

const是类型限定符. constant和device是地址空间.

const阻止您修改其适用的内容:

const prevents you from modifying the thing to which it applies:

int a = 15;
a = 16; // Fine; reassigning to a non-const variable

const int b = 15;
b = a; // Error: attempt to write to a read-only constant

int d = 18;
int const* c = &a;
*c = 17; // Error: attempt to write value through a pointer to const int
c = &d;  // Fine; reassigning (a different address) to a (non-const) pointer

int *const e = &d;
*e = a; // Fine; assigning to pointee through pointer to non-const int
e = c;  // Error: attempt to reassign const pointer

希望这些示例充分说明了变量和常量的语义，以及在使用指针的情况下规则如何工作.

Hopefully these examples adequately illustrate the semantics of variables and constants, and how the rules work in the case of pointers.

在Metal中，指针始终位于特定的地址空间中.如果您在Metal着色器函数中获取具有自动存储的变量的地址(即本地"变量)，则该指针位于 thread 地址空间中.另一方面，缓冲区参数始终位于恒定或设备地址空间中.

In Metal, pointers always reside in a particular address space. If you take the address of a variable with automatic storage in a Metal shader function (i.e. a "local" variable), that pointer is in the thread address space. Buffer parameters, on the other hand, are always in the constant or device address space.

device缓冲区用于保存其元素将被大致访问一次的内存，就像在顶点函数中按顺序获取顶点数据时所执行的操作一样.另一方面，constant缓冲区保存的数据可能会被函数的许多调用访问，就像统一数据一样.

device buffers are used to hold memory whose elements will be accessed roughly once, as you might do when fetching vertex data sequentially in a vertex function. On the other hand, constant buffers hold data that might be accessed by many invocations of a function, as with uniform data.

您不能在constant地址空间中写入缓冲区.这是此答案中最重要的一句话: constant地址空间中的所有指针都是隐式const限定的.

You cannot write to a buffer in the constant address space. Here's the most important sentence in this answer: All pointers in the constant address space are implicitly const-qualified.

您可以在恒定地址空间中形成新的指针，并且根据上述规则，您可以重新分配它们.但是尝试写给他们的对象将产生编译器错误.

You can form new pointers in the constant address space, and by the rules explained above, you can reassign them. But attempting to write to their pointee will produce a compiler error.

假设您使用以下参数编写一个片段函数:

Suppose you write a fragment function with the following parameter:

constant Light *lights [[buffer(0)]]

然后在函数主体中可以这样说:

Then in the function body you could say this:

constant Light *light = &lights[0];

这:

light = &lights[1];

但不是这样:

light->color = float4(1, 1, 1, 1); // Error: attempt to write to variable with const-qualified type "const constant Light *"

同样，请注意，在最后一个示例中，即使我们没有说常量指针应该是指向const的指针，也是如此.因此，用const(在星号之前)进一步限定constant指针是多余的.

Again, note that in this last example, even though we didn't say that the constant pointer should be a pointer-to-const, it is. For this reason, further qualifying a constant pointer with const (before the asterisk) is redundant.

现在让我们谈谈device指针.

Now let's talk a bit about device pointers.

与始终是只读的constant缓冲区相反，在许多情况下都可以写入device缓冲区.但是，您通常会将设备缓冲区视为只读(例如，在大多数顶点函数中).为了向编译器表明此意图，可以将const添加到设备缓冲区指针参数.这将防止您无意中写入只打算读取的缓冲区.如果在不适当的上下文中使用device指向非常量类型的指针，则Metal着色器编译器的最新版本会发出警告，这就是为什么养成为此类参数编写const device的习惯通常是一个好主意的原因.

In contrast to constant buffers, which are always read-only, it is possible in many contexts to write to device buffers. However, you often treat device buffers as read-only (e.g., in most vertex functions). To indicate this intent to the compiler, you can add const to a device buffer pointer parameter. This will prevent you from inadvertently writing to a buffer you are only intending to read. Recent versions of the Metal shader compiler emit a warning if you take a device pointer to non-const type in an inappropriate context, which is why it's generally a good idea to get in the habit of writing const device for such parameters.

但是写const constant是多余的，而且从不需要.

But writing const constant is redundant and never necessary.

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