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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 : 14147457 : return (u32)(((u64)A * R) >> 32);
31 : : }
32 : : #endif
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