什么是比std :: pow更快？ [英] What is faster than std::pow?

问题描述

我的程序在 std :: pow（double，int）函数中花费了90％的CPU时间。准确性不是这里的主要关注点，所以我想知道是否有更快的选择。一个我想的尝试是转换为浮动，执行操作，然后回到双（还没有尝试过）;我担心这不是一种提高性能的可移植方法（不是大多数CPU本质上都是双向操作的）

My program spends 90% of CPU time in the std::pow(double,int) function. Accuracy is not a primary concern here, so I was wondering if there were any faster alternatives. One thing I was thinking of trying is casting to float, performing the operation and then back to double (haven't tried this yet); I am concerned that this is not a portable way of improving performance (don't most CPUs operate on doubles intrinsically anyway?)

干杯

推荐答案

看起来像马丁·安克尔有一些关于此的文章，

It looks like Martin Ankerl has a few of articles on this, Optimized Approximative pow() in C / C++ is one and it has two fast versions, one is as follows:

inline double fastPow(double a, double b) {
  union {
    double d;
    int x[2];
  } u = { a };
  u.x[1] = (int)(b * (u.x[1] - 1072632447) + 1072632447);
  u.x[0] = 0;
  return u.d;
}

这取决于在C ++中未定义行为的联合类型， 9.5 [class.union] ：



which relies on type punning through a union which is undefined behavior in C++, from the draft standard section 9.5 [class.union]:

在并集中，最多一个非静态数据成员可以在任何时间处于活动状态，也就是说，
的值可以将大多数非静态数据成员存储在联合随时。 [...]

In a union, at most one of the non-static data members can be active at any time, that is, the value of at most one of the non-static data members can be stored in a union at any time. [...]

但大多数编译器包括 gcc支持此定义的行为：

从与最近写入的成员不同的成员（称为类型冲击）读取的做法是常见的。即使使用-fstrict别名，只要通过联合类型

The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type

访问内存，则允许类型划分，但这不是通用的作为本文指出和我在我的回答在这里指出使用 memcpy 应该生成相同的代码，不会调用未定义的行为。

but this is not universal as this article points out and as I point out in my answer here using memcpy should generate identical code and does not invoke undefined behavior.

他也链接到第二个为Java，C / C ++和C＃优化pow（）近似。

He also links to a second one Optimized pow() approximation for Java, C / C++, and C#.

第一篇文章还链接到他的microbenchmarks < a href =http://pastebin.com/DRvPJL2K =nofollow>此处

The first article also links to his microbenchmarks here

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