很早以前看到过这篇文章, 最近学牛顿迭代法, 于是又想到了, 经典啊经典.
Quake III公开源码后,有人在game/code/q_math.c里发现了这样一段代码。它的作用是将一个数开平方并取倒,经测试这段代码比(float) (1.0/sqrt(x))快4倍:
float Q_rsqrt( float number )
{
long i;
float x2, y;
const float threehalfs = 1.5F;
x2 = number * 0.5F;
y = number;
i = * ( long * ) &y; // evil floating point bit level hacking
i = 0x5f3759df - ( i >> 1 ); // what the fuck?
y = * ( float * ) &i;
y = y * ( threehalfs - ( x2 * y * y ) ); // 1st iteration
// y = y * ( threehalfs - ( x2 * y * y ) ); // 2nd iteration, this can be removed
#ifndef Q3_VM
#ifdef __linux__
assert( !isnan(y) ); // bk010122 - FPE?
No comments:
Post a Comment