Compute the minimum or maximum of two integers without branching

转载于 2013-06-09 14:12:13 发布 · 952 阅读

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本文介绍了一种不使用条件分支来计算两个整数的最大值和最小值的方法。通过巧妙地利用位运算和布尔表达式的特性，该方法能够在不需要传统条件判断的情况下高效地完成计算。

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reference:

http://www.geeksforgeeks.org/compute-the-minimum-or-maximum-max-of-two-integers-without-branching/

Problem Definition:

The key idea is to incorporate the branch into one equation.

Solution:

Minimum of x and y will be

y ^ ((x ^ y) & -(x < y))

It works because if x < y, then -(x < y) will be all ones, so r = y ^ (x ^ y) & ~0 = y ^ x ^ y = x. Otherwise, if x >= y, then -(x < y) will be all zeros, so r = y ^ ((x ^ y) & 0) = y.On some machines, evaluating (x < y) as 0 or 1 requires a branch instruction, so there may be no advantage.

To find the maximum, use

x ^ ((x ^ y) & -(x < y));

Code:

/*Function to find minimum of x and y*/
int min(int x, int y)
{
  return y ^ ((x ^ y) & -(x < y));
}
 
/*Function to find maximum of x and y*/
int max(int x, int y)
{
  return x ^ ((x ^ y) & -(x < y)); 
}