android-kvm / linux / 6bbfa44116689469267f1a6e3d233b52114139d2 / . / include / linux / reciprocal_div.h

/* SPDX-License-Identifier: GPL-2.0 */ | |

#ifndef _LINUX_RECIPROCAL_DIV_H | |

#define _LINUX_RECIPROCAL_DIV_H | |

#include <linux/types.h> | |

/* | |

* This algorithm is based on the paper "Division by Invariant | |

* Integers Using Multiplication" by Torbjörn Granlund and Peter | |

* L. Montgomery. | |

* | |

* The assembler implementation from Agner Fog, which this code is | |

* based on, can be found here: | |

* http://www.agner.org/optimize/asmlib.zip | |

* | |

* This optimization for A/B is helpful if the divisor B is mostly | |

* runtime invariant. The reciprocal of B is calculated in the | |

* slow-path with reciprocal_value(). The fast-path can then just use | |

* a much faster multiplication operation with a variable dividend A | |

* to calculate the division A/B. | |

*/ | |

struct reciprocal_value { | |

u32 m; | |

u8 sh1, sh2; | |

}; | |

/* "reciprocal_value" and "reciprocal_divide" together implement the basic | |

* version of the algorithm described in Figure 4.1 of the paper. | |

*/ | |

struct reciprocal_value reciprocal_value(u32 d); | |

static inline u32 reciprocal_divide(u32 a, struct reciprocal_value R) | |

{ | |

u32 t = (u32)(((u64)a * R.m) >> 32); | |

return (t + ((a - t) >> R.sh1)) >> R.sh2; | |

} | |

struct reciprocal_value_adv { | |

u32 m; | |

u8 sh, exp; | |

bool is_wide_m; | |

}; | |

/* "reciprocal_value_adv" implements the advanced version of the algorithm | |

* described in Figure 4.2 of the paper except when "divisor > (1U << 31)" whose | |

* ceil(log2(d)) result will be 32 which then requires u128 divide on host. The | |

* exception case could be easily handled before calling "reciprocal_value_adv". | |

* | |

* The advanced version requires more complex calculation to get the reciprocal | |

* multiplier and other control variables, but then could reduce the required | |

* emulation operations. | |

* | |

* It makes no sense to use this advanced version for host divide emulation, | |

* those extra complexities for calculating multiplier etc could completely | |

* waive our saving on emulation operations. | |

* | |

* However, it makes sense to use it for JIT divide code generation for which | |

* we are willing to trade performance of JITed code with that of host. As shown | |

* by the following pseudo code, the required emulation operations could go down | |

* from 6 (the basic version) to 3 or 4. | |

* | |

* To use the result of "reciprocal_value_adv", suppose we want to calculate | |

* n/d, the pseudo C code will be: | |

* | |

* struct reciprocal_value_adv rvalue; | |

* u8 pre_shift, exp; | |

* | |

* // handle exception case. | |

* if (d >= (1U << 31)) { | |

* result = n >= d; | |

* return; | |

* } | |

* | |

* rvalue = reciprocal_value_adv(d, 32) | |

* exp = rvalue.exp; | |

* if (rvalue.is_wide_m && !(d & 1)) { | |

* // floor(log2(d & (2^32 -d))) | |

* pre_shift = fls(d & -d) - 1; | |

* rvalue = reciprocal_value_adv(d >> pre_shift, 32 - pre_shift); | |

* } else { | |

* pre_shift = 0; | |

* } | |

* | |

* // code generation starts. | |

* if (imm == 1U << exp) { | |

* result = n >> exp; | |

* } else if (rvalue.is_wide_m) { | |

* // pre_shift must be zero when reached here. | |

* t = (n * rvalue.m) >> 32; | |

* result = n - t; | |

* result >>= 1; | |

* result += t; | |

* result >>= rvalue.sh - 1; | |

* } else { | |

* if (pre_shift) | |

* result = n >> pre_shift; | |

* result = ((u64)result * rvalue.m) >> 32; | |

* result >>= rvalue.sh; | |

* } | |

*/ | |

struct reciprocal_value_adv reciprocal_value_adv(u32 d, u8 prec); | |

#endif /* _LINUX_RECIPROCAL_DIV_H */ |