Eric Dumazet | 6a2d7a9 | 2006-12-13 00:34:27 -0800 | [diff] [blame^] | 1 | #ifndef _LINUX_RECIPROCAL_DIV_H |
| 2 | #define _LINUX_RECIPROCAL_DIV_H |
| 3 | |
| 4 | #include <linux/types.h> |
| 5 | |
| 6 | /* |
| 7 | * This file describes reciprocical division. |
| 8 | * |
| 9 | * This optimizes the (A/B) problem, when A and B are two u32 |
| 10 | * and B is a known value (but not known at compile time) |
| 11 | * |
| 12 | * The math principle used is : |
| 13 | * Let RECIPROCAL_VALUE(B) be (((1LL << 32) + (B - 1))/ B) |
| 14 | * Then A / B = (u32)(((u64)(A) * (R)) >> 32) |
| 15 | * |
| 16 | * This replaces a divide by a multiply (and a shift), and |
| 17 | * is generally less expensive in CPU cycles. |
| 18 | */ |
| 19 | |
| 20 | /* |
| 21 | * Computes the reciprocal value (R) for the value B of the divisor. |
| 22 | * Should not be called before each reciprocal_divide(), |
| 23 | * or else the performance is slower than a normal divide. |
| 24 | */ |
| 25 | extern u32 reciprocal_value(u32 B); |
| 26 | |
| 27 | |
| 28 | static inline u32 reciprocal_divide(u32 A, u32 R) |
| 29 | { |
| 30 | return (u32)(((u64)A * R) >> 32); |
| 31 | } |
| 32 | #endif |