为已知的更常见路径优化分支

2021-12-20 00:00:00 compiler-optimization optimization c c++

请考虑以下代码:

void error_handling();
bool method_impl();

bool method()
{
    const bool res = method_impl();
    if (res == false) {
        error_handling();
        return false;
    }
    return true;
}

我知道 method_impl() 会返回 true 99.999%(是的，三位小数)，但我的编译器不会.method() 在时间消耗方面是部分关键的.

I know method_impl() will return true 99.999% (yes, three decimal places) of the time, but my compiler doesn't. method() is partially critical in term of time-consumption.

  1. 我是否应该重写method()(并降低可读性)以确保仅在method_impl() 返回false 时才可能发生跳转?如果是，如何?
  2. 我应该让编译器为我做这些工作吗?
  3. 我应该让 CPU 的分支预测为我做这些工作吗?
  1. Should I rewrite method() (and make it less readable) to ensure a jump may only occur when method_impl() returns false? If yes, how?
  2. Should I let the compiler do the work for me?
  3. Should I let the branch prediction of my CPU do the work for me?

推荐答案

底层硬件已经执行了这种优化.第一次预测它会失败"，但在它会命中正确的选项 en.wikipedia.org/wiki/Branch_predictor 之后.

The underlying hardware already performs this optimizations. It will "fail" to predict it the first times, but after it will hit the correct option en.wikipedia.org/wiki/Branch_predictor.

您可以尝试应用 GCC 扩展并检查使用它是否更快，但我认为您几乎看不出有和没有它的任何区别.总是应用分支预测，它不是你启用的东西

You can try applying the GCC extension and check if it is faster with it or not, but I think you will barely see any difference with it and without it. The branch prediction is applied always, it is not something you enable

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