有效生成字节缓冲区而不会破坏严格的别名 [英] Efficiently generating byte buffer without breaking strict aliasing

问题描述

这是一个简单的模式，必须采用一种不错的方式进行分类。

This is such a simple pattern, there has to be a "nice" way of sorting it out.

我有一个需要生成动态尺寸的函数包含算术数据的字节数组。

I have a function that needs to generate a dynamically sized byte array containing arithmetic data.

// Given that I have a function that kinda looks like this:
void dispatch(std::vector<char> data); //Will take possesion of data.

// The behavior I want, but this breaks strict aliasing
void bad_foo(int c) {
  std::vector<char> real_data(c * sizeof(float));
  float* raw_data = reinterpret_cast<float*>(real_data.data());

  //Fill raw_data with usefull stuff...

  dispatch(std::move(real_data));
}

void correct_but_slow_foo(int c) {
  std::vector<float> raw_data(c);

  //Fill raw_data with usefull stuff...

  std::vector<char> real_data(c * sizeof(float));
  std::memcpy(real_data.data(), raw_data.data(), c * sizeof(float));

  dispatch(std::move(real_data));
}

不幸的是，似乎连clang的堆清除都无法解决需要的东西可以在这里完成：在Godbolt上查看

Unfortunately, it seems even clang's heap elision is not managing to sort out what needs to be done here: see on godbolt

在最坏的情况下，我可以创建 dispatch（）模板，但这会变得非常混乱，我很想知道是否有解决办法

At the very worst, I can make dispatch() a template, but that would become very messy, and I'm curious to see if there is a way out of this mess I'm overlooking anyways.

谢谢！

编辑：一个想法刚刚想到了这一点（立即发布了问题之后……）：我可以将 real_data 视为分配池，并在其上放置新的算术数据：

A thought just crossed my mind (immediately after posting the question of course...) : I could treat real_data as an allocation pool and in-place new the arithmetic data on top of it:

void fixed_foo(int c) {
  std::vector<char> real_data(c * sizeof(float));
  float* raw_data = new (real_data.data()) float[c];

  //Fill raw_data with usefull stuff...

  dispatch(std::move(real_data));
}

这看起来很像地狱，但我认为这可能是合法的。

This looks funky as hell, but I "think" it might be legal. Maybe?

推荐答案

避开别名规则的最安全方法是使用 memcpy（），但您不需要对整个第二个数据副本执行此操作。我建议对本地 float 变量进行所有 float 工作，然后对 memcpy（ ）一次将其存储到 real_data 缓冲区中的适当位置。根据我的经验，大多数编译器都会对此进行有效优化。

The safest way to get around aliasing rules is to use memcpy() but you don't need to do that on a whole second copy of the data. I'd suggest doing all your float work on a local float variable and then memcpy()ing that to the appropriate location in your real_data buffer an element at a time. Most compilers will optimize that effectively in my experience.

void better_foo(int c) {
  std::vector<char> real_data(c * sizeof(float));

  //Fill raw_data with usefull stuff...
  for (int i = 0; i < c; ++i) {
    float x = my_complicated_calculation(i);
    memcpy(&real_data[i * sizeof(float)], &x, sizeof(x));
  }

  dispatch(std::move(real_data));
}

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