缓冲区中填充了不同类型的数据，并严格混叠 [英] Buffer filled with different types of data, and strict aliasing

问题描述

根据标准，它始终是未定义的行为例如，使用C ++将 float * 指向与 int * 相同的内存位置，然后读取

According to the standard, it is always undefined behavior in C++ to make, for example, a float* point to the same memory location as a int*, and then read/write from them.

在我拥有的应用程序中，可以有一个填充有32位整数元素的缓冲区，该缓冲区被32位浮点元素覆盖。 （它实际上包含一张图像的表示，该图像通过GPU内核在多个阶段中进行了转换，但是还应该有一个主机执行该过程，以进行验证）。

In the application I have, there can be a buffer filled with 32-bit integer elements, that are overwritten by 32-bit floating point elements. (It actually contains a representation of an image, that gets transformed in multiple stages by GPU kernels, but there should also be a host implementation that does the same processing, for verification.)

程序基本上是这样做的（不是实际的源代码）：

The program basically does this (not actual source code):

void* buffer = allocate_buffer(); // properly aligned buffer

static_assert(sizeof(std::int32_t) == sizeof(float), "must have same size");
const std::int32_t* in = reinterpret_cast<const std::int32_t*>(buffer); 
float* out = reinterpret_cast<float*>(buffer); 
for(int i = 0; i < num_items; ++i)
   out[i] = transform(in[i]);

有没有办法使 reinterpret_cast 指针情况在C ++标准中定义良好，而不需要对整个缓冲区进行额外的内存复制，也不进行额外的按元素复制（例如，使用 std :: bit_cast ）？

Is there a way to make the reinterpret_cast pointer cases well-defined, within the C++ standard, without doing additional memory copies of the whole buffer, or additional per-element copies (for example with std::bit_cast)?

推荐答案

即使我一直希望有一种不错的方法，但目前还没有。您必须使用所选编译器的 no-strict-aliasing 标志。

Even though I wished all the time there would be a nice way, currently there is non. You will have to use no-strict-aliasing flag of the compiler of your choice.

对于 std :: bit_cast ，您将不得不等到 C ++ 20 。据我所知，没有使用 memcpy 就是没有标准的遵循方式。

For std::bit_cast you will have to wait until C++20. There is no standard conform way without using memcpy as far as I know.

也可以看看< a href = http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0476r1.html rel = nofollow noreferrer> bit_cast提案和此网站。

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