1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
|
/* Optimization of repeated integer division.
Copyright (C) 2019 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <stdint.h>
#include <sys/param.h>
/* Precompute *MULTIPLIER for dividing by DIVISOR, which must be two
or larger, and return the shift count (non-negative and less than
32), for use with divopt_32 below. */
static int __attribute__ ((used))
precompute_divopt_32 (uint32_t divisor, uint32_t *multiplier)
{
if (divisor == 1)
{
*multiplier = 1;
return 0;
}
int log2 = 32 - __builtin_clz (divisor);
/* Handle powers-of-two first, so that we do not need to deal with
the clz corner cases below. */
if (powerof2 (divisor))
{
*multiplier = 1;
return log2 - 2;
}
if (log2 != 32)
{
/* Compute ceil (2**(32 + log2) / divisor). The
most-significant bit is always set and is discarded. */
*multiplier = (((uint64_t) 1 << (32 + log2)) + divisor) / divisor;
return log2 - 1;
}
else
{
/* Perform a long division of 2**64 + (divisor - 1) by the
divisor, encoded in base-2**32, using a 64-by-32 division.
Start out with the first two digits, which are (1, 0). 2**32
divided by the divisor is 1 because the divisor is larger
than 2**31. This set bit is discarded. */
uint64_t remainder = -divisor;
/* Combine the remainder of the first division with the third
and final base 2**32 digit. */
*multiplier = ((remainder << 32) | (divisor - 1)) / divisor;
return 31;
}
}
/* Return the quotient of DIVIDEND devided by the divisor that was
used to compute MULTIPLIER and SHIFT via precompute_divopt_32. */
static inline uint32_t
divopt_32 (uint32_t dividend, uint32_t multiplier, int shift)
{
/* Approximation to the quotient. */
uint32_t quotient = ((uint64_t) dividend * multiplier) >> 32;
/* Compute (dividend + quotient) / 2 without overflow. */
uint32_t temp = ((dividend - quotient) >> 1) + quotient;
/* The result is in the higher-order bits. */
return temp >> shift;
}
|
